|
|
|
|
|
|
IETF RFC 8551
Last modified on Wednesday, May 1st, 2019
 Permanent link to RFC 8551
 Search GitHub Wiki for RFC 8551
 Show other RFCs mentioning RFC 8551
Internet Engineering Task Force (IETF) J. Schaad
Request for Comments: 8551 August Cellars
Obsoletes: 5751 B. Ramsdell
Category: Standards Track Brute Squad Labs, Inc.
ISSN: 2070-1721 S. Turner
sn3rd
April 2019
Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
Message Specification
 Abstract
This document defines Secure/Multipurpose Internet Mail Extensions
(S/MIME) version 4.0. S/MIME provides a consistent way to send and
receive secure MIME data. Digital signatures provide authentication,
message integrity, and non-repudiation with proof of origin.
Encryption provides data confidentiality. Compression can be used to
reduce data size. This document obsoletes RFC 5751.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/RFC 8551.
Schaad, et al. Standards Track PAGE 1 
RFC 8551 S/MIME 4.0 Message Specification April 2019
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Schaad, et al. Standards Track PAGE 2
RFC 8551 S/MIME 4.0 Message Specification April 2019
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Specification Overview . . . . . . . . . . . . . . . . . 5
1.2. Definitions . . . . . . . . . . . . . . . . . . . . . . . 6
1.3. Conventions Used in This Document . . . . . . . . . . . . 7
1.4. Compatibility with Prior Practice of S/MIME . . . . . . . 8
1.5. Changes from S/MIME v3 to S/MIME v3.1 . . . . . . . . . . 9
1.6. Changes from S/MIME v3.1 to S/MIME v3.2 . . . . . . . . . 9
1.7. Changes for S/MIME v4.0 . . . . . . . . . . . . . . . . . 11
2. CMS Options . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.1. DigestAlgorithmIdentifier . . . . . . . . . . . . . . . . 12
2.2. SignatureAlgorithmIdentifier . . . . . . . . . . . . . . 12
2.3. KeyEncryptionAlgorithmIdentifier . . . . . . . . . . . . 13
2.4. General Syntax . . . . . . . . . . . . . . . . . . . . . 13
2.4.1. Data Content Type . . . . . . . . . . . . . . . . . . 14
2.4.2. SignedData Content Type . . . . . . . . . . . . . . . 14
2.4.3. EnvelopedData Content Type . . . . . . . . . . . . . 14
2.4.4. AuthEnvelopedData Content Type . . . . . . . . . . . 14
2.4.5. CompressedData Content Type . . . . . . . . . . . . . 14
2.5. Attributes and the SignerInfo Type . . . . . . . . . . . 15
2.5.1. Signing Time Attribute . . . . . . . . . . . . . . . 15
2.5.2. SMIMECapabilities Attribute . . . . . . . . . . . . . 16
2.5.3. Encryption Key Preference Attribute . . . . . . . . . 17
2.6. SignerIdentifier SignerInfo Type . . . . . . . . . . . . 19
2.7. ContentEncryptionAlgorithmIdentifier . . . . . . . . . . 19
2.7.1. Deciding Which Encryption Method to Use . . . . . . . 19
2.7.2. Choosing Weak Encryption . . . . . . . . . . . . . . 21
2.7.3. Multiple Recipients . . . . . . . . . . . . . . . . . 21
3. Creating S/MIME Messages . . . . . . . . . . . . . . . . . . 21
3.1. Preparing the MIME Entity for Signing, Enveloping, or
Compressing . . . . . . . . . . . . . . . . . . . . . . . 22
3.1.1. Canonicalization . . . . . . . . . . . . . . . . . . 23
3.1.2. Transfer Encoding . . . . . . . . . . . . . . . . . . 24
3.1.3. Transfer Encoding for Signing Using multipart/signed 25
3.1.4. Sample Canonical MIME Entity . . . . . . . . . . . . 25
3.2. The application/pkcs7-mime Media Type . . . . . . . . . . 26
3.2.1. The name and filename Parameters . . . . . . . . . . 27
3.2.2. The smime-type Parameter . . . . . . . . . . . . . . 28
3.3. Creating an Enveloped-Only Message . . . . . . . . . . . 29
3.4. Creating an Authenticated Enveloped-Only Message . . . . 30
3.5. Creating a Signed-Only Message . . . . . . . . . . . . . 31
3.5.1. Choosing a Format for Signed-Only Messages . . . . . 32
3.5.2. Signing Using application/pkcs7-mime with SignedData 32
3.5.3. Signing Using the multipart/signed Format . . . . . . 33
3.6. Creating a Compressed-Only Message . . . . . . . . . . . 36
3.7. Multiple Operations . . . . . . . . . . . . . . . . . . . 37
3.8. Creating a Certificate Management Message . . . . . . . . 38
Schaad, et al. Standards Track PAGE 3
RFC 8551 S/MIME 4.0 Message Specification April 2019
3.9. Registration Requests . . . . . . . . . . . . . . . . . . 38
3.10. Identifying an S/MIME Message . . . . . . . . . . . . . . 39
4. Certificate Processing . . . . . . . . . . . . . . . . . . . 39
4.1. Key Pair Generation . . . . . . . . . . . . . . . . . . . 40
4.2. Signature Generation . . . . . . . . . . . . . . . . . . 40
4.3. Signature Verification . . . . . . . . . . . . . . . . . 40
4.4. Encryption . . . . . . . . . . . . . . . . . . . . . . . 41
4.5. Decryption . . . . . . . . . . . . . . . . . . . . . . . 41
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 41
5.1. Media Type for application/pkcs7-mime . . . . . . . . . . 42
5.2. Media Type for application/pkcs7-signature . . . . . . . 43
5.3. authEnveloped-data smime-type . . . . . . . . . . . . . . 44
5.4. Reference Updates . . . . . . . . . . . . . . . . . . . . 44
6. Security Considerations . . . . . . . . . . . . . . . . . . . 44
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 48
7.1. Reference Conventions . . . . . . . . . . . . . . . . . . 48
7.2. Normative References . . . . . . . . . . . . . . . . . . 49
7.3. Informative References . . . . . . . . . . . . . . . . . 52
Appendix A. ASN.1 Module . . . . . . . . . . . . . . . . . . . . 57
Appendix B. Historic Mail Considerations . . . . . . . . . . . . 59
B.1. DigestAlgorithmIdentifier . . . . . . . . . . . . . . . . 59
B.2. Signature Algorithms . . . . . . . . . . . . . . . . . . 59
B.3. ContentEncryptionAlgorithmIdentifier . . . . . . . . . . 61
B.4. KeyEncryptionAlgorithmIdentifier . . . . . . . . . . . . 62
Appendix C. Moving S/MIME v2 Message Specification to Historic
Status . . . . . . . . . . . . . . . . . . . . . . . 62
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 62
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 63
Schaad, et al. Standards Track PAGE 4
RFC 8551 S/MIME 4.0 Message Specification April 2019
1. Introduction
S/MIME (Secure/Multipurpose Internet Mail Extensions) provides a
consistent way to send and receive secure MIME data. Based on the
popular Internet MIME standard, S/MIME provides the following
cryptographic security services for electronic messaging
applications: authentication, message integrity, and non-repudiation
of origin (using digital signatures), and data confidentiality (using
encryption). As a supplementary service, S/MIME provides message
compression.
S/MIME can be used by traditional mail user agents (MUAs) to add
cryptographic security services to mail that is sent, and to
interpret cryptographic security services in mail that is received.
However, S/MIME is not restricted to mail; it can be used with any
transport mechanism that transports MIME data, such as HTTP or SIP.
As such, S/MIME takes advantage of the object-based features of MIME
and allows secure messages to be exchanged in mixed-transport
systems.
Further, S/MIME can be used in automated message transfer agents that
use cryptographic security services that do not require any human
intervention, such as the signing of software-generated documents and
the encryption of FAX messages sent over the Internet.
This document defines version 4.0 of the S/MIME Message
Specification. As such, this document obsoletes version 3.2 of the
S/MIME Message Specification [RFC 5751].
This specification contains a number of references to documents that
have been obsoleted or replaced. This is intentional, as the updated
documents often do not have the same information or protocol
requirements in them.
1.1. Specification Overview
This document describes a protocol for adding cryptographic signature
and encryption services to MIME data. The MIME standard [MIME-SPEC]
provides a general structure for the content of Internet messages and
allows extensions for new applications based on content-type.
This specification defines how to create a MIME body part that has
been cryptographically enhanced according to the Cryptographic
Message Syntax (CMS) [CMS], which is derived from PKCS #7 [RFC 2315].
This specification also defines the application/pkcs7-mime media
type, which can be used to transport those body parts.
Schaad, et al. Standards Track PAGE 5
RFC 8551 S/MIME 4.0 Message Specification April 2019
This document also discusses how to use the multipart/signed media
type defined in [RFC 1847] to transport S/MIME signed messages.
multipart/signed is used in conjunction with the
application/pkcs7-signature media type, which is used to transport a
detached S/MIME signature.
In order to create S/MIME messages, an S/MIME agent MUST follow the
specifications in this document, as well as the specifications listed
in [CMS], [RFC 3370], [RFC 4056], [RFC 3560], and [RFC 5754].
Throughout this specification, there are requirements and
recommendations made for how receiving agents handle incoming
messages. There are separate requirements and recommendations for
how sending agents create outgoing messages. In general, the best
strategy is to follow the Robustness Principle (be liberal in what
you receive and conservative in what you send). Most of the
requirements are placed on the handling of incoming messages, while
the recommendations are mostly on the creation of outgoing messages.
The separation for requirements on receiving agents and sending
agents also derives from the likelihood that there will be S/MIME
systems that involve software other than traditional Internet mail
clients. S/MIME can be used with any system that transports MIME
data. An automated process that sends an encrypted message might not
be able to receive an encrypted message at all, for example. Thus,
the requirements and recommendations for the two types of agents are
listed separately when appropriate.
1.2. Definitions
For the purposes of this specification, the following definitions
apply.
ASN.1:
Abstract Syntax Notation One, as defined in ITU-T Recommendations
X.680, X.681, X.682, and X.683 [ASN.1].
BER:
Basic Encoding Rules for ASN.1, as defined in ITU-T Recommendation
X.690 [X.690].
Certificate:
A type that binds an entity's name to a public key with a digital
signature.
DER:
Distinguished Encoding Rules for ASN.1, as defined in ITU-T
Recommendation X.690 [X.690].
Schaad, et al. Standards Track PAGE 6
RFC 8551 S/MIME 4.0 Message Specification April 2019
7-bit data:
Text data with lines less than 998 characters long, where none of
the characters have the 8th bit set, and there are no NULL
characters. <CR> and <LF> occur only as part of a <CR><LF>
end-of-line delimiter.
8-bit data:
Text data with lines less than 998 characters, and where none of
the characters are NULL characters. <CR> and <LF> occur only as
part of a <CR><LF> end-of-line delimiter.
Binary data:
Arbitrary data.
Transfer encoding:
A reversible transformation made on data so 8-bit or binary data
can be sent via a channel that only transmits 7-bit data.
Receiving agent:
Software that interprets and processes S/MIME CMS objects, MIME
body parts that contain CMS content types, or both.
Sending agent:
Software that creates S/MIME CMS content types, MIME body parts
that contain CMS content types, or both.
S/MIME agent:
User software that is a receiving agent, a sending agent, or both.
Data integrity service:
A security service that protects against unauthorized changes to
data by ensuring that changes to the data are detectable
[RFC 4949].
Data confidentiality:
The property that data is not disclosed to system entities unless
they have been authorized to know the data [RFC 4949].
1.3. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC 2119] [RFC 8174] when, and only when, they appear in all
capitals, as shown here.
Schaad, et al. Standards Track PAGE 7
RFC 8551 S/MIME 4.0 Message Specification April 2019
We define the additional requirement levels:
SHOULD+ This term means the same as SHOULD. However, the authors
expect that a requirement marked as SHOULD+ will be
promoted at some future time to be a MUST.
SHOULD- This term means the same as SHOULD. However, the authors
expect that a requirement marked as SHOULD- will be demoted
to a MAY in a future version of this document.
MUST- This term means the same as MUST. However, the authors
expect that this requirement will no longer be a MUST in a
future document. Although its status will be determined at
a later time, it is reasonable to expect that if a future
revision of a document alters the status of a MUST-
requirement, it will remain at least a SHOULD or a SHOULD-.
The term "RSA" in this document almost always refers to the
PKCS #1 v1.5 RSA [RFC 2313] signature or encryption algorithms even
when not qualified as such. There are a couple of places where it
refers to the general RSA cryptographic operation; these can be
determined from the context where it is used.
1.4. Compatibility with Prior Practice of S/MIME
S/MIME version 4.0 agents ought to attempt to have the greatest
interoperability possible with agents for prior versions of S/MIME.
- S/MIME version 2 is described in RFC 2311 through RFC 2315
inclusive [SMIMEv2].
- S/MIME version 3 is described in RFC 2630 through RFC 2634
inclusive and RFC 5035 [SMIMEv3].
- S/MIME version 3.1 is described in RFC 2634, RFC 3850, RFC 3851,
RFC 3852, and RFC 5035 [SMIMEv3.1].
- S/MIME version 3.2 is described in RFC 2634, RFC 5035, RFC 5652,
RFC 5750, and RFC 5751 [SMIMEv3.2].
- [RFC 2311] also has historical information about the development of
S/MIME.
Schaad, et al. Standards Track PAGE 8
RFC 8551 S/MIME 4.0 Message Specification April 2019
1.5. Changes from S/MIME v3 to S/MIME v3.1
This section describes the changes made between S/MIME v3 and
S/MIME v3.1. Note that the requirement levels indicated by the
capitalized key words ("MUST", "SHOULD", etc.) may have changed in
later versions of S/MIME.
- The RSA public key algorithm was changed to a MUST implement. The
key wrap algorithm and the Diffie-Hellman (DH) algorithm [RFC 2631]
were changed to a SHOULD implement.
- The AES symmetric encryption algorithm has been included as a
SHOULD implement.
- The RSA public key algorithm was changed to a MUST implement
signature algorithm.
- Ambiguous language about the use of "empty" SignedData messages to
transmit certificates was clarified to reflect that transmission
of Certificate Revocation Lists is also allowed.
- The use of binary encoding for some MIME entities is now
explicitly discussed.
- Header protection through the use of the message/RFC 822 media type
has been added.
- Use of the CompressedData CMS type is allowed, along with required
media type and file extension additions.
1.6. Changes from S/MIME v3.1 to S/MIME v3.2
This section describes the changes made between S/MIME v3.1 and
S/MIME v3.2. Note that the requirement levels indicated by the
capitalized key words ("MUST", "SHOULD", etc.) may have changed in
later versions of S/MIME. Note that the section numbers listed here
(e.g., 3.4.3.2) are from [RFC 5751].
- Made editorial changes, e.g., replaced "MIME type" with "media
type", "content-type" with "Content-Type".
- Moved "Conventions Used in This Document" to Section 1.3. Added
definitions for SHOULD+, SHOULD-, and MUST-.
- Section 1.1 and Appendix A: Added references to RFCs for
RSASSA-PSS, RSAES-OAEP, and SHA2 CMS algorithms. Added CMS
Multiple Signers Clarification to CMS reference.
Schaad, et al. Standards Track PAGE 9
RFC 8551 S/MIME 4.0 Message Specification April 2019
- Section 1.2: Updated references to ASN.1 to X.680, and BER and DER
to X.690.
- Section 1.4: Added references to S/MIME v3.1 RFCs.
- Section 2.1 (digest algorithm): SHA-256 added as MUST, SHA-1 and
MD5 made SHOULD-.
- Section 2.2 (signature algorithms): RSA with SHA-256 added as
MUST; DSA with SHA-256 added as SHOULD+; RSA with SHA-1, DSA with
SHA-1, and RSA with MD5 changed to SHOULD-; and RSASSA-PSS with
SHA-256 added as SHOULD+. Also added note about what S/MIME v3.1
clients support.
- Section 2.3 (key encryption): DH changed to SHOULD-, and RSAES-
OAEP added as SHOULD+. Elaborated on requirements for key wrap
algorithm.
- Section 2.5.1: Added requirement that receiving agents MUST
support both GeneralizedTime and UTCTime.
- Section 2.5.2: Replaced reference "sha1WithRSAEncryption" with
"sha256WithRSAEncryption", replaced "DES-3EDE-CBC" with "AES-128
CBC", and deleted the RC5 example.
- Section 2.5.2.1: Deleted entire section (discussed
deprecated RC2).
- Section 2.7, Section 2.7.1, and Appendix A: References to RC2/40
removed.
- Section 2.7 (content encryption): AES-128 CBC added as MUST,
AES-192 and AES-256 CBC SHOULD+, and tripleDES now SHOULD-.
- Section 2.7.1: Updated pointers from 2.7.2.1 through 2.7.2.4 to
2.7.1.1 and 2.7.1.2.
- Section 3.1.1: Removed text about MIME character sets.
- Sections 3.2.2 and 3.6: Replaced "encrypted" with "enveloped".
Updated OID example to use AES-128 CBC OID.
- Section 3.4.3.2: Replaced "micalg" parameter for "SHA-1" with
"sha-1".
- Section 4: Updated reference to CERT v3.2.
Schaad, et al. Standards Track PAGE 10
RFC 8551 S/MIME 4.0 Message Specification April 2019
- Section 4.1: Updated RSA and DSA key size discussion. Moved last
four sentences to security considerations. Updated reference to
randomness requirements for security.
- Section 5: Added IANA registration templates to update media type
registry to point to this document as opposed to RFC 2311.
- Section 6: Updated security considerations.
- Section 7: Moved references from Appendix B to this section.
Updated references. Added informative references to SMIMEv2,
SMIMEv3, and SMIMEv3.1.
- Appendix B: Added Appendix B to move S/MIME v2 to Historic status.
1.7. Changes for S/MIME v4.0
This section describes the changes made between S/MIME v3.2 and
S/MIME v4.0.
- Added the use of AuthEnvelopedData, including defining and
registering an smime-type value (Sections 2.4.4 and 3.4).
- Updated the content-encryption algorithms (Sections 2.7 and
2.7.1.2): added AES-256 Galois/Counter Mode (GCM), added
ChaCha20-Poly1305, removed mention of AES-192 Cipher Block
Chaining (CBC), and marked tripleDES as historic.
- Updated the set of signature algorithms (Section 2.2): added the
Edwards-curve Digital Signature Algorithm (EdDSA), added the
Elliptic Curve Digital Signature Algorithm (ECDSA), and marked DSA
as historic.
- Updated the set of digest algorithms (Section 2.1): added SHA-512,
and marked SHA-1 as historic.
- Updated the size of keys to be used for RSA encryption and RSA
signing (Section 4).
- Created Appendix B, which discusses considerations for dealing
with historic email messages.
Schaad, et al. Standards Track PAGE 11
RFC 8551 S/MIME 4.0 Message Specification April 2019
2. CMS Options
CMS allows for a wide variety of options in content, attributes, and
algorithm support. This section puts forth a number of support
requirements and recommendations in order to achieve a base level of
interoperability among all S/MIME implementations. [RFC 3370] and
[RFC 5754] provide additional details regarding the use of the
cryptographic algorithms. [ESS] provides additional details
regarding the use of additional attributes.
2.1. DigestAlgorithmIdentifier
The algorithms here are used for digesting the body of the message
and are not the same as the digest algorithms used as part of the
signature algorithms. The result of this is placed in the
message-digest attribute of the signed attributes. It is RECOMMENDED
that the algorithm used for digesting the body of the message be of
similar strength to, or greater strength than, the signature
algorithm.
Sending and receiving agents:
- MUST support SHA-256.
- MUST support SHA-512.
[RFC 5754] provides the details for using these algorithms with
S/MIME.
2.2. SignatureAlgorithmIdentifier
There are different sets of requirements placed on receiving and
sending agents. By having the different requirements, the maximum
amount of interoperability is achieved, as it allows for specialized
protection of private key material but maximum signature validation.
Receiving agents:
- MUST support ECDSA with curve P-256 and SHA-256.
- MUST support EdDSA with curve25519 using PureEdDSA mode [RFC 8419].
- MUST- support RSA PKCS #1 v1.5 with SHA-256.
- SHOULD support the RSA Probabilistic Signature Scheme (RSASSA-PSS)
with SHA-256.
Schaad, et al. Standards Track PAGE 12
RFC 8551 S/MIME 4.0 Message Specification April 2019
Sending agents:
- MUST support at least one of the following algorithms: ECDSA with
curve P-256 and SHA-256, or EdDSA with curve25519 using PureEdDSA
mode.
- MUST- support RSA PKCS #1 v1.5 with SHA-256.
- SHOULD support RSASSA-PSS with SHA-256.
See Section 4.1 for information on key size and algorithm references.
2.3. KeyEncryptionAlgorithmIdentifier
Receiving and sending agents:
- MUST support Elliptic Curve Diffie-Hellman (ECDH) ephemeral-static
mode for P-256, as specified in [RFC 5753].
- MUST support ECDH ephemeral-static mode for X25519 using HKDF-256
("HKDF" stands for "HMAC-based Key Derivation Function") for the
KDF, as specified in [RFC 8418].
- MUST- support RSA encryption, as specified in [RFC 3370].
- SHOULD+ support RSA Encryption Scheme - Optimal Asymmetric
Encryption Padding (RSAES-OAEP), as specified in [RFC 3560].
When ECDH ephemeral-static is used, a key wrap algorithm is also
specified in the KeyEncryptionAlgorithmIdentifier [RFC 5652]. The
underlying encryption functions for the key wrap and content-
encryption algorithms [RFC 3370] [RFC 3565] and the key sizes for the
two algorithms MUST be the same (e.g., AES-128 key wrap algorithm
with AES-128 content-encryption algorithm). As both 128-bit and
256-bit AES modes are mandatory to implement as content-encryption
algorithms (Section 2.7), both the AES-128 and AES-256 key wrap
algorithms MUST be supported when ECDH ephemeral-static is used.
Recipients MAY enforce this but MUST use the weaker of the two as
part of any cryptographic strength computations they might do.
Appendix B provides information on algorithm support in older
versions of S/MIME.
2.4. General Syntax
There are several CMS content types. Of these, only the Data,
SignedData, EnvelopedData, AuthEnvelopedData, and CompressedData
content types are currently used for S/MIME.
Schaad, et al. Standards Track PAGE 13
RFC 8551 S/MIME 4.0 Message Specification April 2019
2.4.1. Data Content Type
Sending agents MUST use the id-data content type identifier to
identify the "inner" MIME message content. For example, when
applying a digital signature to MIME data, the CMS SignedData
encapContentInfo eContentType MUST include the id-data object
identifier (OID), and the media type MUST be stored in the SignedData
encapContentInfo eContent OCTET STRING (unless the sending agent is
using multipart/signed, in which case the eContent is absent, per
Section 3.5.3 of this document). As another example, when applying
encryption to MIME data, the CMS EnvelopedData encryptedContentInfo
contentType MUST include the id-data OID and the encrypted MIME
content MUST be stored in the EnvelopedData encryptedContentInfo
encryptedContent OCTET STRING.
2.4.2. SignedData Content Type
Sending agents MUST use the SignedData content type to apply a
digital signature to a message or, in a degenerate case where there
is no signature information, to convey certificates. Applying a
signature to a message provides authentication, message integrity,
and non-repudiation of origin.
2.4.3. EnvelopedData Content Type
This content type is used to apply data confidentiality to a message.
In order to distribute the symmetric key, a sender needs to have
access to a public key for each intended message recipient to use
this service.
2.4.4. AuthEnvelopedData Content Type
This content type is used to apply data confidentiality and message
integrity to a message. This content type does not provide
authentication or non-repudiation. In order to distribute the
symmetric key, a sender needs to have access to a public key for each
intended message recipient to use this service.
2.4.5. CompressedData Content Type
This content type is used to apply data compression to a message.
This content type does not provide authentication, message integrity,
non-repudiation, or data confidentiality; it is only used to reduce
the message's size.
See Section 3.7 for further guidance on the use of this type in
conjunction with other CMS types.
Schaad, et al. Standards Track PAGE 14
RFC 8551 S/MIME 4.0 Message Specification April 2019
2.5. Attributes and the SignerInfo Type
The SignerInfo type allows the inclusion of unsigned and signed
attributes along with a signature. These attributes can be required
for the processing of messages (e.g., message digest), information
the signer supplied (e.g., SMIME capabilities) that should be
processed, or attributes that are not relevant to the current
situation (e.g., mlExpansionHistory [RFC 2634] for mail viewers).
Receiving agents MUST be able to handle zero or one instance of each
of the signed attributes listed here. Sending agents SHOULD generate
one instance of each of the following signed attributes in each
S/MIME message:
- Signing time (Section 2.5.1 in this document)
- SMIME capabilities (Section 2.5.2 in this document)
- Encryption key Preference (Section 2.5.3 in this document)
- Message digest (Section 11.2 in [RFC 5652])
- Content type (Section 11.1 in [RFC 5652])
Further, receiving agents SHOULD be able to handle zero or one
instance of the signingCertificate and signingCertificateV2 signed
attributes, as defined in Section 5 of RFC 2634 [ESS] and Section 3
of RFC 5035 [ESS], respectively.
Sending agents SHOULD generate one instance of the signingCertificate
or signingCertificateV2 signed attribute in each SignerInfo
structure.
Additional attributes and values for these attributes might be
defined in the future. Receiving agents SHOULD handle attributes or
values that they do not recognize in a graceful manner.
Interactive sending agents that include signed attributes that are
not listed here SHOULD display those attributes to the user, so that
the user is aware of all of the data being signed.
2.5.1. Signing Time Attribute
The signingTime attribute is used to convey the time that a message
was signed. The time of signing will most likely be created by a
signer and therefore is only as trustworthy as that signer.
Schaad, et al. Standards Track PAGE 15
RFC 8551 S/MIME 4.0 Message Specification April 2019
Sending agents MUST encode signing time through the year 2049 as
UTCTime; signing times in 2050 or later MUST be encoded as
GeneralizedTime. When the UTCTime CHOICE is used, S/MIME agents MUST
interpret the year field (YY) as follows:
If YY is greater than or equal to 50, the year is interpreted as
19YY; if YY is less than 50, the year is interpreted as 20YY.
Receiving agents MUST be able to process signingTime attributes that
are encoded in either UTCTime or GeneralizedTime.
2.5.2. SMIMECapabilities Attribute
The SMIMECapabilities attribute includes signature algorithms (such
as "sha256WithRSAEncryption"), symmetric algorithms (such as "AES-128
CBC"), authenticated symmetric algorithms (such as "AES-128 GCM"),
and key encipherment algorithms (such as "rsaEncryption"). The
presence of an SMIMECapability attribute containing an algorithm
implies that the sender can deal with the algorithm as well as
understand the ASN.1 structures associated with that algorithm.
There are also several identifiers that indicate support for other
optional features such as binary encoding and compression. The
SMIMECapabilities attribute was designed to be flexible and
extensible so that, in the future, a means of identifying other
capabilities and preferences such as certificates can be added in a
way that will not cause current clients to break.
If present, the SMIMECapabilities attribute MUST be a
SignedAttribute. CMS defines SignedAttributes as a SET OF Attribute.
The SignedAttributes in a signerInfo MUST include a single instance
of the SMIMECapabilities attribute. CMS defines the ASN.1 syntax for
Attribute to include attrValues SET OF AttributeValue. An
SMIMECapabilities attribute MUST only include a single instance of
AttributeValue. If a signature is detected as violating these
requirements, the signature SHOULD be treated as failing.
The semantics of the SMIMECapabilities attribute specify a partial
list as to what the client announcing the SMIMECapabilities can
support. A client does not have to list every capability it
supports, and it need not list all its capabilities so that the
capabilities list doesn't get too long. In an SMIMECapabilities
attribute, the OIDs are listed in order of their preference but
SHOULD be separated logically along the lines of their categories
(signature algorithms, symmetric algorithms, key encipherment
algorithms, etc.).
Schaad, et al. Standards Track PAGE 16
RFC 8551 S/MIME 4.0 Message Specification April 2019
The structure of the SMIMECapabilities attribute is to facilitate
simple table lookups and binary comparisons in order to determine
matches. For instance, the encoding for the SMIMECapability for
sha256WithRSAEncryption includes rather than omits the NULL
parameter. Because of the requirement for identical encoding,
individuals documenting algorithms to be used in the
SMIMECapabilities attribute SHOULD explicitly document the correct
byte sequence for the common cases.
For any capability, the associated parameters for the OID MUST
specify all of the parameters necessary to differentiate between two
instances of the same algorithm.
The same OID that is used to identify an algorithm SHOULD also be
used in the SMIMECapability for that algorithm. There are cases
where a single OID can correspond to multiple algorithms. In these
cases, a single algorithm MUST be assigned to the SMIMECapability
using that OID. Additional OIDs from the smimeCapabilities OID tree
are then allocated for the other algorithms usages. For instance, in
an earlier specification, rsaEncryption was ambiguous because it
could refer to either a signature algorithm or a key encipherment
algorithm. In the event that an OID is ambiguous, it needs to be
arbitrated by the maintainer of the registered SMIMECapabilities list
as to which type of algorithm will use the OID, and a new OID MUST be
allocated under the smimeCapabilities OID to satisfy the other use of
the OID.
The registered SMIMECapabilities list specifies the parameters for
OIDs that need them, most notably key lengths in the case of
variable-length symmetric ciphers. In the event that there are no
differentiating parameters for a particular OID, the parameters MUST
be omitted and MUST NOT be encoded as NULL. Additional values for
the SMIMECapabilities attribute might be defined in the future.
Receiving agents MUST handle an SMIMECapabilities object that has
values that it does not recognize in a graceful manner.
Section 2.7.1 explains a strategy for caching capabilities.
2.5.3. Encryption Key Preference Attribute
The encryption key preference attribute allows the signer to
unambiguously describe which of the signer's certificates has the
signer's preferred encryption key. This attribute is designed to
enhance behavior for interoperating with those clients that use
separate keys for encryption and signing. This attribute is used to
convey to anyone viewing the attribute which of the listed
certificates is appropriate for encrypting a session key for future
encrypted messages.
Schaad, et al. Standards Track PAGE 17
RFC 8551 S/MIME 4.0 Message Specification April 2019
If present, the SMIMEEncryptionKeyPreference attribute MUST be a
SignedAttribute. CMS defines SignedAttributes as a SET OF Attribute.
The SignedAttributes in a signerInfo MUST include a single instance
of the SMIMEEncryptionKeyPreference attribute. CMS defines the ASN.1
syntax for Attribute to include attrValues SET OF AttributeValue. An
SMIMEEncryptionKeyPreference attribute MUST only include a single
instance of AttributeValue. If a signature is detected as violating
these requirements, the signature SHOULD be treated as failing.
The sending agent SHOULD include the referenced certificate in the
set of certificates included in the signed message if this attribute
is used. The certificate MAY be omitted if it has been previously
made available to the receiving agent. Sending agents SHOULD use
this attribute if the commonly used or preferred encryption
certificate is not the same as the certificate used to sign the
message.
Receiving agents SHOULD store the preference data if the signature on
the message is valid and the signing time is greater than the
currently stored value. (As with the SMIMECapabilities, the clock
skew SHOULD be checked and the data not used if the skew is too
great.) Receiving agents SHOULD respect the sender's encryption key
preference attribute if possible. This, however, represents only a
preference, and the receiving agent can use any certificate in
replying to the sender that is valid.
Section 2.7.1 explains a strategy for caching preference data.
2.5.3.1. Selection of Recipient Key Management Certificate
In order to determine the key management certificate to be used when
sending a future CMS EnvelopedData message for a particular
recipient, the following steps SHOULD be followed:
- If an SMIMEEncryptionKeyPreference attribute is found in a
SignedData object received from the desired recipient, this
identifies the X.509 certificate that SHOULD be used as the X.509
key management certificate for the recipient.
- If an SMIMEEncryptionKeyPreference attribute is not found in a
SignedData object received from the desired recipient, the set of
X.509 certificates SHOULD be searched for an X.509 certificate
with the same subject name as the signer of an X.509 certificate
that can be used for key management.
Schaad, et al. Standards Track PAGE 18
RFC 8551 S/MIME 4.0 Message Specification April 2019
- Or, use some other method of determining the user's key management
key. If an X.509 key management certificate is not found, then
encryption cannot be done with the signer of the message. If
multiple X.509 key management certificates are found, the S/MIME
agent can make an arbitrary choice between them.
2.6. SignerIdentifier SignerInfo Type
S/MIME v4.0 implementations MUST support both issuerAndSerialNumber
and subjectKeyIdentifier. Messages that use the subjectKeyIdentifier
choice cannot be read by S/MIME v2 clients.
It is important to understand that some certificates use a value for
subjectKeyIdentifier that is not suitable for uniquely identifying a
certificate. Implementations MUST be prepared for multiple
certificates for potentially different entities to have the same
value for subjectKeyIdentifier and MUST be prepared to try each
matching certificate during signature verification before indicating
an error condition.
2.7. ContentEncryptionAlgorithmIdentifier
Sending and receiving agents:
- MUST support encryption and decryption with AES-128 GCM and
AES-256 GCM [RFC 5084].
- MUST- support encryption and decryption with AES-128 CBC
[RFC 3565].
- SHOULD+ support encryption and decryption with ChaCha20-Poly1305
[RFC 7905].
2.7.1. Deciding Which Encryption Method to Use
When a sending agent creates an encrypted message, it has to decide
which type of encryption to use. The decision process involves using
information garnered from the capabilities lists included in messages
received from the recipient, as well as out-of-band information such
as private agreements, user preferences, legal restrictions, and
so on.
Section 2.5.2 defines a method by which a sending agent can
optionally announce, among other things, its decrypting capabilities
in its order of preference. The following method for processing and
remembering the encryption capabilities attribute in incoming signed
messages SHOULD be used.
Schaad, et al. Standards Track PAGE 19
RFC 8551 S/MIME 4.0 Message Specification April 2019
- If the receiving agent has not yet created a list of capabilities
for the sender's public key, then, after verifying the signature
on the incoming message and checking the timestamp, the receiving
agent SHOULD create a new list containing at least the signing
time and the symmetric capabilities.
- If such a list already exists, the receiving agent SHOULD verify
that the signing time in the incoming message is greater than the
signing time stored in the list and that the signature is valid.
If so, the receiving agent SHOULD update both the signing time and
capabilities in the list. Values of the signing time that lie far
in the future (that is, a greater discrepancy than any reasonable
clock skew), or a capabilities list in messages whose signature
could not be verified, MUST NOT be accepted.
The list of capabilities SHOULD be stored for future use in creating
messages.
Before sending a message, the sending agent MUST decide whether it is
willing to use weak encryption for the particular data in the
message. If the sending agent decides that weak encryption is
unacceptable for this data, then the sending agent MUST NOT use a
weak algorithm. The decision to use or not use weak encryption
overrides any other decision in this section about which encryption
algorithm to use.
Sections 2.7.1.1 and 2.7.1.2 describe the decisions a sending agent
SHOULD use when choosing which type of encryption will be applied to
a message. These rules are ordered, so the sending agent SHOULD make
its decision in the order given.
2.7.1.1. Rule 1: Known Capabilities
If the sending agent has received a set of capabilities from the
recipient for the message the agent is about to encrypt, then the
sending agent SHOULD use that information by selecting the first
capability in the list (that is, the capability most preferred by the
intended recipient) that the sending agent knows how to encrypt. The
sending agent SHOULD use one of the capabilities in the list if the
agent reasonably expects the recipient to be able to decrypt the
message.
2.7.1.2. Rule 2: Unknown Capabilities, Unknown Version of S/MIME
If the following two conditions are met, the sending agent SHOULD use
AES-256 GCM, as AES-256 GCM is a stronger algorithm and is required
by S/MIME v4.0:
Schaad, et al. Standards Track PAGE 20
RFC 8551 S/MIME 4.0 Message Specification April 2019
- The sending agent has no knowledge of the encryption capabilities
of the recipient.
- The sending agent has no knowledge of the version of S/MIME used
or supported by the recipient.
If the sending agent chooses not to use AES-256 GCM in this step,
given the presumption is that a client implementing AES-GCM would do
both AES-256 and AES-128, it SHOULD use AES-128 CBC.
2.7.2. Choosing Weak Encryption
Algorithms such as RC2 are considered to be weak encryption
algorithms. Algorithms such as TripleDES are not state of the art
and are considered to be weaker algorithms than AES. A sending agent
that is controlled by a human SHOULD allow a human sender to
determine the risks of sending data using a weaker encryption
algorithm before sending the data, and possibly allow the human to
use a stronger encryption algorithm such as AES GCM or AES CBC even
if there is a possibility that the recipient will not be able to
process that algorithm.
2.7.3. Multiple Recipients
If a sending agent is composing an encrypted message to a group of
recipients where the encryption capabilities of some of the
recipients do not overlap, the sending agent is forced to send more
than one message. Please note that if the sending agent chooses to
send a message encrypted with a strong algorithm and then send the
same message encrypted with a weak algorithm, someone watching the
communications channel could learn the contents of the strongly
encrypted message simply by decrypting the weakly encrypted message.
3. Creating S/MIME Messages
This section describes the S/MIME message formats and how they are
created. S/MIME messages are a combination of MIME bodies and CMS
content types. Several media types as well as several CMS content
types are used. The data to be secured is always a canonical MIME
entity. The MIME entity and other data, such as certificates and
algorithm identifiers, are given to CMS processing facilities that
produce a CMS object. Finally, the CMS object is wrapped in MIME.
The "Enhanced Security Services for S/MIME" documents [ESS] provide
descriptions of how nested, secured S/MIME messages are formatted.
ESS provides a description of how a triple-wrapped S/MIME message is
formatted using multipart/signed and application/pkcs7-mime for the
signatures.
Schaad, et al. Standards Track PAGE 21
RFC 8551 S/MIME 4.0 Message Specification April 2019
S/MIME provides one format for enveloped-only data, several formats
for signed-only data, and several formats for signed and enveloped
data. Several formats are required to accommodate several
environments -- in particular, for signed messages. The criteria for
choosing among these formats are also described.
Anyone reading this section is expected to understand MIME as
described in [MIME-SPEC] and [RFC 1847].
3.1. Preparing the MIME Entity for Signing, Enveloping, or Compressing
S/MIME is used to secure MIME entities. A MIME message is composed
of a MIME header and a MIME body. A body can consist of a single
MIME entity or a tree of MIME entities (rooted with a multipart).
S/MIME can be used to secure either a single MIME entity or a tree of
MIME entities. These entities can be in locations other than the
root. S/MIME can be applied multiple times to different entities in
a single message. A MIME entity that is the whole message includes
only the MIME message headers and MIME body and does not include the
RFC 822 header. Note that S/MIME can also be used to secure MIME
entities used in applications other than Internet mail. For cases
where protection of the RFC 822 header is required, the use of the
message/RFC 822 media type is explained later in this section.
The MIME entity that is secured and described in this section can be
thought of as the "inside" MIME entity. That is, it is the
"innermost" object in what is possibly a larger MIME message.
Processing "outside" MIME entities into CMS EnvelopedData,
CompressedData, and AuthEnvelopedData content types is described in
Sections 3.2 and 3.5. Other documents define additional CMS content
types; those documents should be consulted for processing those CMS
content types.
The procedure for preparing a MIME entity is given in [MIME-SPEC].
The same procedure is used here with some additional restrictions
when signing. The description of the procedures from [MIME-SPEC] is
repeated here, but it is suggested that the reader refer to those
documents for the exact procedures. This section also describes
additional requirements.
A single procedure is used for creating MIME entities that are to
have any combination of signing, enveloping, and compressing applied.
Some additional steps are recommended to defend against known
corruptions that can occur during mail transport that are of
particular importance for clear-signing using the multipart/signed
format. It is recommended that these additional steps be performed
on enveloped messages, or signed and enveloped messages, so that the
messages can be forwarded to any environment without modification.
Schaad, et al. Standards Track PAGE 22
RFC 8551 S/MIME 4.0 Message Specification April 2019
These steps are descriptive rather than prescriptive. The
implementer is free to use any procedure as long as the result is
the same.
Step 1. The MIME entity is prepared according to local conventions.
Step 2. The leaf parts of the MIME entity are converted to
canonical form.
Step 3. Appropriate transfer encoding is applied to the leaves
of the MIME entity.
When an S/MIME message is received, the security services on the
message are processed, and the result is the MIME entity. That MIME
entity is typically passed to a MIME-capable user agent where it is
further decoded and presented to the user or receiving application.
In order to protect outer, non-content-related message header fields
(for instance, the "Subject", "To", "From", and "Cc" fields), the
sending client MAY wrap a full MIME message in a message/RFC 822
wrapper in order to apply S/MIME security services to these header
fields. It is up to the receiving client to decide how to present
this "inner" header along with the unprotected "outer" header. Given
the security difference between headers, it is RECOMMENDED that the
receiving client provide a distinction between header fields,
depending on where they are located.
When an S/MIME message is received, if the top-level protected MIME
entity has a Content-Type of message/RFC 822, it can be assumed that
the intent was to provide header protection. This entity SHOULD be
presented as the top-level message, taking into account
header-merging issues as previously discussed.
3.1.1. Canonicalization
Each MIME entity MUST be converted to a canonical form that is
uniquely and unambiguously representable in the environment where the
signature is created and the environment where the signature will be
verified. MIME entities MUST be canonicalized for enveloping and
compressing as well as signing.
The exact details of canonicalization depend on the actual media type
and subtype of an entity and are not described here. Instead, the
standard for the particular media type SHOULD be consulted. For
example, canonicalization of type text/plain is different from
canonicalization of audio/basic. Other than text types, most types
have only one representation, regardless of computing platform or
environment, that can be considered their canonical representation.
Schaad, et al. Standards Track PAGE 23
RFC 8551 S/MIME 4.0 Message Specification April 2019
In general, canonicalization will be performed by the non-security
part of the sending agent rather than the S/MIME implementation.
The most common and important canonicalization is for text, which is
often represented differently in different environments. MIME
entities of major type "text" MUST have both their line endings and
character set canonicalized. The line ending MUST be the pair of
characters <CR><LF>, and the charset SHOULD be a registered charset
[CHARSETS]. The details of the canonicalization are specified in
[MIME-SPEC].
Note that some charsets such as ISO-2022 have multiple
representations for the same characters. When preparing such text
for signing, the canonical representation specified for the charset
MUST be used.
3.1.2. Transfer Encoding
When generating any of the secured MIME entities below, except the
signing using the multipart/signed format, no transfer encoding is
required at all. S/MIME implementations MUST be able to deal with
binary MIME objects. If no Content-Transfer-Encoding header field is
present, the transfer encoding is presumed to be 7BIT.
As a rule, S/MIME implementations SHOULD use transfer encoding as
described in Section 3.1.3 for all MIME entities they secure. The
reason for securing only 7-bit MIME entities, even for enveloped data
that is not exposed to the transport, is that it allows the MIME
entity to be handled in any environment without changing it. For
example, a trusted gateway might remove the envelope, but not the
signature, of a message, and then forward the signed message on to
the end recipient so that they can verify the signatures directly.
If the transport internal to the site is not 8-bit clean, such as on
a wide-area network with a single mail gateway, verifying the
signature will not be possible unless the original MIME entity was
only 7-bit data.
In the case where S/MIME implementations can determine that all
intended recipients are capable of handling inner (all but the
outermost) binary MIME objects, implementations SHOULD use binary
encoding as opposed to a 7-bit-safe transfer encoding for the inner
entities. The use of a 7-bit-safe encoding (such as base64)
unnecessarily expands the message size. Implementations MAY
determine that recipient implementations are capable of
handling inner binary MIME entities by (1) interpreting the
id-cap-preferBinaryInside SMIMECapabilities attribute, (2) prior
agreement, or (3) other means.
Schaad, et al. Standards Track PAGE 24
RFC 8551 S/MIME 4.0 Message Specification April 2019
If one or more intended recipients are unable to handle inner binary
MIME objects or if this capability is unknown for any of the intended
recipients, S/MIME implementations SHOULD use transfer encoding as
described in Section 3.1.3 for all MIME entities they secure.
3.1.3. Transfer Encoding for Signing Using multipart/signed
If a multipart/signed entity is ever to be transmitted over the
standard Internet SMTP infrastructure or other transport that is
constrained to 7-bit text, it MUST have transfer encoding applied so
that it is represented as 7-bit text. MIME entities that are already
7-bit data need no transfer encoding. Entities such as 8-bit text
and binary data can be encoded with quoted-printable or base64
transfer encoding.
The primary reason for the 7-bit requirement is that the Internet
mail transport infrastructure cannot guarantee transport of 8-bit or
binary data. Even though many segments of the transport
infrastructure now handle 8-bit and even binary data, it is sometimes
not possible to know whether the transport path is 8-bit clean. If a
mail message with 8-bit data were to encounter a message transfer
agent that cannot transmit 8-bit or binary data, the agent has three
options, none of which are acceptable for a clear-signed message:
- The agent could change the transfer encoding; this would
invalidate the signature.
- The agent could transmit the data anyway, which would most likely
result in the 8th bit being corrupted; this too would invalidate
the signature.
- The agent could return the message to the sender.
[RFC 1847] prohibits an agent from changing the transfer encoding of
the first part of a multipart/signed message. If a compliant agent
that cannot transmit 8-bit or binary data encountered a
multipart/signed message with 8-bit or binary data in the first part,
it would have to return the message to the sender as undeliverable.
3.1.4. Sample Canonical MIME Entity
This example shows a multipart/mixed message with full transfer
encoding. This message contains a text part and an attachment. The
sample message text includes characters that are not ASCII and thus
need to be transfer encoded. Though not shown here, the end of each
line is <CR><LF>. The line ending of the MIME headers, the text, and
the transfer-encoded parts all MUST be <CR><LF>.
Schaad, et al. Standards Track PAGE 25
RFC 8551 S/MIME 4.0 Message Specification April 2019
Note that this example is not an example of an S/MIME message.
Content-Type: multipart/mixed; boundary=bar
--bar
Content-Type: text/plain; charset=iso-8859-1
Content-Transfer-Encoding: quoted-printable
=A1Hola Michael!
How do you like the new S/MIME specification?
It's generally a good idea to encode lines that begin with
From=20because some mail transport agents will insert a
greater-than (>) sign, thus invalidating the signature.
Also, in some cases it might be desirable to encode any =20
trailing whitespace that occurs on lines in order to ensure =20
that the message signature is not invalidated when passing =20
a gateway that modifies such whitespace (like BITNET). =20
--bar
Content-Type: image/jpeg
Content-Transfer-Encoding: base64
iQCVAwUBMJrRF2N9oWBghPDJAQE9UQQAtl7LuRVndBjrk4EqYBIb3h5QXIX/LC//
jJV5bNvkZIGPIcEmI5iFd9boEgvpirHtIREEqLQRkYNoBActFBZmh9GC3C041WGq
uMbrbxc+nIs1TIKlA08rVi9ig/2Yh7LFrK5Ein57U/W72vgSxLhe/zhdfolT9Brn
HOxEa44b+EI=
--bar--
3.2. The application/pkcs7-mime Media Type
The application/pkcs7-mime media type is used to carry CMS content
types, including EnvelopedData, SignedData, and CompressedData. The
details of constructing these entities are described in subsequent
sections. This section describes the general characteristics of the
application/pkcs7-mime media type.
The carried CMS object always contains a MIME entity that is prepared
as described in Section 3.1 if the eContentType is id-data. Other
contents MAY be carried when the eContentType contains different
values. See [ESS] for an example of this with signed receipts.
Since CMS content types are binary data, in most cases base64
transfer encoding is appropriate -- in particular, when used with
SMTP transport. The transfer encoding used depends on the transport
Schaad, et al. Standards Track PAGE 26
RFC 8551 S/MIME 4.0 Message Specification April 2019
through which the object is to be sent and is not a characteristic of
the media type.
Note that this discussion refers to the transfer encoding of the CMS
object or "outside" MIME entity. It is completely distinct from, and
unrelated to, the transfer encoding of the MIME entity secured by the
CMS object -- the "inside" object, which is described in Section 3.1.
Because there are several types of application/pkcs7-mime objects, a
sending agent SHOULD do as much as possible to help a receiving agent
know about the contents of the object without forcing the receiving
agent to decode the ASN.1 for the object. The Content-Type header
field of all application/pkcs7-mime objects SHOULD include the
optional "smime-type" parameter, as described in the following
sections.
3.2.1. The name and filename Parameters
For application/pkcs7-mime, sending agents SHOULD emit the
optional "name" parameter to the Content-Type field for compatibility
with older systems. Sending agents SHOULD also emit the optional
Content-Disposition field [RFC 2183] with the "filename" parameter.
If a sending agent emits the above parameters, the value of the
parameters SHOULD be a filename with the appropriate extension:
File
Media Type Extension
-------------------------------------------------------------------
application/pkcs7-mime (SignedData, EnvelopedData, .p7m
AuthEnvelopedData)
application/pkcs7-mime (degenerate SignedData certificate .p7c
management message)
application/pkcs7-mime (CompressedData) .p7z
application/pkcs7-signature (SignedData) .p7s
In addition, the filename SHOULD be limited to eight characters
followed by a three-letter extension. The eight-character filename
base can be any distinct name; the use of the filename base "smime"
SHOULD be used to indicate that the MIME entity is associated with
S/MIME.
Including a filename serves two purposes. It facilitates easier use
of S/MIME objects as files on disk. It also can convey type
information across gateways. When a MIME entity of type
application/pkcs7-mime (for example) arrives at a gateway that has no
special knowledge of S/MIME, it will default the entity's media type
to application/octet-stream and treat it as a generic attachment,
thus losing the type information. However, the suggested filename
Schaad, et al. Standards Track PAGE 27
RFC 8551 S/MIME 4.0 Message Specification April 2019
for an attachment is often carried across a gateway. This often
allows the receiving systems to determine the appropriate application
to hand the attachment off to -- in this case, a standalone S/MIME
processing application. Note that this mechanism is provided as a
convenience for implementations in certain environments. A proper
S/MIME implementation MUST use the media types and MUST NOT rely on
the file extensions.
3.2.2. The smime-type Parameter
The application/pkcs7-mime content type defines the optional
"smime-type" parameter. The intent of this parameter is to convey
details about the security applied (signed or enveloped) along with
information about the contained content. This specification defines
the following smime-types.
Name CMS Type Inner Content
----------------------------------------------------------
enveloped-data EnvelopedData id-data
signed-data SignedData id-data
certs-only SignedData id-data
compressed-data CompressedData id-data
authEnveloped-data AuthEnvelopedData id-data
In order for consistency to be obtained with future specifications,
the following guidelines SHOULD be followed when assigning a new
smime-type parameter.
1. If both signing and encryption can be applied to the content,
then three values for smime-type SHOULD be assigned: "signed-*",
"authEnv-*", and "enveloped-*". If one operation can be
assigned, then this can be omitted. Thus, since "certs-only" can
only be signed, "signed-" is omitted.
2. A common string for a content OID SHOULD be assigned. We use
"data" for the id-data content OID when MIME is the inner
content.
3. If no common string is assigned, then the common string of
"OID.<oid>" is recommended (for example,
"OID.2.16.840.1.101.3.4.1.2" would be AES-128 CBC).
It is explicitly intended that this field be a suitable hint for mail
client applications to indicate whether a message is "signed",
"authEnveloped", or "enveloped" without having to tunnel into the CMS
payload.
Schaad, et al. Standards Track PAGE 28
RFC 8551 S/MIME 4.0 Message Specification April 2019
A registry for additional smime-type parameter values has been
defined in [RFC 7114].
3.3. Creating an Enveloped-Only Message
This section describes the format for enveloping a MIME entity
without signing it. It is important to note that sending enveloped
but not signed messages does not provide for data integrity. The
"enveloped-only" structure does not support authenticated symmetric
algorithms. Use the "authenticated enveloped" structure for these
algorithms. Thus, it is possible to replace ciphertext in such a way
that the processed message will still be valid, but the meaning can
be altered.
Step 1. The MIME entity to be enveloped is prepared according to
Section 3.1.
Step 2. The MIME entity and other required data are processed into a
CMS object of type EnvelopedData. In addition to encrypting
a copy of the content-encryption key (CEK) for each
recipient, a copy of the CEK SHOULD be encrypted for the
originator and included in the EnvelopedData (see [RFC 5652],
Section 6).
Step 3. The EnvelopedData object is wrapped in a CMS ContentInfo
object.
Step 4. The ContentInfo object is inserted into an
application/pkcs7-mime MIME entity.
The smime-type parameter for enveloped-only messages is
"enveloped-data". The file extension for this type of message
is ".p7m".
A sample message would be:
Content-Type: application/pkcs7-mime; name=smime.p7m;
smime-type=enveloped-data
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=smime.p7m
MIIBHgYJKoZIhvcNAQcDoIIBDzCCAQsCAQAxgcAwgb0CAQAwJjASMRAwDgYDVQQDEw
dDYXJsUlNBAhBGNGvHgABWvBHTbi7NXXHQMA0GCSqGSIb3DQEBAQUABIGAC3EN5nGI
iJi2lsGPcP2iJ97a4e8kbKQz36zg6Z2i0yx6zYC4mZ7mX7FBs3IWg+f6KgCLx3M1eC
bWx8+MDFbbpXadCDgO8/nUkUNYeNxJtuzubGgzoyEd8Ch4H/dd9gdzTd+taTEgS0ip
dSJuNnkVY4/M652jKKHRLFf02hosdR8wQwYJKoZIhvcNAQcBMBQGCCqGSIb3DQMHBA
gtaMXpRwZRNYAgDsiSf8Z9P43LrY4OxUk660cu1lXeCSFOSOpOJ7FuVyU=
Schaad, et al. Standards Track PAGE 29
RFC 8551 S/MIME 4.0 Message Specification April 2019
3.4. Creating an Authenticated Enveloped-Only Message
This section describes the format for enveloping a MIME entity
without signing it. Authenticated enveloped messages provide
confidentiality and data integrity. It is important to note that
sending authenticated enveloped messages does not provide for proof
of origination when using S/MIME. It is possible for a third party
to replace ciphertext in such a way that the processed message will
still be valid, but the meaning can be altered. However, this is
substantially more difficult than it is for an enveloped-only
message, as the algorithm does provide a level of authentication.
Any recipient for whom the message is encrypted can replace it
without detection.
Step 1. The MIME entity to be enveloped is prepared according to
Section 3.1.
Step 2. The MIME entity and other required data are processed into a
CMS object of type AuthEnvelopedData. In addition to
encrypting a copy of the CEK for each recipient, a copy of
the CEK SHOULD be encrypted for the originator and included
in the AuthEnvelopedData (see [RFC 5083]).
Step 3. The AuthEnvelopedData object is wrapped in a CMS ContentInfo
object.
Step 4. The ContentInfo object is inserted into an
application/pkcs7-mime MIME entity.
The smime-type parameter for authenticated enveloped-only messages is
"authEnveloped-data". The file extension for this type of message
is ".p7m".
Schaad, et al. Standards Track PAGE 30
RFC 8551 S/MIME 4.0 Message Specification April 2019
A sample message would be:
Content-Type: application/pkcs7-mime; smime-type=authEnveloped-data;
name=smime.p7m
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=smime.p7m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3.5. Creating a Signed-Only Message
There are two formats for signed messages defined for S/MIME:
- application/pkcs7-mime with SignedData.
- multipart/signed.
In general, the multipart/signed form is preferred for sending, and
receiving agents MUST be able to handle both.
Schaad, et al. Standards Track PAGE 31
RFC 8551 S/MIME 4.0 Message Specification April 2019
3.5.1. Choosing a Format for Signed-Only Messages
There are no hard-and-fast rules as to when a particular signed-only
format is chosen. It depends on the capabilities of all the
receivers and the relative importance of receivers with S/MIME
facilities being able to verify the signature versus the importance
of receivers without S/MIME software being able to view the message.
Messages signed using the multipart/signed format can always be
viewed by the receiver whether or not they have S/MIME software.
They can also be viewed whether they are using a MIME-native user
agent or they have messages translated by a gateway. In this
context, "be viewed" means the ability to process the message
essentially as if it were not a signed message, including any other
MIME structure the message might have.
Messages signed using the SignedData format cannot be viewed by a
recipient unless they have S/MIME facilities. However, the
SignedData format protects the message content from being changed by
benign intermediate agents. Such agents might do line wrapping or
content-transfer encoding changes that would break the signature.
3.5.2. Signing Using application/pkcs7-mime with SignedData
This signing format uses the application/pkcs7-mime media type. The
steps to create this format are as follows:
Step 1. The MIME entity is prepared according to Section 3.1.
Step 2. The MIME entity and other required data are processed into a
CMS object of type SignedData.
Step 3. The SignedData object is wrapped in a CMS ContentInfo
object.
Step 4. The ContentInfo object is inserted into an
application/pkcs7-mime MIME entity.
The smime-type parameter for messages using application/pkcs7-mime
with SignedData is "signed-data". The file extension for this type
of message is ".p7m".
Schaad, et al. Standards Track PAGE 32
RFC 8551 S/MIME 4.0 Message Specification April 2019
A sample message would be:
Content-Type: application/pkcs7-mime; smime-type=signed-data;
name=smime.p7m
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=smime.p7m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3.5.3. Signing Using the multipart/signed Format
This format is a clear-signing format. Recipients without any S/MIME
or CMS processing facilities are able to view the message. It makes
use of the multipart/signed media type described in [RFC 1847]. The
multipart/signed media type has two parts. The first part contains
the MIME entity that is signed; the second part contains the
"detached signature" CMS SignedData object in which the
encapContentInfo eContent field is absent.
3.5.3.1. The application/pkcs7-signature Media Type
This media type always contains a CMS ContentInfo containing a single
CMS object of type SignedData. The SignedData encapContentInfo
eContent field MUST be absent. The signerInfos field contains the
signatures for the MIME entity.
The file extension for signed-only messages using
application/pkcs7-signature is ".p7s".
Schaad, et al. Standards Track PAGE 33
RFC 8551 S/MIME 4.0 Message Specification April 2019
3.5.3.2. Creating a multipart/signed Message
Step 1. The MIME entity to be signed is prepared according to
Section 3.1, taking special care for clear-signing.
Step 2. The MIME entity is presented to CMS processing in order to
obtain an object of type SignedData in which the
encapContentInfo eContent field is absent.
Step 3. The MIME entity is inserted into the first part of a
multipart/signed message with no processing other than that
described in Section 3.1.
Step 4. Transfer encoding is applied to the "detached signature" CMS
SignedData object, and it is inserted into a MIME entity of
type application/pkcs7-signature.
Step 5. The MIME entity of the application/pkcs7-signature is
inserted into the second part of the multipart/signed
entity.
The multipart/signed Content-Type has two required parameters: the
protocol parameter and the micalg parameter.
The protocol parameter MUST be "application/pkcs7-signature". Note
that quotation marks are required around the protocol parameter
because MIME requires that the "/" character in the parameter value
MUST be quoted.
Schaad, et al. Standards Track PAGE 34
RFC 8551 S/MIME 4.0 Message Specification April 2019
The micalg parameter allows for one-pass processing when the
signature is being verified. The value of the micalg parameter is
dependent on the message digest algorithm(s) used in the calculation
of the Message Integrity Check. If multiple message digest
algorithms are used, they MUST be separated by commas per [RFC 1847].
The values to be placed in the micalg parameter SHOULD be from the
following:
Algorithm Value Used
-----------------------------------------------------------
MD5* md5
SHA-1* sha-1
SHA-224 sha-224
SHA-256 sha-256
SHA-384 sha-384
SHA-512 sha-512
Any other (defined separately in the algorithm profile
or "unknown" if not defined)
*Note: MD5 and SHA-1 are historical and no longer considered secure.
See Appendix B for details.
(Historical note: Some early implementations of S/MIME emitted and
expected "rsa-md5", "rsa-sha1", and "sha1" for the micalg parameter.)
Receiving agents SHOULD be able to recover gracefully from a micalg
parameter value that they do not recognize. Future values for this
parameter will be taken from the IANA "Hash Function Textual Names"
registry.
Schaad, et al. Standards Track PAGE 35
RFC 8551 S/MIME 4.0 Message Specification April 2019
3.5.3.3. Sample multipart/signed Message
Content-Type: multipart/signed;
micalg=sha-256;
boundary="----=_NextBoundary____Fri,_06_Sep_2002_00:25:21";
protocol="application/pkcs7-signature"
This is a multipart message in MIME format.
------=_NextBoundary____Fri,_06_Sep_2002_00:25:21
This is some sample content.
------=_NextBoundary____Fri,_06_Sep_2002_00:25:21
Content-Type: application/pkcs7-signature; name=smime.p7s
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=smime.p7s
MIIBJgYJKoZIhvcNAQcCoIIBFzCCARMCAQExADALBgkqhkiG9w0BBwExgf4w
gfsCAQIwJjASMRAwDgYDVQQDEwdDYXJsUlNBAhBGNGvHgABWvBHTbi7EELOw
MAsGCWCGSAFlAwQCAaAxMC8GCSqGSIb3DQEJBDEiBCCxwpZGNZzTSsugsn+f
lEidzQK4mf/ozKqfmbxhcIkKqjALBgkqhkiG9w0BAQsEgYB0XJV7fjPa5Nuh
oth5msDfP8A5urYUMjhNpWgXG8ae3XpppqVrPi2nVO41onHnkByjkeD/wc31
A9WH8MzFQgSTsrJ65JvffTTXkOpRPxsSHn3wJFwP/atWHkh8YK/jR9bULhUl
Mv5jQEDiwVX5DRasxu6Ld8zv9u5/TsdBNiufGw==
------=_NextBoundary____Fri,_06_Sep_2002_00:25:21--
The content that is digested (the first part of the multipart/signed)
consists of the bytes:
54 68 69 73 20 69 73 20 73 6f 6d 65 20 73 61 6d 70 6c 65 20 63 6f 6e
74 65 6e 74 2e 0d 0a
3.6. Creating a Compressed-Only Message
This section describes the format for compressing a MIME entity.
Please note that versions of S/MIME prior to version 3.1 did not
specify any use of CompressedData and will not recognize it. The use
of a capability to indicate the ability to receive CompressedData is
described in [RFC 3274] and is the preferred method for compatibility.
Step 1. The MIME entity to be compressed is prepared according to
Section 3.1.
Step 2. The MIME entity and other required data are processed into a
CMS object of type CompressedData.
Schaad, et al. Standards Track PAGE 36
RFC 8551 S/MIME 4.0 Message Specification April 2019
Step 3. The CompressedData object is wrapped in a CMS ContentInfo
object.
Step 4. The ContentInfo object is inserted into an
application/pkcs7-mime MIME entity.
The smime-type parameter for compressed-only messages is
"compressed-data". The file extension for this type of message
is ".p7z".
A sample message would be:
Content-Type: application/pkcs7-mime; smime-type=compressed-data;
name=smime.p7z
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=smime.p7z
eNoLycgsVgCi4vzcVIXixNyCnFSF5Py8ktS8Ej0AlCkKVA==
3.7. Multiple Operations
The signed-only, enveloped-only, and compressed-only MIME formats can
be nested. This works because these formats are all MIME entities
that encapsulate other MIME entities.
An S/MIME implementation MUST be able to receive and process
arbitrarily nested S/MIME within reasonable resource limits of the
recipient computer.
It is possible to apply any of the signing, encrypting, and
compressing operations in any order. It is up to the implementer and
the user to choose. When signing first, the signatories are then
securely obscured by the enveloping. When enveloping first, the
signatories are exposed, but it is possible to verify signatures
without removing the enveloping. This can be useful in an
environment where automatic signature verification is desired, as no
private key material is required to verify a signature.
There are security ramifications related to choosing whether to sign
first or encrypt first. A recipient of a message that is encrypted
and then signed can validate that the encrypted block was unaltered
but cannot determine any relationship between the signer and the
unencrypted contents of the message. A recipient of a message that
is signed and then encrypted can assume that the signed message
itself has not been altered but that a careful attacker could have
changed the unauthenticated portions of the encrypted message.
Schaad, et al. Standards Track PAGE 37
RFC 8551 S/MIME 4.0 Message Specification April 2019
When using compression, keep the following guidelines in mind:
- Compression of encrypted data that is transferred as binary data
is discouraged, since it will not yield significant compression.
Encrypted data that is transferred as base64-encoded data could
benefit as well.
- If a lossy compression algorithm is used with signing, you will
need to compress first, then sign.
3.8. Creating a Certificate Management Message
The certificate management message or MIME entity is used to
transport certificates and/or Certificate Revocation Lists (CRLs),
such as in response to a registration request.
Step 1. The certificates and/or CRLs are made available to the CMS
generating process that creates a CMS object of type
SignedData. The SignedData encapContentInfo eContent field
MUST be absent, and the signerInfos field MUST be empty.
Step 2. The SignedData object is wrapped in a CMS ContentInfo
object.
Step 3. The ContentInfo object is enclosed in an
application/pkcs7-mime MIME entity.
The smime-type parameter for a certificate management message is
"certs-only". The file extension for this type of message is ".p7c".
3.9. Registration Requests
A sending agent that signs messages MUST have a certificate for the
signature so that a receiving agent can verify the signature. There
are many ways of getting certificates, such as through an exchange
with a certification authority, through a hardware token or diskette,
and so on.
S/MIME v2 [SMIMEv2] specified a method for "registering" public keys
with certificate authorities using an application/pkcs10 body part.
Since that time, the IETF PKIX Working Group has developed other
methods for requesting certificates. However, S/MIME v4.0 does not
require a particular certificate request mechanism.
Schaad, et al. Standards Track PAGE 38
RFC 8551 S/MIME 4.0 Message Specification April 2019
3.10. Identifying an S/MIME Message
Because S/MIME takes into account interoperation in non-MIME
environments, several different mechanisms are employed to carry the
type information, and it becomes a bit difficult to identify S/MIME
messages. The following table lists criteria for determining whether
or not a message is an S/MIME message. A message is considered an
S/MIME message if it matches any of the criteria listed below.
The file suffix in the table below comes from the "name" parameter in
the Content-Type header field or the "filename" parameter in the
Content-Disposition header field. The MIME parameters that carry the
file suffix are not listed below.
Media Type Parameters File Suffix
---------------------------------------------------------------------
application/pkcs7-mime N/A N/A
multipart/signed protocol= N/A
"application/pkcs7-signature"
application/octet-stream N/A p7m, p7s,
p7c, p7z
4. Certificate Processing
A receiving agent MUST provide some certificate retrieval mechanism
in order to gain access to certificates for recipients of digital
envelopes. This specification does not cover how S/MIME agents
handle certificates -- only what they do after a certificate has been
validated or rejected. S/MIME certificate issues are covered in
[RFC 5750].
At a minimum, for initial S/MIME deployment, a user agent could
automatically generate a message to an intended recipient requesting
that recipient's certificate in a signed return message. Receiving
and sending agents SHOULD also provide a mechanism to allow a user to
"store and protect" certificates for correspondents in such a way as
to guarantee their later retrieval.
Schaad, et al. Standards Track PAGE 39
RFC 8551 S/MIME 4.0 Message Specification April 2019
4.1. Key Pair Generation
All key pairs MUST be generated from a good source of
non-deterministic random input [RFC 4086], and the private key MUST be
protected in a secure fashion.
An S/MIME user agent MUST NOT generate asymmetric keys less than
2048 bits for use with an RSA signature algorithm.
For 2048-bit through 4096-bit RSA with SHA-256, see [RFC 5754] and
[FIPS186-4]. The first reference provides the signature algorithm's
OID, and the second provides the signature algorithm's definition.
For RSASSA-PSS with SHA-256, see [RFC 4056]. For RSAES-OAEP, see
[RFC 3560].
4.2. Signature Generation
The following are the requirements for an S/MIME agent when
generating RSA and RSASSA-PSS signatures:
key size <= 2047 : SHOULD NOT (Note 2)
2048 <= key size <= 4096 : SHOULD (Note 1)
4096 < key size : MAY (Note 1)
Note 1: See Security Considerations in Section 6.
Note 2: See Historical Mail Considerations in Appendix B.
Key sizes for ECDSA and EdDSA are fixed by the curve.
4.3. Signature Verification
The following are the requirements for S/MIME receiving agents during
verification of RSA and RSASSA-PSS signatures:
key size <= 2047 : SHOULD NOT (Note 2)
2048 <= key size <= 4096 : MUST (Note 1)
4096 < key size : MAY (Note 1)
Note 1: See Security Considerations in Section 6.
Note 2: See Historical Mail Considerations in Appendix B.
Key sizes for ECDSA and EdDSA are fixed by the curve.
Schaad, et al. Standards Track PAGE 40
RFC 8551 S/MIME 4.0 Message Specification April 2019
4.4. Encryption
The following are the requirements for an S/MIME agent when
establishing keys for content encryption using the RSA and RSA-OAEP
algorithms:
key size <= 2047 : SHOULD NOT (Note 2)
2048 <= key size <= 4096 : SHOULD (Note 1)
4096 < key size : MAY (Note 1)
Note 1: See Security Considerations in Section 6.
Note 2: See Historical Mail Considerations in Appendix B.
Key sizes for ECDH are fixed by the curve.
4.5. Decryption
The following are the requirements for an S/MIME agent when
establishing keys for content decryption using the RSA and RSAES-OAEP
algorithms:
key size <= 2047 : MAY (Note 2)
2048 <= key size <= 4096 : MUST (Note 1)
4096 < key size : MAY (Note 1)
Note 1: See Security Considerations in Section 6.
Note 2: See Historical Mail Considerations in Appendix B.
Key sizes for ECDH are fixed by the curve.
5. IANA Considerations
This section (1) updates the media type registrations for
application/pkcs7-mime and application/pkcs7-signature to refer to
this document as opposed to RFC 5751, (2) adds authEnveloped-data to
the list of values for smime-type, and (3) updates references from
RFC 5751 to this document in general.
Note that other documents can define additional media types for
S/MIME.
Schaad, et al. Standards Track PAGE 41
RFC 8551 S/MIME 4.0 Message Specification April 2019
5.1. Media Type for application/pkcs7-mime
Type name: application
Subtype Name: pkcs7-mime
Required Parameters: NONE
Optional Parameters: smime-type
name
Encoding Considerations: See Section 3 of this document
Security Considerations: See Section 6 of this document
Interoperability Considerations: See Sections 1-6 of this document
Published Specification: RFC 2311, RFC 2633, RFC 5751,
and this document
Applications that use this media type: Security applications
Fragment identifier considerations: N/A
Additional information:
Deprecated alias names for this type: N/A
Magic number(s): N/A
File extensions(s): See Section 3.2.1 of this document
Macintosh file type code(s): N/A
Person & email address to contact for further information:
The IESG <iesg@ietf.org>
Intended usage: COMMON
Restrictions on usage: NONE
Author: Sean Turner
Change Controller: LAMPS working group delegated from the IESG
Schaad, et al. Standards Track PAGE 42
RFC 8551 S/MIME 4.0 Message Specification April 2019
5.2. Media Type for application/pkcs7-signature
Type name: application
Subtype Name: pkcs7-signature
Required Parameters: N/A
Optional Parameters: N/A
Encoding Considerations: See Section 3 of this document
Security Considerations: See Section 6 of this document
Interoperability Considerations: See Sections 1-6 of this document
Published Specification: RFC 2311, RFC 2633, RFC 5751,
and this document
Applications that use this media type: Security applications
Fragment identifier considerations: N/A
Additional information:
Deprecated alias names for this type: N/A
Magic number(s): N/A
File extensions(s): See Section 3.2.1 of this document
Macintosh file type code(s): N/A
Person & email address to contact for further information:
The IESG <iesg@ietf.org>
Intended usage: COMMON
Restrictions on usage: N/A
Author: Sean Turner
Change Controller: LAMPS working group delegated from the IESG
Schaad, et al. Standards Track PAGE 43
RFC 8551 S/MIME 4.0 Message Specification April 2019
5.3. authEnveloped-data smime-type
IANA has registered the following value in the "Parameter Values for
the smime-type Parameter" registry.
smime-type value: authEnveloped-data
Reference: RFC 8551, Section 3.2.2
5.4. Reference Updates
IANA is to update all references to RFC 5751 to this document. Known
registries to be updated are "CoAP Content-Formats" and "media-
types".
6. Security Considerations
Cryptographic algorithms will be broken or weakened over time.
Implementers and users need to check that the cryptographic
algorithms listed in this document continue to provide the expected
level of security. The IETF from time to time may issue documents
dealing with the current state of the art. For example:
- The Million Message Attack described in RFC 3218 [RFC 3218].
- The Diffie-Hellman "small-subgroup" attacks described in RFC 2785
[RFC 2785].
- The attacks against hash algorithms described in RFC 4270
[RFC 4270].
This specification uses Public-Key Cryptography technologies. It is
assumed that the private key is protected to ensure that it is not
accessed or altered by unauthorized parties.
It is impossible for most people or software to estimate the value of
a message's content. Further, it is impossible for most people or
software to estimate the actual cost of recovering an encrypted
message's content that is encrypted with a key of a particular size.
Further, it is quite difficult to determine the cost of a failed
decryption if a recipient cannot process a message's content. Thus,
choosing between different key sizes (or choosing whether to just use
plaintext) is also impossible for most people or software. However,
decisions based on these criteria are made all the time, and
therefore this specification gives a framework for using those
estimates in choosing algorithms.
Schaad, et al. Standards Track PAGE 44
RFC 8551 S/MIME 4.0 Message Specification April 2019
The choice of 2048 bits as an RSA asymmetric key size in this
specification is based on the desire to provide at least 100 bits of
security. The key sizes that must be supported to conform to this
specification seem appropriate for the Internet, based on [RFC 3766].
Of course, there are environments, such as financial and medical
systems, that may select different key sizes. For this reason, an
implementation MAY support key sizes beyond those recommended in this
specification.
Receiving agents that validate signatures and sending agents that
encrypt messages need to be cautious of cryptographic processing
usage when validating signatures and encrypting messages using keys
larger than those mandated in this specification. An attacker could
send certificates with keys that would result in excessive
cryptographic processing -- for example, keys larger than those
mandated in this specification, as such keys could swamp the
processing element. Agents that use such keys without first
validating the certificate to a trust anchor are advised to have some
sort of cryptographic resource management system to prevent such
attacks.
Some cryptographic algorithms such as RC2 offer little actual
security over sending plaintext. Other algorithms such as TripleDES
provide security but are no longer considered to be state of the art.
S/MIME requires the use of current state-of-the-art algorithms such
as AES and provides the ability to announce cryptographic
capabilities to parties with whom you communicate. This allows the
sender to create messages that can use the strongest common
encryption algorithm. Using algorithms such as RC2 is never
recommended unless the only alternative is no cryptography.
RSA and DSA keys of less than 2048 bits are now considered by many
experts to be cryptographically insecure (due to advances in
computing power) and should no longer be used to protect messages.
Such keys were previously considered secure, so processing previously
received signed and encrypted mail will often result in the use of
weak keys. Implementations that wish to support previous versions of
S/MIME or process old messages need to consider the security risks
that result from smaller key sizes (e.g., spoofed messages) versus
the costs of denial of service. If an implementation supports
verification of digital signatures generated with RSA and DSA keys of
less than 1024 bits, it MUST warn the user. Implementers should
consider providing different warnings for newly received messages and
previously stored messages. Server implementations (e.g., secure
mail list servers) where user warnings are not appropriate SHOULD
reject messages with weak signatures.
Schaad, et al. Standards Track PAGE 45
RFC 8551 S/MIME 4.0 Message Specification April 2019
Implementers SHOULD be aware that multiple active key pairs can be
associated with a single individual. For example, one key pair can
be used to support confidentiality, while a different key pair can be
used for digital signatures.
If a sending agent is sending the same message using different
strengths of cryptography, an attacker watching the communications
channel might be able to determine the contents of the strongly
encrypted message by decrypting the weakly encrypted version. In
other words, a sender SHOULD NOT send a copy of a message using
weaker cryptography than they would use for the original of the
message.
Modification of the ciphertext in EnvelopedData can go undetected if
authentication is not also used, which is the case when sending
EnvelopedData without wrapping it in SignedData or enclosing
SignedData within it. This is one of the reasons for moving from
EnvelopedData to AuthEnvelopedData, as the authenticated encryption
algorithms provide the authentication without needing the SignedData
layer.
If an implementation is concerned about compliance with National
Institute of Standards and Technology (NIST) key size
recommendations, then see [SP800-57].
If messaging environments make use of the fact that a message is
signed to change the behavior of message processing (examples would
be running rules or UI display hints), without first verifying that
the message is actually signed and knowing the state of the
signature, this can lead to incorrect handling of the message.
Visual indicators on messages may need to have the signature
validation code checked periodically if the indicator is supposed to
give information on the current status of a message.
Many people assume that the use of an authenticated encryption
algorithm is all that is needed for the sender of the message to be
authenticated. In almost all cases, this is not a correct statement.
There are a number of preconditions that need to hold for an
authenticated encryption algorithm to provide this service:
- The starting key must be bound to a single entity. The use of a
group key only would allow for the statement that a message was
sent by one of the entities that held the key but will not
identify a specific entity.
Schaad, et al. Standards Track PAGE 46
RFC 8551 S/MIME 4.0 Message Specification April 2019
- The message must have exactly one sender and one recipient.
Having more than one recipient would allow for the second
recipient to create a message that the first recipient would
believe is from the sender by stripping the second recipient from
the message.
- A direct path needs to exist from the starting key to the key used
as the CEK. That path needs to guarantee that no third party
could have seen the resulting CEK. This means that one needs to
be using an algorithm that is called a "Direct Encryption" or a
"Direct Key Agreement" algorithm in other contexts. This means
that the starting key is (1) used directly as the CEK or (2) used
to create a secret that is then transformed into the CEK via a
KDF step.
S/MIME implementations almost universally use ephemeral-static rather
than static-static key agreement and do not use a shared secret for
encryption. This means that the first precondition is not met.
[RFC 6278] defines how to use static-static key agreement with CMS, so
the first precondition can be met. Currently, all S/MIME key
agreement methods derive a key-encryption key (KEK) and wrap a CEK.
This violates the third precondition above. New key agreement
algorithms that directly created the CEK without creating an
intervening KEK would need to be defined.
Even when all of the preconditions are met and origination of a
message is established by the use of an authenticated encryption
algorithm, users need to be aware that there is no way to prove this
to a third party. This is because either of the parties can
successfully create the message (or just alter the content) based on
the fact that the CEK is going to be known to both parties. Thus,
the origination is always built on a presumption that "I did not send
this message to myself."
All of the authenticated encryption algorithms in this document use
counter mode for the encryption portion of the algorithm. This means
that the length of the plaintext will always be known, as the
ciphertext length and the plaintext length are always the same. This
information can enable passive observers to infer information based
solely on the length of the message. Applications for which this is
a concern need to provide some type of padding so that the length of
the message does not provide this information.
When compression is used with encryption, it has the potential to
provide an additional layer of security. However, care needs to be
taken when designing a protocol that relies on using compression, so
as not to create a compression oracle. Compression oracle attacks
require an adaptive input to the process and attack the unknown
Schaad, et al. Standards Track PAGE 47
RFC 8551 S/MIME 4.0 Message Specification April 2019
content of a message based on the length of the compressed output.
This means that no attack on the encryption key is necessarily
required.
A recent paper on S/MIME and OpenPGP email security [Efail] has
pointed out a number of problems with the current S/MIME
specifications and how people have implemented mail clients. Due to
the nature of how CBC mode operates, the modes allow for malleability
of plaintexts. This malleability allows for attackers to make
changes in the ciphertext and, if parts of the plaintext are known,
create arbitrary blocks of plaintext. These changes can be made
without the weak integrity check in CBC mode being triggered. This
type of attack can be prevented by the use of an Authenticated
Encryption with Associated Data (AEAD) algorithm with a more robust
integrity check on the decryption process. It is therefore
recommended that mail systems migrate to using AES-GCM as quickly as
possible and that the decrypted content not be acted on prior to
finishing the integrity check.
The other attack that is highlighted in [Efail] is due to an error in
how mail clients deal with HTML and multipart/mixed messages.
Clients MUST require that a text/html content type be a complete HTML
document (per [RFC 1866]). Clients SHOULD treat each of the different
pieces of the multipart/mixed construct as being of different
origins. Clients MUST treat each encrypted or signed piece of a MIME
message as being of different origins both from unprotected content
and from each other.
7. References
7.1. Reference Conventions
[ASN.1] refers to [X.680], [X.681], [X.682], and [X.683].
[CMS] refers to [RFC 5083] and [RFC 5652].
[ESS] refers to [RFC 2634] and [RFC 5035].
[MIME-SPEC] refers to [RFC 2045], [RFC 2046], [RFC 2047], [RFC 2049],
[RFC 6838], and [RFC 4289].
[SMIMEv2] refers to [RFC 2311], [RFC 2312], [RFC 2313], [RFC 2314], and
[RFC 2315].
[SMIMEv3] refers to [RFC 2630], [RFC 2631], [RFC 2632], [RFC 2633],
[RFC 2634], and [RFC 5035].
Schaad, et al. Standards Track PAGE 48
RFC 8551 S/MIME 4.0 Message Specification April 2019
[SMIMEv3.1] refers to [RFC 2634], [RFC 5035], [RFC 5652], [RFC 5750], and
[RFC 5751].
[SMIMEv3.2] refers to [RFC 2634], [RFC 3850], [RFC 3851], [RFC 3852], and
[RFC 5035].
[SMIMEv4] refers to [RFC 2634], [RFC 5035], [RFC 5652], [RFC 8550], and
this document.
7.2. Normative References
[CHARSETS] IANA, "Character sets assigned by IANA",
<http://www.iana.org/assignments/character-sets>.
[FIPS186-4]
National Institute of Standards and Technology (NIST),
"Digital Signature Standard (DSS)", Federal Information
Processing Standards Publication 186-4,
DOI 10.6028/NIST.FIPS.186-4, July 2013,
<https://nvlpubs.nist.gov/nistpubs/fips/
nist.fips.186-4.pdf>.
[RFC 1847] Galvin, J., Murphy, S., Crocker, S., and N. Freed,
"Security Multiparts for MIME: Multipart/Signed and
Multipart/Encrypted", RFC 1847, DOI 10.17487/RFC 1847,
October 1995, <https://www.rfc-editor.org/info/RFC 1847>.
[RFC 2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, DOI 10.17487/RFC 2045, November 1996,
<https://www.rfc-editor.org/info/RFC 2045>.
[RFC 2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
DOI 10.17487/RFC 2046, November 1996,
<https://www.rfc-editor.org/info/RFC 2046>.
[RFC 2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII Text",
RFC 2047, DOI 10.17487/RFC 2047, November 1996,
<https://www.rfc-editor.org/info/RFC 2047>.
[RFC 2049] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Five: Conformance Criteria and
Examples", RFC 2049, DOI 10.17487/RFC 2049, November 1996,
<https://www.rfc-editor.org/info/RFC 2049>.
Schaad, et al. Standards Track PAGE 49
RFC 8551 S/MIME 4.0 Message Specification April 2019
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<https://www.rfc-editor.org/info/RFC 2119>.
[RFC 2183] Troost, R., Dorner, S., and K. Moore, Ed., "Communicating
Presentation Information in Internet Messages: The
Content-Disposition Header Field", RFC 2183,
DOI 10.17487/RFC 2183, August 1997,
<https://www.rfc-editor.org/info/RFC 2183>.
[RFC 2634] Hoffman, P., Ed., "Enhanced Security Services for S/MIME",
RFC 2634, DOI 10.17487/RFC 2634, June 1999,
<https://www.rfc-editor.org/info/RFC 2634>.
[RFC 3274] Gutmann, P., "Compressed Data Content Type for
Cryptographic Message Syntax (CMS)", RFC 3274,
DOI 10.17487/RFC 3274, June 2002,
<https://www.rfc-editor.org/info/RFC 3274>.
[RFC 3370] Housley, R., "Cryptographic Message Syntax (CMS)
Algorithms", RFC 3370, DOI 10.17487/RFC 3370, August 2002,
<https://www.rfc-editor.org/info/RFC 3370>.
[RFC 3560] Housley, R., "Use of the RSAES-OAEP Key Transport
Algorithm in Cryptographic Message Syntax (CMS)",
RFC 3560, DOI 10.17487/RFC 3560, July 2003,
<https://www.rfc-editor.org/info/RFC 3560>.
[RFC 3565] Schaad, J., "Use of the Advanced Encryption Standard (AES)
Encryption Algorithm in Cryptographic Message Syntax
(CMS)", RFC 3565, DOI 10.17487/RFC 3565, July 2003,
<https://www.rfc-editor.org/info/RFC 3565>.
[RFC 4289] Freed, N. and J. Klensin, "Multipurpose Internet Mail
Extensions (MIME) Part Four: Registration Procedures",
BCP 13, RFC 4289, DOI 10.17487/RFC 4289, December 2005,
<https://www.rfc-editor.org/info/RFC 4289>.
[RFC 4056] Schaad, J., "Use of the RSASSA-PSS Signature Algorithm in
Cryptographic Message Syntax (CMS)", RFC 4056,
DOI 10.17487/RFC 4056, June 2005,
<https://www.rfc-editor.org/info/RFC 4056>.
[RFC 4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC 4086, June 2005,
<https://www.rfc-editor.org/info/RFC 4086>.
Schaad, et al. Standards Track PAGE 50
RFC 8551 S/MIME 4.0 Message Specification April 2019
[RFC 5083] Housley, R., "Cryptographic Message Syntax (CMS)
Authenticated-Enveloped-Data Content Type", RFC 5083,
DOI 10.17487/RFC 5083, November 2007,
<https://www.rfc-editor.org/info/RFC 5083>.
[RFC 5084] Housley, R., "Using AES-CCM and AES-GCM Authenticated
Encryption in the Cryptographic Message Syntax (CMS)",
RFC 5084, DOI 10.17487/RFC 5084, November 2007,
<https://www.rfc-editor.org/info/RFC 5084>.
[RFC 5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, DOI 10.17487/RFC 5652, September 2009,
<https://www.rfc-editor.org/info/RFC 5652>.
[RFC 5753] Turner, S. and D. Brown, "Use of Elliptic Curve
Cryptography (ECC) Algorithms in Cryptographic Message
Syntax (CMS)", RFC 5753, DOI 10.17487/RFC 5753,
January 2010, <https://www.rfc-editor.org/info/RFC 5753>.
[RFC 5754] Turner, S., "Using SHA2 Algorithms with Cryptographic
Message Syntax", RFC 5754, DOI 10.17487/RFC 5754,
January 2010, <https://www.rfc-editor.org/info/RFC 5754>.
[RFC 6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC 6838, January 2013,
<https://www.rfc-editor.org/info/RFC 6838>.
[RFC 8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC 8174,
May 2017, <https://www.rfc-editor.org/info/RFC 8174>.
[RFC 8418] Housley, R., "Use of the Elliptic Curve Diffie-Hellman Key
Agreement Algorithm with X25519 and X448 in the
Cryptographic Message Syntax (CMS)", RFC 8418,
DOI 10.17487/RFC 8418, August 2018,
<https://www.rfc-editor.org/info/RFC 8418>.
[RFC 8419] Housley, R., "Use of Edwards-Curve Digital Signature
Algorithm (EdDSA) Signatures in the Cryptographic Message
Syntax (CMS)", RFC 8419, DOI 10.17487/RFC 8419,
August 2018, <https://www.rfc-editor.org/info/RFC 8419>.
[RFC 8550] Schaad, J., Ramsdell, B., and S. Turner,
"Secure/Multipurpose Internet Mail Extensions (S/MIME)
Version 4.0 Certificate Handling", RFC 8550,
DOI 10.17487/RFC 8550, April 2019,
<https://www.rfc-editor.org/info/RFC 8550>.
Schaad, et al. Standards Track PAGE 51
RFC 8551 S/MIME 4.0 Message Specification April 2019
[X.680] "Information Technology - Abstract Syntax Notation One
(ASN.1): Specification of basic notation", ITU-T
Recommendation X.680, ISO/IEC 8824-1:2015, August 2015,
<https://www.itu.int/rec/T-REC-X.680>.
[X.681] "Information Technology - Abstract Syntax Notation One
(ASN.1): Information object specification", ITU-T
Recommendation X.681, ISO/IEC 8824-2:2015, August 2015,
<https://www.itu.int/rec/T-REC-X.681>.
[X.682] "Information Technology - Abstract Syntax Notation One
(ASN.1): Constraint specification", ITU-T
Recommendation X.682, ISO/IEC 8824-3:2015, August 2015,
<https://www.itu.int/rec/T-REC-X.682>.
[X.683] "Information Technology - Abstract Syntax Notation One
(ASN.1): Parameterization of ASN.1 specifications", ITU-T
Recommendation X.683, ISO/IEC 8824-4:2015, August 2015,
<https://www.itu.int/rec/T-REC-X.683>.
[X.690] "Information Technology - ASN.1 encoding rules:
Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules
(DER)", ITU-T Recommendation X.690, ISO/IEC 8825-1:2015,
August 2015, <https://www.itu.int/rec/T-REC-X.690>.
7.3. Informative References
[Efail] Poddebniak, D., Dresen, C., Muller, J., Ising, F.,
Schinzel, S., Friedberger, S., Somorovsky, J., and J.
Schwenk, "Efail: Breaking S/MIME and OpenPGP Email
Encryption using Exfiltration Channels",
UsenixSecurity 2018, August 2018,
<https://www.usenix.org/system/files/conference/
usenixsecurity18/sec18-poddebniak.pdf>.
[FIPS186-2]
National Institute of Standards and Technology (NIST),
"Digital Signature Standard (DSS) (also with Change
Notice 1)", Federal Information Processing Standards
Publication 186-2, January 2000,
<https://csrc.nist.gov/publications/detail/fips/186/2/
archive/2000-01-27>.
[RFC 1866] Berners-Lee, T. and D. Connolly, "Hypertext Markup
Language - 2.0", RFC 1866, DOI 10.17487/RFC 1866,
November 1995, <https://www.rfc-editor.org/info/RFC 1866>.
Schaad, et al. Standards Track PAGE 52
RFC 8551 S/MIME 4.0 Message Specification April 2019
[RFC 2268] Rivest, R., "A Description of the RC2(r) Encryption
Algorithm", RFC 2268, DOI 10.17487/RFC 2268, March 1998,
<https://www.rfc-editor.org/info/RFC 2268>.
[RFC 2311] Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L., and
L. Repka, "S/MIME Version 2 Message Specification",
RFC 2311, DOI 10.17487/RFC 2311, March 1998,
<https://www.rfc-editor.org/info/RFC 2311>.
[RFC 2312] Dusse, S., Hoffman, P., Ramsdell, B., and J. Weinstein,
"S/MIME Version 2 Certificate Handling", RFC 2312, DOI
10.17487/RFC 2312, March 1998,
<https://www.rfc-editor.org/info/RFC 2312>.
[RFC 2313] Kaliski, B., "PKCS #1: RSA Encryption Version 1.5",
RFC 2313, DOI 10.17487/RFC 2313, March 1998,
<https://www.rfc-editor.org/info/RFC 2313>.
[RFC 2314] Kaliski, B., "PKCS #10: Certification Request Syntax
Version 1.5", RFC 2314, DOI 10.17487/RFC 2314, March 1998,
<https://www.rfc-editor.org/info/RFC 2314>.
[RFC 2315] Kaliski, B., "PKCS #7: Cryptographic Message Syntax
Version 1.5", RFC 2315, DOI 10.17487/RFC 2315, March 1998,
<https://www.rfc-editor.org/info/RFC 2315>.
[RFC 2630] Housley, R., "Cryptographic Message Syntax", RFC 2630,
DOI 10.17487/RFC 2630, June 1999,
<https://www.rfc-editor.org/info/RFC 2630>.
[RFC 2631] Rescorla, E., "Diffie-Hellman Key Agreement Method",
RFC 2631, DOI 10.17487/RFC 2631, June 1999,
<https://www.rfc-editor.org/info/RFC 2631>.
[RFC 2632] Ramsdell, B., Ed., "S/MIME Version 3 Certificate
Handling", RFC 2632, DOI 10.17487/RFC 2632, June 1999,
<https://www.rfc-editor.org/info/RFC 2632>.
[RFC 2633] Ramsdell, B., Ed., "S/MIME Version 3 Message
Specification", RFC 2633, DOI 10.17487/RFC 2633, June 1999,
<https://www.rfc-editor.org/info/RFC 2633>.
[RFC 2785] Zuccherato, R., "Methods for Avoiding the "Small-Subgroup"
Attacks on the Diffie-Hellman Key Agreement Method for
S/MIME", RFC 2785, DOI 10.17487/RFC 2785, March 2000,
<https://www.rfc-editor.org/info/RFC 2785>.
Schaad, et al. Standards Track PAGE 53
RFC 8551 S/MIME 4.0 Message Specification April 2019
[RFC 3218] Rescorla, E., "Preventing the Million Message Attack on
Cryptographic Message Syntax", RFC 3218,
DOI 10.17487/RFC 3218, January 2002,
<https://www.rfc-editor.org/info/RFC 3218>.
[RFC 3766] Orman, H. and P. Hoffman, "Determining Strengths For
Public Keys Used For Exchanging Symmetric Keys", BCP 86,
RFC 3766, DOI 10.17487/RFC 3766, April 2004,
<https://www.rfc-editor.org/info/RFC 3766>.
[RFC 3850] Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail
Extensions (S/MIME) Version 3.1 Certificate Handling",
RFC 3850, DOI 10.17487/RFC 3850, July 2004,
<https://www.rfc-editor.org/info/RFC 3850>.
[RFC 3851] Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail
Extensions (S/MIME) Version 3.1 Message Specification",
RFC 3851, DOI 10.17487/RFC 3851, July 2004,
<https://www.rfc-editor.org/info/RFC 3851>.
[RFC 3852] Housley, R., "Cryptographic Message Syntax (CMS)",
RFC 3852, DOI 10.17487/RFC 3852, July 2004,
<https://www.rfc-editor.org/info/RFC 3852>.
[RFC 4134] Hoffman, P., Ed., "Examples of S/MIME Messages", RFC 4134,
DOI 10.17487/RFC 4134, July 2005,
<https://www.rfc-editor.org/info/RFC 4134>.
[RFC 4270] Hoffman, P. and B. Schneier, "Attacks on Cryptographic
Hashes in Internet Protocols", RFC 4270,
DOI 10.17487/RFC 4270, November 2005,
<https://www.rfc-editor.org/info/RFC 4270>.
[RFC 4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC 4949, August 2007,
<https://www.rfc-editor.org/info/RFC 4949>.
[RFC 5035] Schaad, J., "Enhanced Security Services (ESS) Update:
Adding CertID Algorithm Agility", RFC 5035, DOI
10.17487/RFC 5035, August 2007,
<https://www.rfc-editor.org/info/RFC 5035>.
[RFC 5750] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Certificate
Handling", RFC 5750, DOI 10.17487/RFC 5750, January 2010,
<https://www.rfc-editor.org/info/RFC 5750>.
Schaad, et al. Standards Track PAGE 54
RFC 8551 S/MIME 4.0 Message Specification April 2019
[RFC 5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, DOI 10.17487/RFC 5751,
January 2010, <https://www.rfc-editor.org/info/RFC 5751>.
[RFC 6151] Turner, S. and L. Chen, "Updated Security Considerations
for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
RFC 6151, DOI 10.17487/RFC 6151, March 2011,
<https://www.rfc-editor.org/info/RFC 6151>.
[RFC 6194] Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
Considerations for the SHA-0 and SHA-1 Message-Digest
Algorithms", RFC 6194, DOI 10.17487/RFC 6194, March 2011,
<https://www.rfc-editor.org/info/RFC 6194>.
[RFC 6268] Schaad, J. and S. Turner, "Additional New ASN.1 Modules
for the Cryptographic Message Syntax (CMS) and the Public
Key Infrastructure Using X.509 (PKIX)", RFC 6268,
DOI 10.17487/RFC 6268, July 2011,
<https://www.rfc-editor.org/info/RFC 6268>.
[RFC 6278] Herzog, J. and R. Khazan, "Use of Static-Static Elliptic
Curve Diffie-Hellman Key Agreement in Cryptographic
Message Syntax", RFC 6278, DOI 10.17487/RFC 6278,
June 2011, <https://www.rfc-editor.org/info/RFC 6278>.
[RFC 7114] Leiba, B., "Creation of a Registry for smime-type
Parameter Values", RFC 7114, DOI 10.17487/RFC 7114,
January 2014, <https://www.rfc-editor.org/info/RFC 7114>.
[RFC 7905] Langley, A., Chang, W., Mavrogiannopoulos, N.,
Strombergson, J., and S. Josefsson, "ChaCha20-Poly1305
Cipher Suites for Transport Layer Security (TLS)",
RFC 7905, DOI 10.17487/RFC 7905, June 2016,
<https://www.rfc-editor.org/info/RFC 7905>.
[SP800-56A]
National Institute of Standards and Technology (NIST),
"Recommendation for Pair-Wise Key Establishment Schemes
Using Discrete Logarithm Cryptography", NIST Special
Publication 800-56A Revision 2,
DOI 10.6028/NIST.SP.800-56Ar2, May 2013,
<https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
NIST.SP.800-56Ar2.pdf>.
Schaad, et al. Standards Track PAGE 55
RFC 8551 S/MIME 4.0 Message Specification April 2019
[SP800-57] National Institute of Standards and Technology (NIST),
"Recommendation for Key Management - Part 1: General",
NIST Special Publication 800-57 Revision 4,
DOI 10.6028/NIST.SP.800-57pt1r4, January 2016,
<https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
NIST.SP.800-57pt1r4.pdf>.
[TripleDES]
Tuchman, W., "Hellman Presents No Shortcut Solutions to
the DES", IEEE Spectrum v. 16, n. 7, pp. 40-41,
DOI 10.1109/MSPEC.1979.6368160, July 1979.
Schaad, et al. Standards Track PAGE 56
RFC 8551 S/MIME 4.0 Message Specification April 2019
Appendix A. ASN.1 Module
Note: The ASN.1 module contained herein is unchanged from RFC 5751
[SMIMEv2] and RFC 3851 [SMIMEv3.1], with the exception of a change to
the preferBinaryInside ASN.1 comment in RFC 3851 [SMIMEv3.1]. If a
module is needed that is compatible with current ASN.1 standards, one
can be found in [RFC 6268]. This module uses the 1988 version
of ASN.1.
SecureMimeMessageV3dot1
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) msg-v3dot1(21) }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
IMPORTS
-- Cryptographic Message Syntax [CMS]
SubjectKeyIdentifier, IssuerAndSerialNumber,
RecipientKeyIdentifier
FROM CryptographicMessageSyntax
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) cms-2001(14) };
-- id-aa is the arc with all new authenticated and unauthenticated
-- attributes produced by the S/MIME Working Group.
id-aa OBJECT IDENTIFIER ::= {iso(1) member-body(2) usa(840)
rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) attributes(2)}
-- S/MIME Capabilities provides a method of broadcasting the
-- symmetric capabilities understood. Algorithms SHOULD be ordered
-- by preference and grouped by type.
smimeCapabilities OBJECT IDENTIFIER ::= {iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 15}
SMIMECapability ::= SEQUENCE {
capabilityID OBJECT IDENTIFIER,
parameters ANY DEFINED BY capabilityID OPTIONAL }
SMIMECapabilities ::= SEQUENCE OF SMIMECapability
-- Encryption Key Preference provides a method of broadcasting the
-- preferred encryption certificate.
Schaad, et al. Standards Track PAGE 57
RFC 8551 S/MIME 4.0 Message Specification April 2019
id-aa-encrypKeyPref OBJECT IDENTIFIER ::= {id-aa 11}
SMIMEEncryptionKeyPreference ::= CHOICE {
issuerAndSerialNumber [0] IssuerAndSerialNumber,
receipentKeyId [1] RecipientKeyIdentifier,
subjectAltKeyIdentifier [2] SubjectKeyIdentifier
}
-- "receipentKeyId" is spelled incorrectly but is kept for
-- historical reasons.
id-smime OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs-9(9) 16 }
id-cap OBJECT IDENTIFIER ::= { id-smime 11 }
-- The preferBinaryInside OID indicates an ability to receive
-- messages with binary encoding inside the CMS wrapper.
-- The preferBinaryInside attribute's value field is ABSENT.
id-cap-preferBinaryInside OBJECT IDENTIFIER ::= { id-cap 1 }
-- The following is a list of OIDs to be used with S/MIME v3.
-- Signature Algorithms Not Found in [RFC 3370], [RFC 5754], [RFC 4056],
-- and [RFC 3560]
--
-- md2WithRSAEncryption OBJECT IDENTIFIER ::=
-- {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1)
-- 2}
--
-- Other Signed Attributes
--
-- signingTime OBJECT IDENTIFIER ::=
-- {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
-- 5}
-- See [CMS] for a description of how to encode the attribute
-- value.
SMIMECapabilitiesParametersForRC2CBC ::= INTEGER
-- (RC2 Key Length (number of bits))
END
Schaad, et al. Standards Track PAGE 58
RFC 8551 S/MIME 4.0 Message Specification April 2019
Appendix B. Historic Mail Considerations
Over the course of updating the S/MIME specifications, the set of
recommended algorithms has been modified each time the documents have
been updated. This means that if a user has historic emails and
their user agent has been updated to only support the current set of
recommended algorithms, some of those old emails will no longer be
accessible. It is strongly suggested that user agents implement some
of the following algorithms for dealing with historic emails.
This appendix contains a number of references to documents that have
been obsoleted or replaced. This is intentional, as the updated
documents often do not have the same information in them.
B.1. DigestAlgorithmIdentifier
The following algorithms have been called out for some level of
support by previous S/MIME specifications:
- SHA-1 was dropped in [SMIMEv4]. SHA-1 is no longer considered to
be secure, as it is no longer collision resistant. The IETF
statement on SHA-1 can be found in [RFC 6194], but it is out of
date relative to the most recent advances.
- MD5 was dropped in [SMIMEv4]. MD5 is no longer considered to be
secure, as it is no longer collision resistant. Details can be
found in [RFC 6151].
B.2. Signature Algorithms
There are a number of problems with validating signatures on
sufficiently historic messages. For this reason, it is strongly
suggested that user agents treat these signatures differently from
those on current messages. These problems include the following:
- Certification authorities are not required to keep certificates on
a CRL beyond one update after a certificate has expired. This
means that unless CRLs are cached as part of the message it is not
always possible to check to see if a certificate has been revoked.
The same problems exist with Online Certificate Status Protocol
(OCSP) responses, as they may be based on a CRL rather than on the
certificate database.
Schaad, et al. Standards Track PAGE 59
RFC 8551 S/MIME 4.0 Message Specification April 2019
- RSA and DSA keys of less than 2048 bits are now considered by many
experts to be cryptographically insecure (due to advances in
computing power). Such keys were previously considered secure, so
the processing of historic signed messages will often result in
the use of weak keys. Implementations that wish to support
previous versions of S/MIME or process old messages need to
consider the security risks that result from smaller key sizes
(e.g., spoofed messages) versus the costs of denial of service.
[SMIMEv3.1] set the lower limit on suggested key sizes for
creating and validation at 1024 bits. Prior to that, the lower
bound on key sizes was 512 bits.
- Hash functions used to validate signatures on historic messages
may no longer be considered to be secure (see below). While there
are not currently any known practical pre-image or second
pre-image attacks against MD5 or SHA-1, the fact that they are no
longer considered to be collision resistant implies that the
security levels of the signatures are generally considered
suspect. If a message is known to be historic and it has been in
the possession of the client for some time, then it might still be
considered to be secure.
- The previous two issues apply to the certificates used to validate
the binding of the public key to the identity that signed the
message as well.
The following algorithms have been called out for some level of
support by previous S/MIME specifications:
- RSA with MD5 was dropped in [SMIMEv4]. MD5 is no longer
considered to be secure, as it is no longer collision resistant.
Details can be found in [RFC 6151].
- RSA and DSA with SHA-1 were dropped in [SMIMEv4]. SHA-1 is no
longer considered to be secure, as it is no longer collision
resistant. The IETF statement on SHA-1 can be found in [RFC 6194],
but it is out of date relative to the most recent advances.
- DSA with SHA-256 was dropped in [SMIMEv4]. DSA has been replaced
by elliptic curve versions.
Schaad, et al. Standards Track PAGE 60
RFC 8551 S/MIME 4.0 Message Specification April 2019
As requirements for "mandatory to implement" have changed over time,
some issues have been created that can cause interoperability
problems:
- S/MIME v2 clients are only required to verify digital signatures
using the rsaEncryption algorithm with SHA-1 or MD5 and might not
implement id-dsa-with-sha1 or id-dsa at all.
- S/MIME v3 clients might only implement signing or signature
verification using id-dsa-with-sha1 and might also use id-dsa as
an AlgorithmIdentifier in this field.
- Note that S/MIME v3.1 clients support verifying id-dsa-with-sha1
and rsaEncryption and might not implement sha256WithRSAEncryption.
NOTE: Receiving clients SHOULD recognize id-dsa as equivalent to
id-dsa-with-sha1.
For 512-bit RSA with SHA-1, see [RFC 3370] and [FIPS186-2] without
Change Notice 1; for 512-bit RSA with SHA-256, see [RFC 5754] and
[FIPS186-2] without Change Notice 1; and for 1024-bit through
2048-bit RSA with SHA-256, see [RFC 5754] and [FIPS186-2] with Change
Notice 1. The first reference provides the signature algorithm's
OID, and the second provides the signature algorithm's definition.
For 512-bit DSA with SHA-1, see [RFC 3370] and [FIPS186-2] without
Change Notice 1; for 512-bit DSA with SHA-256, see [RFC 5754] and
[FIPS186-2] without Change Notice 1; for 1024-bit DSA with SHA-1, see
[RFC 3370] and [FIPS186-2] with Change Notice 1; and for 1024-bit and
above DSA with SHA-256, see [RFC 5754] and [FIPS186-4]. The first
reference provides the signature algorithm's OID, and the second
provides the signature algorithm's definition.
B.3. ContentEncryptionAlgorithmIdentifier
The following algorithms have been called out for some level of
support by previous S/MIME specifications:
- RC2/40 [RFC 2268] was dropped in [SMIMEv3.2]. The algorithm is
known to be insecure and, if supported, should only be used to
decrypt existing email.
- DES EDE3 CBC [TripleDES], also known as "tripleDES", was dropped
in [SMIMEv4]. This algorithm is removed from the list of
supported algorithms because (1) it has a 64-bit block size and
(2) it offers less than 128 bits of security. This algorithm
should be supported only to decrypt existing email; it should not
be used to encrypt new emails.
Schaad, et al. Standards Track PAGE 61
RFC 8551 S/MIME 4.0 Message Specification April 2019
B.4. KeyEncryptionAlgorithmIdentifier
The following algorithms have been called out for some level of
support by previous S/MIME specifications:
- DH ephemeral-static mode, as specified in [RFC 3370] and
[SP800-57], was dropped in [SMIMEv4].
- RSA key sizes have been increased over time. Decrypting old mail
with smaller key sizes is reasonable; however, new mail should use
the updated key sizes.
For 1024-bit DH, see [RFC 3370]. For 1024-bit and larger DH, see
[SP800-56A]; regardless, use the KDF, which is from X9.42, specified
in [RFC 3370].
Appendix C. Moving S/MIME v2 Message Specification to Historic Status
The S/MIME v3 [SMIMEv3], v3.1 [SMIMEv3.1], and v3.2 [SMIMEv3.2]
specifications are backward compatible with the S/MIME v2 Message
Specification [SMIMEv2], with the exception of the algorithms
(dropped RC2/40 requirement and added DSA and RSASSA-PSS
requirements). Therefore, RFC 2311 [SMIMEv2] was moved to Historic
status.
Acknowledgements
Many thanks go out to the other authors of the S/MIME version 2
Message Specification RFC: Steve Dusse, Paul Hoffman, Laurence
Lundblade, and Lisa Repka. Without v2, there wouldn't be a v3, v3.1,
v3.2, or v4.0.
Some of the examples in this document were copied from [RFC 4134].
Thanks go to the people who wrote and verified the examples in that
document.
A number of the members of the S/MIME Working Group have also worked
very hard and contributed to this document. Any list of people is
doomed to omission, and for that I apologize. In alphabetical order,
the following people stand out in my mind because they made direct
contributions to this document:
Tony Capel, Piers Chivers, Dave Crocker, Bill Flanigan, Peter
Gutmann, Alfred Hoenes, Paul Hoffman, Russ Housley, William Ottaway,
and John Pawling.
The version 4 update to the S/MIME documents was done under the
auspices of the LAMPS Working Group.
Schaad, et al. Standards Track PAGE 62
RFC 8551 S/MIME 4.0 Message Specification April 2019
Authors' Addresses
Jim Schaad
August Cellars
Email: ietf@augustcellars.com
Blake Ramsdell
Brute Squad Labs, Inc.
Email: blaker@gmail.com
Sean Turner
sn3rd
Email: sean@sn3rd.com
Schaad, et al. Standards Track PAGE 63
RFC TOTAL SIZE: 136849 bytes
PUBLICATION DATE: Wednesday, May 1st, 2019
LEGAL RIGHTS: The IETF Trust (see BCP 78)
|
