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IETF RFC 8550
Last modified on Wednesday, May 1st, 2019
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Internet Engineering Task Force (IETF) J. Schaad
Request for Comments: 8550 August Cellars
Obsoletes: 5750 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
Certificate Handling
 Abstract
This document specifies conventions for X.509 certificate usage by
Secure/Multipurpose Internet Mail Extensions (S/MIME) v4.0 agents.
S/MIME provides a method to send and receive secure MIME messages,
and certificates are an integral part of S/MIME agent processing.
S/MIME agents validate certificates as described in RFC 5280
("Internet X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile"). S/MIME agents must meet
the certificate-processing requirements in this document as well as
those in RFC 5280. This document obsoletes RFC 5750.
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 8550.
Schaad, et al. Standards Track PAGE 1 
RFC 8550 S/MIME 4.0 Certificate Handling 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
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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 8550 S/MIME 4.0 Certificate Handling April 2019
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Conventions Used in This Document . . . . . . . . . . . . 5
1.3. Compatibility with Prior Practice of S/MIME . . . . . . . 6
1.4. Changes from S/MIME v3 to S/MIME v3.1 . . . . . . . . . . 6
1.5. Changes from S/MIME v3.1 to S/MIME v3.2 . . . . . . . . . 7
1.6. Changes since S/MIME 3.2 . . . . . . . . . . . . . . . . 8
2. CMS Options . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1. Certificate Revocation Lists . . . . . . . . . . . . . . 9
2.2. Certificate Choices . . . . . . . . . . . . . . . . . . . 9
2.2.1. Historical Note about CMS Certificates . . . . . . . 9
2.3. Included Certificates . . . . . . . . . . . . . . . . . . 10
3. Using Distinguished Names for Internet Mail . . . . . . . . . 11
4. Certificate Processing . . . . . . . . . . . . . . . . . . . 12
4.1. Certificate Revocation Lists . . . . . . . . . . . . . . 13
4.2. Certificate Path Validation . . . . . . . . . . . . . . . 13
4.3. Certificate and CRL Signing Algorithms, and Key Sizes . . 14
4.4. PKIX Certificate Extensions . . . . . . . . . . . . . . . 15
4.4.1. Basic Constraints . . . . . . . . . . . . . . . . . . 16
4.4.2. Key Usage Extension . . . . . . . . . . . . . . . . . 16
4.4.3. Subject Alternative Name . . . . . . . . . . . . . . 17
4.4.4. Extended Key Usage Extension . . . . . . . . . . . . 17
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
6. Security Considerations . . . . . . . . . . . . . . . . . . . 18
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
7.1. Reference Conventions . . . . . . . . . . . . . . . . . . 20
7.1. Normative References . . . . . . . . . . . . . . . . . . 20
7.2. Informative References . . . . . . . . . . . . . . . . . 23
Appendix A. Historic Considerations . . . . . . . . . . . . . . 26
A.1. Signature Algorithms and Key Sizes . . . . . . . . . . . 26
Appendix B. Moving S/MIME v2 Certificate Handling to Historic
Status . . . . . . . . . . . . . . . . . . . . . . . 27
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28
Schaad, et al. Standards Track PAGE 3
RFC 8550 S/MIME 4.0 Certificate Handling April 2019
1. Introduction
S/MIME (Secure/Multipurpose Internet Mail Extensions) v4.0, described
in [RFC 8551], provides a method to send and receive secure MIME
messages. Before using a public key to provide security services,
the S/MIME agent MUST verify that the public key is valid. S/MIME
agents MUST use PKIX certificates to validate public keys as
described in [RFC 5280] ("Internet X.509 Public Key Infrastructure
Certificate and Certificate Revocation List (CRL) Profile"). S/MIME
agents MUST meet the certificate-processing requirements specified in
this document in addition to those stated in [RFC 5280].
This specification is compatible with the Cryptographic Message
Syntax (CMS) [RFC 5652] in that it uses the data types defined by CMS.
It also inherits all the varieties of architectures for certificate-
based key management supported by CMS.
This document obsoletes [RFC 5750]. The most significant changes
revolve around changes in recommendations around the cryptographic
algorithms used by the specification. More details can be found in
Section 1.6.
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. Definitions
For the purposes of this document, the following definitions apply.
ASN.1:
Abstract Syntax Notation One, as defined in ITU-T X.680 [X.680].
Attribute certificate (AC):
An X.509 AC is a separate structure from a subject's public key
X.509 certificate. A subject may have multiple X.509 ACs
associated with each of its public key X.509 certificates. Each
X.509 AC binds one or more attributes with one of the subject's
public key X.509 certificates. The X.509 AC syntax is defined in
[RFC 5755].
Schaad, et al. Standards Track PAGE 4
RFC 8550 S/MIME 4.0 Certificate Handling April 2019
Certificate:
A type that binds an entity's name to a public key with a digital
signature. This type is defined in [RFC 5280]. This type also
contains the distinguished name of the certificate issuer (the
signer), an issuer-specific serial number, the issuer's signature
algorithm identifier, a validity period, and extensions also
defined in that document.
Certificate Revocation List (CRL):
A type that contains information about certificates whose validity
an issuer has revoked. The information consists of an issuer
name, the time of issue, the next scheduled time of issue, a list
of certificate serial numbers and their associated revocation
times, and extensions as defined in [RFC 5280]. The CRL is signed
by the issuer. The type intended by this specification is the one
defined in [RFC 5280].
Receiving agent:
Software that interprets and processes S/MIME CMS objects, MIME
body parts that contain CMS objects, or both.
Sending agent:
Software that creates S/MIME CMS objects, MIME body parts that
contain CMS objects, or both.
S/MIME agent:
User software that is a receiving agent, a sending agent, or both.
1.2. 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.
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.
Schaad, et al. Standards Track PAGE 5
RFC 8550 S/MIME 4.0 Certificate Handling April 2019
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 signature algorithm 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.
1.3. 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.
Appendix A contains information about algorithms that were used for
prior versions of S/MIME but are no longer considered to meet modern
security standards. Support of these algorithms may be needed to
support historic S/MIME artifacts such as messages or files but
SHOULD NOT be used for new artifacts.
1.4. Changes from S/MIME v3 to S/MIME v3.1
This section reflects the changes that were made when S/MIME v3.1 was
released. The language of RFC 2119 ("MUST", "SHOULD", etc.) used for
S/MIME v3 may have been superseded in later versions.
- Version 1 and version 2 CRLs MUST be supported.
Schaad, et al. Standards Track PAGE 6
RFC 8550 S/MIME 4.0 Certificate Handling April 2019
- Multiple certification authority (CA) certificates with the same
subject and public key, but with overlapping validity periods,
MUST be supported.
- Version 2 ACs SHOULD be supported, and version 1 ACs MUST NOT be
used.
- The use of the MD2 digest algorithm for certificate signatures is
discouraged, and security language was added.
- Clarified email address use in certificates. Certificates that do
not contain an email address have no requirements for verifying
the email address associated with the certificate.
- Receiving agents SHOULD display certificate information when
displaying the results of signature verification.
- Receiving agents MUST NOT accept a signature made with a
certificate that does not have at least one of the
digitalSignature or nonRepudiation bits set.
- Added clarifications for the interpretation of the key usage and
extended key usage extensions.
1.5. Changes from S/MIME v3.1 to S/MIME v3.2
This section reflects the changes that were made when S/MIME v3.2 was
released. The language of RFC 2119 ("MUST", "SHOULD", etc.) used for
S/MIME v3.1 may have been superseded in later versions.
Note that the section numbers listed here (e.g., "Section 6") are
from [RFC 5750].
- Moved "Conventions Used in This Document" to Section 1.2. Added
definitions for SHOULD+, SHOULD-, and MUST-.
- Section 1.1: Updated ASN.1 definition and reference.
- Section 1.3: Added text about v3.1 RFCs.
- Section 3: Aligned email address text with RFC 5280. Updated note
to indicate that the emailAddress IA5String upper bound is
255 characters. Added text about matching email addresses.
- Section 4.2: Added text to indicate how S/MIME agents locate the
correct user certificate.
Schaad, et al. Standards Track PAGE 7
RFC 8550 S/MIME 4.0 Certificate Handling April 2019
- Section 4.3: RSA with SHA-256 (PKCS #1 v1.5) 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+. Updated key sizes and changed pointer to PKIX RFCs.
- Section 4.4.1: Aligned with PKIX on the use of a basicConstraints
extension in CA certificates. Clarified which extension is used
to constrain end entities from using their keys to perform
issuing-authority operations.
- Section 5: Updated security considerations.
- Section 6: Moved references from Appendix A of RFC 3850 to this
section. Updated the references.
- Appendix A: Added Appendix A to move S/MIME v2 Certificate
Handling to Historic status.
1.6. Changes since S/MIME 3.2
This section reflects the changes that were made when S/MIME v4.0 was
released. The language of RFC 2119 ("MUST", "SHOULD", etc.) used for
S/MIME v3.2 may have been superseded by S/MIME v4.0 and may be
superseded by future versions.
- Section 3: Support for internationalized email addresses is
required.
- Section 4.3: Mandated support for the Elliptic Curve Digital
Signature Algorithm (ECDSA) with P-256 and the Edwards-curve
Digital Signature Algorithm (EdDSA) with curve25519 [RFC 8410].
SHA-1 and MD5 algorithms are marked as historical, as they are no
longer considered secure. As the Digital Signature Algorithm
(DSA) has been replaced by elliptic curve versions, support for
DSA is now considered historical. Increased lower bounds on RSA
key sizes.
- Appendix A: Added Appendix A for algorithms that are now
considered to be historical.
2. CMS Options
The CMS message format allows for a wide variety of options in
content 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. Most of
the CMS format for S/MIME messages is defined in [RFC 8551].
Schaad, et al. Standards Track PAGE 8
RFC 8550 S/MIME 4.0 Certificate Handling April 2019
2.1. Certificate Revocation Lists
Receiving agents MUST support the CRL format defined in [RFC 5280].
If sending agents include CRLs in outgoing messages, the CRL format
defined in [RFC 5280] MUST be used. Receiving agents MUST support
both v1 and v2 CRLs.
All agents MUST be capable of performing revocation checks using CRLs
as specified in [RFC 5280]. All agents MUST perform revocation status
checking in accordance with [RFC 5280]. Receiving agents MUST
recognize CRLs in received S/MIME messages.
Agents SHOULD store CRLs received in messages for use in processing
later messages.
2.2. Certificate Choices
Receiving agents MUST support v1 X.509 and v3 X.509 certificates as
profiled in [RFC 5280]. End-entity certificates MAY include an
Internet mail address, as described in Section 3.
Receiving agents SHOULD support X.509 version 2 ACs. See [RFC 5755]
for details about the profile for ACs.
2.2.1. Historical Note about CMS Certificates
The CMS message format supports a choice of certificate formats for
public key content types: PKIX, PKCS #6 extended certificates
[PKCS6], and PKIX ACs.
The PKCS #6 format is not in widespread use. In addition, PKIX
certificate extensions address much of the same functionality and
flexibility as was intended in the PKCS #6 certificate extensions.
Thus, sending and receiving agents MUST NOT use PKCS #6 extended
certificates. Receiving agents MUST be able to parse and process a
message containing PKCS #6 extended certificates, although ignoring
those certificates is expected behavior.
X.509 version 1 ACs are also not widely implemented and have
been superseded by version 2 ACs. Sending agents MUST NOT send
version 1 ACs.
Schaad, et al. Standards Track PAGE 9
RFC 8550 S/MIME 4.0 Certificate Handling April 2019
2.3. Included Certificates
Receiving agents MUST be able to handle an arbitrary number of
certificates of arbitrary relationship to the message sender and to
each other in arbitrary order. In many cases, the certificates
included in a signed message may represent a chain of certification
from the sender to a particular root. There may be, however,
situations where the certificates in a signed message may be
unrelated and included for convenience.
Sending agents SHOULD include any certificates for the user's public
key(s) and associated issuer certificates. This increases the
likelihood that the intended recipient can establish trust in the
originator's public key(s). This is especially important when
sending a message to recipients that may not have access to the
sender's public key through any other means or when sending a signed
message to a new recipient. The inclusion of certificates in
outgoing messages can be omitted if S/MIME objects are sent within a
group of correspondents that have established access to each other's
certificates by some other means such as a shared directory or manual
certificate distribution. Receiving S/MIME agents SHOULD be able to
handle messages without certificates by using a database or directory
lookup scheme to find them.
A sending agent SHOULD include at least one chain of certificates up
to, but not including, a CA that it believes that the recipient may
trust as authoritative. A receiving agent MUST be able to handle an
arbitrarily large number of certificates and chains.
Agents MAY send CA certificates -- that is, cross-certificates,
self-issued certificates, and self-signed certificates. Note that
receiving agents SHOULD NOT simply trust any self-signed certificates
as valid CAs but SHOULD use some other mechanism to determine if this
is a CA that should be trusted. Also note that when certificates
contain DSA public keys the parameters may be located in the root
certificate. This would require that the recipient possess both the
end-entity certificate and the root certificate to perform a
signature verification, and is a valid example of a case where
transmitting the root certificate may be required.
Receiving agents MUST support chaining based on the distinguished
name fields. Other methods of building certificate chains MAY be
supported.
Receiving agents SHOULD support the decoding of X.509 ACs included in
CMS objects. All other issues regarding the generation and use of
X.509 ACs are outside the scope of this specification. One
specification that addresses AC use is defined in [RFC 3114].
Schaad, et al. Standards Track PAGE 10
RFC 8550 S/MIME 4.0 Certificate Handling April 2019
3. Using Distinguished Names for Internet Mail
End-entity certificates MAY contain an Internet mail address.
Email addresses restricted to 7-bit ASCII characters use the
pkcs-9-at-emailAddress object identifier (OID) (see below) and are
encoded as described in Section 4.2.1.6 of [RFC 5280].
Internationalized email address names use the OID defined in
[RFC 8398] and are encoded as described therein. The email address
SHOULD be in the subjectAltName extension and SHOULD NOT be in the
subject distinguished name.
Receiving agents MUST recognize and accept certificates that contain
no email address. Agents are allowed to provide an alternative
mechanism for associating an email address with a certificate that
does not contain an email address, such as through the use of the
agent's address book, if available. Receiving agents MUST recognize
both ASCII and internationalized email addresses in the
subjectAltName extension. Receiving agents MUST recognize email
addresses in the distinguished name field in the PKCS #9 [RFC 2985]
emailAddress attribute:
pkcs-9-at-emailAddress OBJECT IDENTIFIER ::=
{ iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 1 }
Note that this attribute MUST be encoded as IA5String and has an
upper bound of 255 characters. The comparing of email addresses is
fraught with peril. [RFC 8398] defines the procedure for doing the
comparison of internationalized email addresses. For ASCII email
addresses, the domain component (right-hand side of the '@') MUST be
compared using a case-insensitive function. The local name
component (left-hand side of the '@') SHOULD be compared using a
case-insensitive function. Some localities may perform other
transformations on the local name component before doing the
comparison; however, an S/MIME client cannot know what specific
localities do.
Sending agents SHOULD make the address in the From or Sender header
in a mail message match an Internet mail address in the signer's
certificate. Receiving agents MUST check that the address in the
From or Sender header of a mail message matches an Internet mail
address in the signer's certificate, if mail addresses are present in
the certificate. A receiving agent SHOULD provide some explicit
alternate processing of the message if this comparison fails; this
might be done by displaying or logging a message that shows the
recipient the mail addresses in the certificate or other certificate
details.
Schaad, et al. Standards Track PAGE 11
RFC 8550 S/MIME 4.0 Certificate Handling April 2019
A receiving agent SHOULD display a subject name or other certificate
details when displaying an indication of successful or unsuccessful
signature verification.
All subject and issuer names MUST be populated (i.e., not an empty
SEQUENCE) in S/MIME-compliant X.509 certificates, except that the
subject distinguished name in a user's (i.e., an end entity's)
certificate MAY be an empty SEQUENCE, in which case the
subjectAltName extension will include the subject's identifier and
MUST be marked as critical.
4. Certificate Processing
S/MIME agents need to provide some certificate retrieval mechanism in
order to gain access to certificates for recipients of digital
envelopes. There are many ways to implement certificate retrieval
mechanisms. [X.500] directory service is an excellent example of a
certificate retrieval-only mechanism that is compatible with classic
X.500 distinguished names. The IETF has published [RFC 8162], which
describes an experimental protocol to retrieve certificates from the
Domain Name System (DNS). Until such mechanisms are widely used,
their utility may be limited by the small number of the
correspondent's certificates that can be retrieved. At a minimum,
for initial S/MIME deployment, a user agent could automatically
generate a message to an intended recipient requesting the
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. In many environments,
it may be desirable to link the certificate retrieval/storage
mechanisms together in some sort of certificate database. In its
simplest form, a certificate database would be local to a particular
user and would function in a way similar to an "address book" that
stores a user's frequent correspondents. In this way, the
certificate retrieval mechanism would be limited to the certificates
that a user has stored (presumably from incoming messages). A
comprehensive certificate retrieval/storage solution might combine
two or more mechanisms to allow the greatest flexibility and utility
to the user. For instance, a secure Internet mail agent might resort
to checking a centralized certificate retrieval mechanism for a
certificate if it cannot be found in a user's local certificate
storage/retrieval database.
Schaad, et al. Standards Track PAGE 12
RFC 8550 S/MIME 4.0 Certificate Handling April 2019
Receiving and sending agents SHOULD provide a mechanism for the
import and export of certificates, using a CMS certs-only message.
This allows for import and export of full certificate chains as
opposed to just a single certificate. This is described in
[RFC 8551].
Agents MUST handle multiple valid CA certificates containing the same
subject name and the same public keys but with overlapping validity
intervals.
4.1. Certificate Revocation Lists
In general, it is always better to get the latest CRL information
from a CA than to get information stored in an incoming message. A
receiving agent SHOULD have access to some CRL retrieval mechanism in
order to gain access to certificate revocation information when
validating certification paths. A receiving or sending agent SHOULD
also provide a mechanism to allow a user to store incoming
certificate revocation information for correspondents in such a way
as to guarantee its later retrieval.
Receiving and sending agents SHOULD retrieve and utilize CRL
information every time a certificate is verified as part of a
certification path validation even if the certificate was already
verified in the past. However, in many instances (such as off-line
verification), access to the latest CRL information may be difficult
or impossible. The use of CRL information, therefore, may be
dictated by the value of the information that is protected. The
value of the CRL information in a particular context is beyond the
scope of this specification but may be governed by the policies
associated with particular certification paths.
All agents MUST be capable of performing revocation checks using CRLs
as specified in [RFC 5280]. All agents MUST perform revocation status
checking in accordance with [RFC 5280]. Receiving agents MUST
recognize CRLs in received S/MIME messages.
4.2. Certificate Path Validation
In creating a user agent for secure messaging, certificate, CRL, and
certification path validation should be highly automated while still
acting in the best interests of the user. Certificate, CRL, and path
validation MUST be performed as per [RFC 5280] when validating a
correspondent's public key. This is necessary before using a public
key to provide security services such as verifying a signature,
encrypting a content-encryption key (e.g., RSA), or forming a
pairwise symmetric key (e.g., Diffie-Hellman) to be used to encrypt
or decrypt a content-encryption key.
Schaad, et al. Standards Track PAGE 13
RFC 8550 S/MIME 4.0 Certificate Handling April 2019
Certificates and CRLs are made available to the path validation
procedure in two ways: a) incoming messages and b) certificate and
CRL retrieval mechanisms. Certificates and CRLs in incoming messages
are not required to be in any particular order, nor are they required
to be in any way related to the sender or recipient of the message
(although in most cases they will be related to the sender).
Incoming certificates and CRLs SHOULD be cached for use in path
validation and optionally stored for later use. This temporary
certificate and CRL cache SHOULD be used to augment any other
certificate and CRL retrieval mechanisms for path validation on
incoming signed messages.
When verifying a signature and the certificates that are included in
the message, if a signingCertificate attribute from RFC 2634 [ESS] or
a signingCertificateV2 attribute from RFC 5035 [ESS] is found in an
S/MIME message, it SHALL be used to identify the signer's
certificate. Otherwise, the certificate is identified in an S/MIME
message, using either (1) the issuerAndSerialNumber, which identifies
the signer's certificate by the issuer's distinguished name and the
certificate serial number or (2) the subjectKeyIdentifier, which
identifies the signer's certificate by a key identifier.
When decrypting an encrypted message, if an
SMIMEEncryptionKeyPreference attribute is found in an encapsulating
SignedData, it SHALL be used to identify the originator's certificate
found in OriginatorInfo. See [RFC 5652] for the CMS fields that
reference the originator's and recipient's certificates.
4.3. Certificate and CRL Signing Algorithms, and Key Sizes
Certificates and CRLs are signed by the certificate issuer.
Receiving agents:
- MUST support ECDSA with curve P-256 with SHA-256.
- MUST support EdDSA with curve25519 using PureEdDSA mode.
- MUST- support RSA PKCS #1 v1.5 with SHA-256.
- SHOULD support the RSA Probabilistic Signature Scheme (RSASSA-PSS)
with SHA-256.
Implementations SHOULD use deterministic generation for the parameter
'k' for ECDSA as outlined in [RFC 6979]. EdDSA is defined to generate
this parameter deterministically.
Schaad, et al. Standards Track PAGE 14
RFC 8550 S/MIME 4.0 Certificate Handling April 2019
The following are the RSA and RSASSA-PSS key size requirements for
S/MIME receiving agents during certificate and CRL signature
verification:
key size <= 2047 : SHOULD NOT (see Appendix A)
2048 <= key size <= 4096 : MUST (see Security Considerations)
4096 < key size : MAY (see Security Considerations)
The signature algorithm OIDs for RSA PKCS #1 v1.5 and RSASSA-PSS with
SHA-256 using 1024-bit through 3072-bit public keys are specified in
[RFC 4055], and the signature algorithm definition is found in
[FIPS186-2] with Change Notice 1.
The signature algorithm OIDs for RSA PKCS #1 v1.5 and RSASSA-PSS with
SHA-256 using 4096-bit public keys are specified in [RFC 4055], and
the signature algorithm definition is found in [RFC 3447].
For RSASSA-PSS with SHA-256, see [RFC 4056].
For ECDSA, see [RFC 5758] and [RFC 6090]. The first reference provides
the signature algorithm's OID, and the second provides the signature
algorithm's definition. Curves other than curve P-256 MAY be used as
well.
For EdDSA, see [RFC 8032] and [RFC 8410]. The first reference provides
the signature algorithm's OID, and the second provides the signature
algorithm's definition. Curves other than curve25519 MAY be used as
well.
4.4. PKIX Certificate Extensions
PKIX describes an extensible framework in which the basic certificate
information can be extended and describes how such extensions can be
used to control the process of issuing and validating certificates.
The LAMPS Working Group has ongoing efforts to identify and create
extensions that have value in particular certification environments.
Further, there are active efforts underway to issue PKIX certificates
for business purposes. This document identifies the minimum required
set of certificate extensions that have the greatest value in the
S/MIME environment. The syntax and semantics of all the identified
extensions are defined in [RFC 5280].
Sending and receiving agents MUST correctly handle the basic
constraints, key usage, authority key identifier, subject key
identifier, and subject alternative name certificate extensions when
they appear in end-entity and CA certificates. Some mechanism SHOULD
exist to gracefully handle other certificate extensions when they
appear in end-entity or CA certificates.
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Certificates issued for the S/MIME environment SHOULD NOT contain any
critical extensions (extensions that have the critical field set to
TRUE) other than those listed here. These extensions SHOULD be
marked as non-critical, unless the proper handling of the extension
is deemed critical to the correct interpretation of the associated
certificate. Other extensions may be included, but those extensions
SHOULD NOT be marked as critical.
Interpretation and syntax for all extensions MUST follow [RFC 5280],
unless otherwise specified here.
4.4.1. Basic Constraints
The basicConstraints extension serves to delimit the role and
position that an issuing-authority or end-entity certificate plays in
a certification path.
For example, certificates issued to CAs and subordinate CAs contain a
basicConstraints extension that identifies them as issuing-authority
certificates. End-entity certificates contain the key usage
extension, which restrains end entities from using the key when
performing issuing-authority operations (see Section 4.4.2).
As per [RFC 5280], certificates MUST contain a basicConstraints
extension in CA certificates and SHOULD NOT contain that extension in
end-entity certificates.
4.4.2. Key Usage Extension
The key usage extension serves to limit the technical purposes for
which a public key listed in a valid certificate may be used.
Issuing-authority certificates may contain a key usage extension that
restricts the key to signing certificates, CRLs, and other data.
For example, a CA may create subordinate issuer certificates that
contain a key usage extension that specifies that the corresponding
public key can be used to sign end-user certificates and CRLs.
If a key usage extension is included in a PKIX certificate, then it
MUST be marked as critical.
S/MIME receiving agents MUST NOT accept the signature of a message if
it was verified using a certificate that contains a key usage
extension without at least one of the digitalSignature or
nonRepudiation bits set. Sometimes S/MIME is used as a secure
message transport for applications beyond interpersonal messaging; in
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such cases, the S/MIME-enabled application can specify additional
requirements concerning the digitalSignature or nonRepudiation bits
within this extension.
If the key usage extension is not specified, receiving clients MUST
presume that both the digitalSignature and nonRepudiation bits
are set.
4.4.3. Subject Alternative Name
The subject alternative name extension is used in S/MIME as the
preferred means to convey the email address or addresses that
correspond to the entity for this certificate. If the local portion
of the email address is ASCII, it MUST be encoded using the
rfc822Name CHOICE of the GeneralName type as described in [RFC 5280],
Section 4.2.1.6. If the local portion of the email address is not
ASCII, it MUST be encoded using the otherName CHOICE of the
GeneralName type as described in [RFC 8398], Section 3. Since the
SubjectAltName type is a SEQUENCE OF GeneralName, multiple email
addresses MAY be present.
4.4.4. Extended Key Usage Extension
The extended key usage extension also serves to limit the technical
purposes for which a public key listed in a valid certificate may be
used. The set of technical purposes for the certificate therefore
are the intersection of the uses indicated in the key usage and
extended key usage extensions.
For example, if the certificate contains a key usage extension
indicating a digital signature and an extended key usage extension
that includes the id-kp-emailProtection OID, then the certificate may
be used for signing but not encrypting S/MIME messages. If the
certificate contains a key usage extension indicating a digital
signature but no extended key usage extension, then the certificate
may also be used to sign but not encrypt S/MIME messages.
If the extended key usage extension is present in the certificate,
then interpersonal-message S/MIME receiving agents MUST check that it
contains either the id-kp-emailProtection OID or the
anyExtendedKeyUsage OID as defined in [RFC 5280]. S/MIME uses other
than interpersonal messaging MAY require the explicit presence of the
extended key usage extension, the presence of other OIDs in the
extension, or both.
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5. IANA Considerations
This document has no IANA actions.
6. Security Considerations
All of the security issues faced by any cryptographic application
must be faced by an S/MIME agent. Among these issues are protecting
the user's private key, preventing various attacks, and helping the
user avoid mistakes such as inadvertently encrypting a message for
the wrong recipient. The entire list of security considerations is
beyond the scope of this document, but some significant concerns are
listed here.
When processing certificates, there are many situations where the
processing might fail. Because the processing may be done by a user
agent, a security gateway, or some other program, there is no single
way to handle such failures. Just because the methods to handle the
failures have not been listed, however, the reader should not assume
that they are not important. The opposite is true: if a certificate
is not provably valid and associated with the message, the processing
software should take immediate and noticeable steps to inform the end
user about it.
Some of the many places where signature and certificate checking
might fail include the following:
- no Internet mail addresses in a certificate match the sender of a
message, if the certificate contains at least one mail address
- no certificate chain leads to a trusted CA
- no ability to check the CRL for a certificate is implemented
- an invalid CRL was received
- the CRL being checked is expired
- the certificate is expired
- the certificate has been revoked
There are certainly other instances where a certificate may be
invalid, and it is the responsibility of the processing software to
check them all thoroughly and decide what to do if the check fails.
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It is possible for there to be multiple unexpired CRLs for a CA. If
an agent is consulting CRLs for certificate validation, it SHOULD
make sure that the most recently issued CRL for that CA is consulted,
since an S/MIME message sender could deliberately include an older
unexpired CRL in an S/MIME message. This older CRL might not include
recently revoked certificates; this scenario might lead an agent to
accept a certificate that has been revoked in a subsequent CRL.
When determining the time for a certificate validity check, agents
have to be careful to use a reliable time. In most cases, the time
used SHOULD be the current time. Some exceptions to this would be as
follows:
- The time the message was received is stored in a secure manner and
is used at a later time to validate the message.
- The time in a SigningTime attribute is found in a countersignature
attribute [RFC 5652] that has been successfully validated.
The signingTime attribute could be deliberately set to a time where
the receiving agent would (1) use a CRL that does not contain a
revocation for the signing certificate or (2) use a certificate that
has expired or is not yet valid. This could be done by either
(1) the sender of the message or (2) an attacker that has compromised
the key of the sender.
In addition to the security considerations identified in [RFC 5280],
caution should be taken when processing certificates that have not
first been validated to a trust anchor. Certificates could be
manufactured by untrusted sources for the purpose of mounting denial-
of-service attacks or other attacks. For example, keys selected to
require excessive cryptographic processing, or extensive lists of CRL
Distribution Point (CDP) and/or Authority Information Access (AIA)
addresses in the certificate, could be used to mount denial-of-
service attacks. Similarly, attacker-specified CDP and/or AIA
addresses could be included in fake certificates to allow the
originator to detect receipt of the message even if signature
verification fails.
RSA 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 sign certificates or CRLs. Such keys
were previously considered secure, so processing previously received
signed and encrypted mail may require processing certificates or CRLs
signed with 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 accepting certificates and CRLs with
smaller key sizes (e.g., spoofed certificates) versus the costs of
Schaad, et al. Standards Track PAGE 19
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denial of service. If an implementation supports verification of
certificates or CRLs generated with RSA and DSA keys of less than
2048 bits, it MUST warn the user. Implementers should consider
providing a stronger warning for weak signatures on certificates and
CRLs associated with newly received messages than the one provided
for certificates and CRLs associated with previously stored messages.
Server implementations (e.g., secure mail list servers) where user
warnings are not appropriate SHOULD reject messages with weak
cryptography.
If an implementation is concerned about compliance with National
Institute of Standards and Technology (NIST) key size
recommendations, then see [SP800-57].
7. References
7.1. Reference Conventions
[ESS] refers to [RFC 2634] and [RFC 5035].
[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].
[SMIMEv3.1] refers to [RFC 2634], [RFC 3850], [RFC 3851], [RFC 3852],
and [RFC 5035].
[SMIMEv3.2] refers to [RFC 2634], [RFC 5035], [RFC 5652], [RFC 5750],
and [RFC 5751].
[SMIMEv4] refers to [RFC 2634], [RFC 5035], [RFC 5652], [RFC 8551], and
this document.
7.2. Normative References
[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>.
Schaad, et al. Standards Track PAGE 20
RFC 8550 S/MIME 4.0 Certificate Handling April 2019
[FIPS186-3]
National Institute of Standards and Technology (NIST),
"Digital Signature Standard (DSS)", Federal Information
Processing Standards Publication 186-3, June 2009,
<https://csrc.nist.gov/csrc/media/publications/fips/186/3/
archive/2009-06-25/documents/fips_186-3.pdf>.
[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 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 2985] Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object
Classes and Attribute Types Version 2.0", RFC 2985,
DOI 10.17487/RFC 2985, November 2000,
<https://www.rfc-editor.org/info/RFC 2985>.
[RFC 3279] Bassham, L., Polk, W., and R. Housley, "Algorithms and
Identifiers for the Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 3279, DOI 10.17487/RFC 3279, April
2002, <https://www.rfc-editor.org/info/RFC 3279>.
[RFC 3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, DOI 10.17487/RFC 3447, February
2003, <https://www.rfc-editor.org/info/RFC 3447>.
[RFC 4055] Schaad, J., Kaliski, B., and R. Housley, "Additional
Algorithms and Identifiers for RSA Cryptography for use in
the Internet X.509 Public Key Infrastructure Certificate
and Certificate Revocation List (CRL) Profile", RFC 4055,
DOI 10.17487/RFC 4055, June 2005,
<https://www.rfc-editor.org/info/RFC 4055>.
[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 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>.
Schaad, et al. Standards Track PAGE 21
RFC 8550 S/MIME 4.0 Certificate Handling April 2019
[RFC 5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC 5280, May 2008,
<https://www.rfc-editor.org/info/RFC 5280>.
[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 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>.
[RFC 5755] Farrell, S., Housley, R., and S. Turner, "An Internet
Attribute Certificate Profile for Authorization",
RFC 5755, DOI 10.17487/RFC 5755, January 2010,
<https://www.rfc-editor.org/info/RFC 5755>.
[RFC 5758] Dang, Q., Santesson, S., Moriarty, K., Brown, D., and T.
Polk, "Internet X.509 Public Key Infrastructure:
Additional Algorithms and Identifiers for DSA and ECDSA",
RFC 5758, DOI 10.17487/RFC 5758, January 2010,
<https://www.rfc-editor.org/info/RFC 5758>.
[RFC 6979] Pornin, T., "Deterministic Usage of the Digital Signature
Algorithm (DSA) and Elliptic Curve Digital Signature
Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC 6979, August
2013, <https://www.rfc-editor.org/info/RFC 6979>.
[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 8398] Melnikov, A., Ed. and W. Chuang, Ed., "Internationalized
Email Addresses in X.509 Certificates", RFC 8398,
DOI 10.17487/RFC 8398, May 2018,
<https://www.rfc-editor.org/info/RFC 8398>.
[RFC 8551] Schaad, J., Ramsdell, B., and S. Turner,
"Secure/Multipurpose Internet Mail Extensions (S/MIME)
Version 4.0 Message Specification", RFC 8551,
DOI 10.17487/RFC 8551, April 2019,
<https://www.rfc-editor.org/info/RFC 8551>.
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RFC 8550 S/MIME 4.0 Certificate Handling 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>.
7.3 Informative References
[PKCS6] RSA Laboratories, "PKCS #6: Extended-Certificate Syntax
Standard", November 1993.
[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>.
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RFC 8550 S/MIME 4.0 Certificate Handling April 2019
[RFC 3114] Nicolls, W., "Implementing Company Classification Policy
with the S/MIME Security Label", RFC 3114,
DOI 10.17487/RFC 3114, May 2002,
<https://www.rfc-editor.org/info/RFC 3114>.
[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 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 6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
Curve Cryptography Algorithms", RFC 6090,
DOI 10.17487/RFC 6090, February 2011,
<https://www.rfc-editor.org/info/RFC 6090>.
[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 8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
Signature Algorithm (EdDSA)", RFC 8032,
DOI 10.17487/RFC 8032, January 2017,
<https://www.rfc-editor.org/info/RFC 8032>.
[RFC 8162] Hoffman, P. and J. Schlyter, "Using Secure DNS to
Associate Certificates with Domain Names for S/MIME",
RFC 8162, DOI 10.17487/RFC 8162, May 2017,
<https://www.rfc-editor.org/info/RFC 8162>.
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[RFC 8410] Josefsson, S. and J. Schaad, "Algorithm Identifiers for
Ed25519, Ed448, X25519, and X448 for Use in the Internet
X.509 Public Key Infrastructure", RFC 8410,
DOI 10.17487/RFC 8410, August 2018,
<https://www.rfc-editor.org/info/RFC 8410>.
[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>.
[X.500] "Information technology - Open Systems Interconnection -
The Directory - Part 1: Overview of concepts, models and
services", ITU-T Recommendation X.500,
ISO/IEC 9594-1:2017.
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Appendix A. Historic Considerations
A.1. Signature Algorithms and Key Sizes
There are a number of problems with validating certificates on
sufficiently historic messages. For this reason, it is strongly
suggested that user agents treat these certificates differently from
those on current messages. These problems include the following:
- CAs 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.
- 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 certificates 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.
[SMIMEv3.2] set the lower limit on suggested key sizes for
creating and validation at 1024 bits. [SMIMEv3.1] set the lower
limit at 768 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 level of any signature that is created with these hash
algorithms should also be considered as suspect.
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].
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- 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 support was dropped in [SMIMEv4]. DSA was
dropped as part of a general movement from finite fields to
elliptic curves. Issues related to dealing with non-deterministic
generation of the parameter 'k' have come up (see [RFC 6979]).
For 512-bit RSA with SHA-1, see [RFC 3279] and [FIPS186-2] without
Change Notice 1; for 512-bit RSA with SHA-256, see [RFC 4055] and
[FIPS186-2] without 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 3279] and [FIPS186-2] without
Change Notice 1; for 512-bit DSA with SHA-256, see [RFC 5758] and
[FIPS186-2] without Change Notice 1; for 1024-bit DSA with SHA-1, see
[RFC 3279] and [FIPS186-2] with Change Notice 1; and for 1024-bit
through 3072-bit DSA with SHA-256, see [RFC 5758] and [FIPS186-3].
The first reference provides the signature algorithm's OID, and the
second provides the signature algorithm's definition.
Appendix B. Moving S/MIME v2 Certificate Handling to Historic Status
The S/MIME v3 [SMIMEv3], v3.1 [SMIMEv3.1], v3.2 [SMIMEv3.2], and v4.0
(this document) specifications are backward compatible with the
S/MIME v2 Certificate Handling Specification [SMIMEv2], with the
exception of the algorithms (dropped RC2/40 requirement, and added
DSA and RSASSA-PSS requirements). Therefore, RFC 2312 [SMIMEv2] was
moved to Historic status.
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RFC 8550 S/MIME 4.0 Certificate Handling April 2019
Acknowledgements
Many thanks go out to the other authors of the S/MIME v2 Certificate
Handling RFC: Steve Dusse, Paul Hoffman, and Jeff Weinstein. Without
v2, there wouldn't be a v3, v3.1, v3.2, or v4.0.
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.
Bill Flanigan, Trevor Freeman, Elliott Ginsburg, Alfred Hoenes, Paul
Hoffman, Russ Housley, David P. Kemp, Michael Myers, John Pawling,
and Denis Pinkas.
The version 4 update to the S/MIME documents was done under the
auspices of the LAMPS Working Group.
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 28
RFC TOTAL SIZE: 63210 bytes
PUBLICATION DATE: Wednesday, May 1st, 2019
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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