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IETF RFC 6010
Cryptographic Message Syntax (CMS) Content Constraints Extension
Last modified on Friday, September 17th, 2010
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Internet Engineering Task Force (IETF) R. Housley
Request for Comments: 6010 Vigil Security, LLC
Category: Standards Track S. Ashmore
ISSN: 2070-1721 National Security Agency
C. Wallace
Cygnacom Solutions
September 2010
Cryptographic Message Syntax (CMS) Content Constraints Extension
 Abstract
This document specifies the syntax and semantics for the
Cryptographic Message Syntax (CMS) content constraints extension.
This extension is used to determine whether a public key is
appropriate to use in the processing of a protected content. In
particular, the CMS content constraints extension is one part of the
authorization decision; it is used when validating a digital
signature on a CMS SignedData content or validating a message
authentication code (MAC) on a CMS AuthenticatedData content or CMS
AuthEnvelopedData content. The signed or authenticated content type
is identified by an ASN.1 object identifier, and this extension
indicates the content types that the public key is authorized to
validate. If the authorization check is successful, the CMS content
constraints extension also provides default values for absent
attributes.
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 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 6010.
Housley, et al. Standards Track PAGE 1 
RFC 6010 CMS Content Constraints September 2010
Copyright Notice
Copyright (c) 2010 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
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described in the Simplified BSD License.
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Contributions published or made publicly available before November
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Without obtaining an adequate license from the person(s) controlling
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not be created outside the IETF Standards Process, except to format
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than English.
Housley, et al. Standards Track PAGE 2
RFC 6010 CMS Content Constraints September 2010
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. CMS Data Structures . . . . . . . . . . . . . . . . . . . 5
1.2. CMS Content Constraints Model . . . . . . . . . . . . . . 10
1.3. Attribute Processing . . . . . . . . . . . . . . . . . . . 11
1.4. Abstract Syntax Notation . . . . . . . . . . . . . . . . . 13
1.5. Terminology . . . . . . . . . . . . . . . . . . . . . . . 13
2. CMS Content Constraints Extension . . . . . . . . . . . . . . 13
3. Certification Path Processing . . . . . . . . . . . . . . . . 16
3.1. Inputs . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.2. Initialization . . . . . . . . . . . . . . . . . . . . . . 18
3.3. Basic Certificate Processing . . . . . . . . . . . . . . . 19
3.4. Preparation for Certificate i+1 . . . . . . . . . . . . . 20
3.5. Wrap-Up Procedure . . . . . . . . . . . . . . . . . . . . 20
3.6. Outputs . . . . . . . . . . . . . . . . . . . . . . . . . 21
4. CMS Content Constraints Processing . . . . . . . . . . . . . . 21
4.1. CMS Processing and CCC Information Collection . . . . . . 22
4.1.1. Collection of Signer or Originator Information . . . . 24
4.1.2. Collection of Attributes . . . . . . . . . . . . . . . 25
4.1.3. Leaf Node Classification . . . . . . . . . . . . . . . 25
4.2. Content Type and Constraint Checking . . . . . . . . . . . 26
4.2.1. Inputs . . . . . . . . . . . . . . . . . . . . . . . . 27
4.2.2. Processing . . . . . . . . . . . . . . . . . . . . . . 27
4.2.3. Outputs . . . . . . . . . . . . . . . . . . . . . . . 27
5. Subordination Processing in TAMP . . . . . . . . . . . . . . . 28
6. Security Considerations . . . . . . . . . . . . . . . . . . . 29
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 32
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 33
8.1. Normative References . . . . . . . . . . . . . . . . . . . 33
8.2. Informative References . . . . . . . . . . . . . . . . . . 34
Appendix A. ASN.1 Modules . . . . . . . . . . . . . . . . . . . . 35
A.1. ASN.1 Module Using 1993 Syntax . . . . . . . . . . . . . . 35
A.2. ASN.1 Module Using 1988 Syntax . . . . . . . . . . . . . . 37
Housley, et al. Standards Track PAGE 3
RFC 6010 CMS Content Constraints September 2010
1. Introduction
The Cryptographic Message Syntax (CMS) SignedData [RFC 5652] construct
is used to sign many things, including cryptographic module firmware
packages [RFC 4108] and certificate management messages [RFC 5272].
Similarly, the CMS AuthenticatedData and CMS AuthEnvelopedData
constructs provide authentication, which can be affiliated with an
originator's static public key. CMS Content Constraints (CCC)
information is conveyed via an extension in a certificate or trust
anchor object that contains the originator's or signer's public key.
This document assumes a particular authorization model, where each
originator is associated with one or more authorized content types.
A CMS SignedData, AuthenticatedData, or AuthEnvelopedData will be
considered valid only if the signature or message authentication code
(MAC) verification process is successful and the originator is
authorized for the encapsulated content type. For example, one
originator might be acceptable for verifying signatures on firmware
packages, but that same originator may be unacceptable for verifying
signatures on certificate management messages.
An originator's constraints are derived from the certification path
used to validate the originator's public key. Constraints are
associated with trust anchors [RFC 5914], and constraints are
optionally included in public key certificates [RFC 5280]. Using the
CMS Content Constraints (CCC) extension, a trust anchor lists the
content types for which it may be used. A trust anchor may also
include further constraints associated with each of the content
types. Certificates in a certification path may contain a CCC
extension that further constrains the authorization for subordinate
certificates in the certification path.
Delegation of authorizations is accomplished using the CCC
certificate extension. An entity may delegate none, some, or all of
its authorizations to another entity by issuing it a certificate with
an appropriate CCC extension. Absence of a CCC certificate extension
in a certificate means that the subject is not authorized for any
content type. If the entity is an end entity, it may perform CCC
delegation, i.e., through the use of proxy certificates. However,
usage of proxy certificates is not described in this specification.
While processing the certification path, relying parties MUST ensure
that authorizations of a subject of a certificate are constrained by
the authorizations of the issuer of that certificate. In other
words, when a content signature or MAC is validated, checks MUST be
performed to ensure that the encapsulated content type is within the
permitted set for the trust anchor (TA) and each certificate in the
Housley, et al. Standards Track PAGE 4
RFC 6010 CMS Content Constraints September 2010
path and that the constraints associated with the specific content
type, if any, are satisfied by the TA and each certificate in the
path.
Additionally, this document provides subordination rules for
processing CCC extensions within the Trust Anchor Management Protocol
(TAMP) and relies on vocabulary from that document [RFC 5934].
1.1. CMS Data Structures
CMS encapsulation can be used to compose structures of arbitrary
breadth and depth. This is achieved using a variety of content types
that achieve different compositional goals. A content type is an
arbitrary structure that is identified using an object identifier.
This document defines two categories of content types: intermediate
content types and leaf content types. Intermediate content types are
those designed specifically to encapsulate one or more additional
content types with the addition of some service (such as a
signature). Leaf content types are those designed to carry specific
information. (Leaf content types may contain other content types.)
CCC is not used to constrain MIME encapsulated data, i.e., CCC
processing stops when a MIME encapsulation layer is encountered.
SignedData [RFC 5652] and ContentCollection [RFC 4073] are examples of
intermediate content types. FirmwarePkgData [RFC 4108] and TSTInfo
[RFC 3161] are examples of leaf content types. Protocol designers may
provide an indication regarding the classification of content types
within the protocol. Four documents define the primary intermediate
content types:
RFC 5652 [RFC 5652]: Cryptographic Message Syntax (CMS)
- SignedData
- EnvelopedData
- EncryptedData
- DigestedData
- AuthenticatedData
RFC 5083 [RFC 5083]: The Cryptographic Message Syntax (CMS)
AuthEnvelopedData Content Type
- AuthEnvelopedData
Housley, et al. Standards Track PAGE 5
RFC 6010 CMS Content Constraints September 2010
RFC 4073 [RFC 4073]: Protecting Multiple Contents with the
Cryptographic Message Syntax (CMS)
- ContentCollection
- ContentWithAttributes
RFC 3274 [RFC 3274]: Compressed Data Content Type for Cryptographic
Message Syntax (CMS)
- CompressedData
Some intermediate nodes can also function as leaf nodes in some
situations. EncryptedData, EnvelopedData, and AuthEnvelopedData
nodes will function as intermediate nodes for recipients that can
decrypt the content and as encrypted leaf nodes for recipients who
cannot decrypt the content.
When using CMS, the outermost structure is always ContentInfo.
ContentInfo consists of an object identifier and an associated
content. The object identifier describes the structure of the
content. Object identifiers are used throughout the CMS family of
specifications to identify structures.
Using the content types listed above, ignoring for the moment
ContentCollection, encapsulation can be used to create structures of
arbitrary depth. Two examples based on [RFC 4108] are shown in Figure
1 and Figure 2.
When ContentCollection is used in conjunction with the other content
types, tree-like structures can be defined, as shown in Figure 3.
The examples in Figures 1, 2, and 3 can each be represented as a
tree: the root node is the outermost ContentInfo, and the leaf nodes
are the encapsulated contents. The trees are shown in Figure 4.
Housley, et al. Standards Track PAGE 6
RFC 6010 CMS Content Constraints September 2010
+---------------------------------------------------------+
| ContentInfo |
| |
| +-----------------------------------------------------+ |
| | SignedData | |
| | | |
| | +-------------------------------------------------+ | |
| | | FirmwarePackage | | |
| | | | | |
| | | | | |
| | +-------------------------------------------------+ | |
| +-----------------------------------------------------+ |
+---------------------------------------------------------+
Figure 1. Example of a Signed Firmware Package
+---------------------------------------------------------+
| ContentInfo |
| |
| +-----------------------------------------------------+ |
| | SignedData | |
| | | |
| | +-------------------------------------------------+ | |
| | | EncryptedData | | |
| | | | | |
| | | +---------------------------------------------+ | | |
| | | | FirmwarePackage | | | |
| | | | | | | |
| | | | | | | |
| | | +---------------------------------------------+ | | |
| | +-------------------------------------------------+ | |
| +-----------------------------------------------------+ |
+---------------------------------------------------------+
Figure 2. Example of a Signed and Encrypted Firmware Package
Housley, et al. Standards Track PAGE 7
RFC 6010 CMS Content Constraints September 2010
+---------------------------------------------------------+
| ContentInfo |
| |
| +-----------------------------------------------------+ |
| | SignedData | |
| | | |
| | +-------------------------------------------------+ | |
| | | ContentCollection | | |
| | | | | |
| | | +----------------------+ +--------------------+ | | |
| | | | SignedData | | SignedData | | | |
| | | | | | | | | |
| | | | +------------------+ | | +----------------+ | | | |
| | | | | EncryptedData | | | | Firmware | | | | |
| | | | | | | | | Package | | | | |
| | | | | +--------------+ | | | | | | | | |
| | | | | | Firmware | | | | +----------------+ | | | |
| | | | | | Package | | | +--------------------+ | | |
| | | | | | | | | | | |
| | | | | +--------------+ | | | | |
| | | | +------------------+ | | | |
| | | +----------------------+ | | |
| | +-------------------------------------------------+ | |
| +-----------------------------------------------------+ |
+---------------------------------------------------------+
Figure 3. Example of Two Firmware Packages in a Collection
Housley, et al. Standards Track PAGE 8
RFC 6010 CMS Content Constraints September 2010
+---------------------------------------------------------+
| |
| CMS PATH RESULTING CMS PATH RESULTING |
| FROM FIGURE 1. FROM FIGURE 2. |
| |
| ContentInfo ContentInfo |
| | | |
| V V |
| SignedData SignedData |
| | | |
| V V |
| FirmwarePackage EncryptedData |
| | |
| V |
| FirmwarePackage |
| |
| |
| CMS PATHS RESULTING FROM FIGURE 3. |
| |
| ContentInfo |
| | |
| V |
| SignedData |
| | |
| V |
| ContentCollection |
| | |
| +----------+--------------+ |
| | | |
| V V |
| SignedData SignedData |
| | | |
| V V |
| EncryptedData FirmwarePackage |
| | |
| V |
| FirmwarePackage |
| |
+---------------------------------------------------------+
Figure 4. Example CMS Path Structures
These examples do not illustrate all of the details of CMS
structures; most CMS protecting content types, and some leaf-node
content types, contain attributes. Attributes from intermediate
nodes can influence processing and handling of the CMS protecting
content type or the encapsulated content type. Attributes from leaf
nodes may be checked independent of the CCC processing, but such
Housley, et al. Standards Track PAGE 9
RFC 6010 CMS Content Constraints September 2010
processing is not addressed in this document. Throughout this
document, paths through the tree structure from a root node to a leaf
node in a CMS-protected message are referred to as CMS paths.
1.2. CMS Content Constraints Model
The CCC extension is used to restrict the types of content for which
a particular public key can be used to verify a signature or MAC.
Trust in a public key is established by building and validating a
certification path from a trust anchor to the subject public key.
Section 6 of [RFC 5280] describes the algorithm for certification path
validation, and the basic path validation algorithm is augmented, as
described in Section 3 of this document, to include processing
required to determine the CMS content constraints that have been
delegated to the subject public key. If the subject public key is
explicitly trusted (the public key belongs to a trust anchor), then
any CMS content constraints associated with the trust anchor are used
directly. If the subject public key is not explicitly trusted, then
the CMS content constraints are determined by calculating the
intersection of the CMS content constraints included in all the
certificates in a valid certification path from the trust anchor to
the subject public key, including those associated with the trust
anchor.
CMS enables the use of multiple nested signatures or MACs. Each
signature or MAC can protect and associate attributes with an
encapsulated data object. The CMS content constraints extension is
associated with a public key, and that public key is used to verify a
signature or a MAC.
The CMS content constraints mechanism can be used to place limits on
the use of the subject public key used for authentication or
signature verification for one or more specific content types.
Furthermore, within each permitted content type, a permitted set of
values can be expressed for one or more specific attribute types.
When a leaf content type is encapsulated by multiple intermediate
authentication layers, the signer or originator closest to a leaf
node must be authorized to serve as a source for the leaf content
type; outer signers or originators need not be authorized to serve as
a source, but must be authorized for the leaf content type. All
signers or originators must be authorized for the attributes that
appear in a CMS path.
A signer or originator may be constrained to use a specific set of
attribute values for some attribute types when producing a particular
content type. If a signer or originator is constrained for a
particular attribute that does not appear in a protected content of
Housley, et al. Standards Track PAGE 10
RFC 6010 CMS Content Constraints September 2010
the type for which the constraint is defined, the constraint serves
as a default attribute, i.e., the payload should be processed as if
an attribute equal to the constraint appeared in the protected
content. However, in some cases, the processing rules for a
particular content type may disallow the usage of default values for
some attribute types and require a signer to explicitly assert the
attribute to satisfy the constraint. Signer constraints are output
for use in leaf node processing or other processing not addressed by
this specification.
Three models for processing attributes were considered:
o Each signer or originator must be authorized for attributes it
asserts.
o Each signer or originator must be authorized for attributes it
asserts and attributes contained in the content it authenticates.
o Each signer or originator must be authorized for attributes it
asserts, attributes contained in the content it authenticates, and
attributes contained in content that authenticates it, i.e., all
signers or originators must be authorized for all attributes
appearing in the CMS path.
The third model is used in this specification.
1.3. Attribute Processing
This specification defines a mechanism for enforcing constraints on
content types and attributes. Where content types are
straightforward to process because there is precisely one content
type of interest for a given CMS path, attributes are more
challenging. Attributes can be asserted at many different points in
a CMS path. Some attributes may, by their nature, be applicable to a
specific node of a CMS path; for example, ContentType and
MessageDigest attributes apply to a specific SignerInfo object.
Other attributes may apply to a less well-defined target; for
example, a ContentCollection may appear as the payload within a
ContentWithAttributes object.
Since there is no automated means of determining what an arbitrary
attribute applies to or how the attribute should be used, CCC
processing simply collects attributes and makes them available for
applications to use during leaf node processing. Implementations
SHOULD refrain from collecting attributes that are known to be
inapplicable to leaf node processing, for example, ContentType and
MessageDigest attributes.
Housley, et al. Standards Track PAGE 11
RFC 6010 CMS Content Constraints September 2010
Some attributes contain multiple values. Attribute constraints
expressed in a CCC extension may contain multiple values. Attributes
expressed in a constraint that do not appear in a CMS path are
returned as default attributes. Default attributes may have multiple
values. Attributes are returned to an application via two output
variables: cms_effective_attributes and cms_default_attributes. An
attribute may be absent, present with one value, or present with
multiple values in a CMS path and/or in CMS content constraints. A
summary of the resulting nine possible combinations is below.
Attribute absent in CMS path; absent in cms_constraints: no
action.
Attribute absent in CMS path; single value in cms_constraints: the
value from cms_constraints is added to cms_default_attributes.
Attribute absent in CMS path; multiple values in cms_constraints:
the values from cms_constraints are added to
cms_default_attributes.
Attribute is present with a single value in CMS path; absent in
cms_constraints: the value from CMS path is returned in
cms_effective_attributes.
Attribute is present with a single value in CMS path; single value
in cms_constraints: the value from CMS path must match the value
from cms_constraints. If successful match, the value is returned
in cms_effective_attribute. If no match, constraints processing
fails.
Attribute is present with a single value in CMS path; multiple
values in cms_constraints: the value from CMS path must match a
value from cms_constraints. If successful match, the value from
the CMS path is returned in cms_effective_attribute. If no match,
constraints processing fails.
Attribute is present with multiple values in CMS path; absent in
cms_constraints: the values from CMS path are returned in
cms_effective_attributes.
Attribute is present with multiple values; single value in
cms_constraints: the values from CMS path must match the value
from cms_constraints (i.e., all values must be identical). If
successful match, the values from the CMS path are returned in
cms_effective_attribute. If no match, constraints processing
fails.
Housley, et al. Standards Track PAGE 12
RFC 6010 CMS Content Constraints September 2010
Attribute is present with multiple values; multiple values in
cms_constraints: each value from CMS path must match a value from
cms_constraints. If each comparison is successful, the values
from the CMS path are returned in cms_effective_attribute. If a
comparison fails, constraints processing fails.
1.4. Abstract Syntax Notation
All X.509 certificate [RFC 5280] extensions are defined using ASN.1
[X.680][X.690].
CMS content types [RFC 5652] are also defined using ASN.1.
CMS uses the Attribute type. The syntax of Attribute is compatible
with X.501 [X.501].
1.5. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC 2119].
2. CMS Content Constraints Extension
The CMS content constraints extension provides a mechanism to
constrain authorization during delegation. If the CMS content
constraints extension is not present, then the subject of the trust
anchor or certificate is not authorized for any content type, with an
exception for apex trust anchors, which are implicitly authorized for
all content types. A certificate issuer may use the CMS content
constraints extension for one or more of the following purposes:
o Limit the certificate subject to a subset of the content types for
which the certificate issuer is authorized.
o Add constraints to a previously unconstrained content type.
o Add additional constraints to a previously constrained content
type.
The CMS content constraints extension MAY be critical, and it MUST
appear at most one time in a trust anchor or certificate. The CMS
content constraints extension is identified by the
id-pe-cmsContentConstraints object identifier:
id-pe-cmsContentConstraints OBJECT IDENTIFIER ::=
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) pe(1) 18 }
Housley, et al. Standards Track PAGE 13
RFC 6010 CMS Content Constraints September 2010
The syntax for the CMS content constraints extension is:
CMSContentConstraints ::= SEQUENCE SIZE (1..MAX) OF
ContentTypeConstraint
ContentTypeGeneration ::= ENUMERATED {
canSource(0),
cannotSource(1)}
ContentTypeConstraint ::= SEQUENCE {
contentType OBJECT IDENTIFIER,
canSource ContentTypeGeneration DEFAULT canSource,
attrConstraints AttrConstraintList OPTIONAL }
AttrConstraintList ::= SEQUENCE SIZE (1..MAX) OF AttrConstraint
AttrConstraint ::= SEQUENCE {
attrType AttributeType,
attrValues SET SIZE (1..MAX) OF AttributeValue }
id-ct-anyContentType OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16)
ct(1) 0 }
The CMSContentConstraints is a list of permitted content types and
associated constraints. A particular content type MUST NOT appear
more than once in a CMSContentConstraints. When the extension is
present, the certificate subject is being authorized by the
certificate issuer to sign or authenticate the content types in the
permitted list as long as the provided constraints, if any, are met.
The relying party MUST ensure that the certificate issuer is
authorized to delegate the privileges. When the extension is absent,
the certificate subject is not authorized for any content type.
The special id-ct-anyContentType value indicates the certificate
subject is being authorized for any content type without any
constraints. Where id-ct-anyContentType appears alongside a specific
content type, the specific content type is authoritative. The
id-ct-anyContentType object identifier can be used in trust anchors
when the trust anchor is unconstrained. Where id-ct-anyContentType
is asserted in the contentType field, the canSource field MUST be
equal to the canSource enumerated value and attrConstraints MUST be
absent, indicating that the trust anchor can serve as a source for
any content type without any constraints.
Housley, et al. Standards Track PAGE 14
RFC 6010 CMS Content Constraints September 2010
The fields of the ContentTypeConstraint type have the following
meanings:
contentType is an object identifier that specifies a permitted
content type. When the extension appears in an end entity
certificate, it indicates that a content of this type can be
verified using the public key in the certificate. When the
extension appears in a certification authority (CA) certificate,
it indicates that a content of this type can be verified using the
public key in the CA certificate or the public key in an
appropriately authorized subordinate certificate. For example,
this field contains id-ct-firmwarePackage when the public key can
be used to verify digital signatures on firmware packages defined
in [RFC 4108]. A particular content type MUST NOT appear more than
once in the list. Intermediate content types MUST NOT be included
in the list of permitted content types. Since the content type of
intermediate nodes is not subject to CMS Constraint Processing,
originators need not be authorized for intermediate node content
types. The intermediate content types are:
id-signedData,
id-envelopedData,
id-digestedData,
id-encryptedData,
id-ct-authEnvelopedData,
id-ct-authData,
id-ct-compressedData,
id-ct-contentCollection, and
id-ct-contentWithAttrs.
canSource is an enumerated value. If the canSource field is equal
to canSource, then the subject can be the innermost authenticator
of the specified content type. For a subject to be authorized to
source a content type, the issuer of the subject certificate MUST
also be authorized to source the content type. Regardless of the
flag value, a subject can sign or authenticate a content that is
already authenticated (when SignedData, AuthenticatedData, or
AuthEnvelopedData is already present).
Housley, et al. Standards Track PAGE 15
RFC 6010 CMS Content Constraints September 2010
attrConstraints is an optional field that contains constraints that
are specific to the content type. If the attrConstraints field is
absent, the public key can be used to verify the specified content
type without further checking. If the attrConstraints field is
present, then the public key can only be used to verify the
specified content type if all of the constraints are satisfied. A
particular constraint type, i.e., attrValues structure for a
particular attribute type, MUST NOT appear more than once in the
attrConstraints for a specified content type. Constraints are
checked by matching the values in the constraint against the
corresponding attribute value(s) in the CMS path. Constraints
processing fails if the attribute is present and the value is not
one of the values provided in the constraint. Constraint checking
is described fully in Section 4.
The fields of the AttrConstraint type have the following meanings:
attrType is an AttributeType, which is an object identifier that
names an attribute. For a content encapsulated in a CMS
SignedData, AuthenticatedData, or AuthEnvelopedData to satisfy
the constraint, if the attributes that are covered by the
signature or MAC include an attribute of the same type, then
the attribute value MUST be equal to one of the values supplied
in the attrValues field. Attributes that are not covered by
the signature or MAC are not checked against constraints.
Attribute types that do not appear as an AttrConstraint are
unconstrained, i.e., the signer or originator is free to assert
any value.
attrValues is a set of AttributeValue. The structure of each of
the values in attrValues is determined by attrType. Constraint
checking is described fully in Section 4.
3. Certification Path Processing
When CMS content constraints are used for authorization, the
processing described in this section SHOULD be included in the
certification path validation. The processing is presented as an
augmentation to the certification path validation algorithm described
in Section 6 of [RFC 5280], as shown in the figure below. Alternative
implementations are allowed but MUST yield the same results as
described below.
Housley, et al. Standards Track PAGE 16
RFC 6010 CMS Content Constraints September 2010
CCC-related inputs
+ inhibitAnyContentType flag
+ absenceEqualsUnconstrained flag
+ Trust anchor CCC extension
+ Content type of interest (cms_content_type)
+ Attributes of interest (cms_effective_attributes)
|
|
_______________V________________________
| |
| CCC-aware Certification Path Processor |
|________________________________________|
|
|
V
CCC-related outputs upon success
+ Applicable content type constraints (subject_constraints)
+ Constrained attributes not present in cms_effective_attributes
(subject_default_attributes)
+ Content types not propagated to end entity (excluded_content_types)
Figure 5. Certification Path Processing Inputs and Outputs
Certification path processing validates the binding between the
subject and subject public key. If a valid certification path cannot
be found, then the corresponding CMS path MUST be rejected.
3.1. Inputs
Two boolean values are provided as input: inhibitAnyContentType and
absenceEqualsUnconstrained.
The inhibitAnyContentType flag is used to govern processing of the
special id-ct-anyContentType value. When inhibitAnyContentType is
true, id-ct-anyContentType is not considered to match a content type.
When inhibitAnyContentType is false, id-ct-anyContentType is
considered to match any content type.
The absenceEqualsUnconstrained flag is used to govern the meaning of
CCC absence. When absenceEqualsUnconstrained is true, a trust anchor
without a CCC extension is considered to be unconstrained and a
certificate without a CCC extension is considered to have the same
CCC privileges as its issuer. When absenceEqualsUnconstrained is
false, a trust anchor or certificate without a CCC extension is not
authorized for any content types.
Neither of these flags has any bearing on an apex trust anchor, which
is always unconstrained by definition.
Housley, et al. Standards Track PAGE 17
RFC 6010 CMS Content Constraints September 2010
If a trust anchor used for path validation is authorized, then the
trust anchor MAY include a CCC extension. A trust anchor may be
constrained or unconstrained. If unconstrained, the trust anchor
MUST either include a CMS Content Constraints extension containing
the special id-ct-anyContentType value and inhibitAnyContentType is
false or the trust anchor MUST have no CCC extension and
absenceEqualsUnconstrained is true. If the trust anchor does not
contain a CMS Content Constraints structure and
absenceEqualsUnconstrained is false, the CMS content constraints
processing fails. If the trust anchor contains a CCC extension with
a single entry containing id-ct-anyContentType and
inhibitAnyContentType is true, the CMS content constraints processing
fails.
The content type of the protected content being verified can be
provided as input along with the set of attributes collected from the
CMS path in order to determine if the certification path is valid for
a given context. Alternatively, the id-ct-anyContentType value can
be provided as the content type input, along with an empty set of
attributes, to determine the full set of constraints associated with
a public key in the end entity certificate in the certification path
being validated.
Trust anchors may produce CMS-protected contents. When validating
messages originated by a trust anchor, certification path validation
as described in Section 6 of [RFC 5280] is not necessary, but
constraints processing MUST still be performed for the trust anchor.
In such cases, the initialization and wrap-up steps described below
can be performed to determine if the public key in the trust anchor
is appropriate to use in the processing of a protected content.
3.2. Initialization
Create an input variable named cms_content_type and set it equal to
the content type provided as input.
Create an input variable named cms_effective_attributes and set it
equal to the set of attributes provided as input.
Create a state variable named working_permitted_content_types. The
initial value of working_permitted_content_types is the permitted
content type list from the trust anchor, including any associated
constraints.
Create a state variable named excluded_content_types. The initial
value of excluded_content_types is empty.
Housley, et al. Standards Track PAGE 18
RFC 6010 CMS Content Constraints September 2010
Create a state variable of type SEQUENCE OF AttrConstraint named
subject_default_attributes and initialize it to empty.
Create a state variable of type SEQUENCE OF ContentTypeConstraint
named subject_constraints and initialize it to empty.
3.3. Basic Certificate Processing
If the CCC extension is not present in the certificate, check the
value of absenceEqualsUnconstrained. If false, set
working_permitted_content_types to empty. If true,
working_permitted_content_types is unchanged. In either case, no
further CCC processing is required for the certificate.
If inhibitAnyContenType is true, discard any entries in the CCC
extension with a content type value equal to id-ct-anyContentType.
For each entry in the permitted content type list sequence in the CMS
content constraints extension, the following steps are performed:
- If the entry contains the special id-ct-anyContentType value, skip
to the next entry.
- If the entry contains a content type that is present in
excluded_content_types, skip to the next entry.
- If the entry includes a content type that is not present in
working_permitted_content_types, determine if
working_permitted_content_types contains an entry equal to the
special id-ct-anyContentType value. If no, no action is taken and
working_permitted_content_types is unchanged. If yes, add the
entry to working_permitted_content_types.
- If the entry includes a content type that is already present in
working_permitted_content_types, then the constraints in the entry
can further reduce the authorization by adding constraints to
previously unconstrained attributes or by removing attribute
values from the attrValues set of a constrained attribute. The
canSource flag is set to cannotSource unless it is canSource in
the working_permitted_content_types entry and in the entry. The
processing actions to be performed for each constraint in the
AttrConstraintList follow:
-- If the constraint includes an attribute type that is not
present in the corresponding working_permitted_content_types
entry, add the attribute type and the associated set of
attribute values to working_permitted_content_types entry.
Housley, et al. Standards Track PAGE 19
RFC 6010 CMS Content Constraints September 2010
-- If the constraint includes an attribute type that is already
present in the corresponding working_permitted_content_types
entry, then compute the intersection of the set of attribute
values from the working_permitted_content_types entry and the
constraint. If the intersection contains at least one
attribute value, then the set of attribute values in
working_permitted_content_types entry is assigned the
intersection. If the intersection is empty, then the entry is
removed from working_permitted_content_types and the content
type from the entry is added to excluded_content_types.
Remove each entry in working_permitted_content_types that includes a
content type that is not present in the CMS content constraints
extension. For values other than id-ct-anyContentType, add the
removed content type to excluded_content_types.
3.4. Preparation for Certificate i+1
No additional action associated with the CMS content constraints
extension is taken during this phase of certification path validation
as described in Section 6 of [RFC 5280].
3.5. Wrap-Up Procedure
If cms_content_type equals the special value anyContentType, the CCC
processing portion of path validation succeeds. Set
subject_constraints equal to working_permitted_content_types. If
cms_content_type is not equal to the special value anyContentType,
perform the following steps:
- If cms_content_type is present in excluded_content_types, the CCC
processing portion of path validation fails.
- If working_permitted_content_types is equal to the special value
anyContentType, set subject_constraints equal to
working_permitted_content_types; the CCC processing portion of
path validation succeeds.
- If cms_content_type does not equal the content type of an entry in
working_permitted_content_types, constraints processing fails and
path validation fails.
Housley, et al. Standards Track PAGE 20
RFC 6010 CMS Content Constraints September 2010
- If cms_content_type equals the content type of an entry in
working_permitted_content_types, add the entry from
working_permitted_content_types to subject_constraints. If the
corresponding entry in working_permitted_content_types contains
the special value anyContentType, set subject_constraints equal to
cms_content_type; the CCC processing portion of path validation
succeeds.
- If the attrConstraints field of the corresponding entry in
working_permitted_content_types is absent; the CCC processing
portion of path validation succeeds.
- If the attrConstraints field of the corresponding entry in
working_permitted_content_types is present, then the constraints
MUST be checked. For each attrType in the attrConstraints, the
constraint is satisfied if either the attribute type is absent
from cms_effective_attributes or each attribute value in the
attrValues field of the corresponding entry in
cms_effective_attributes is equal to one of the values for this
attribute type in the attrConstraints field. If
cms_effective_attributes does not contain an attribute of that
type, then the entry from attrConstraints is added to the
subject_default_attributes for use in processing the payload.
3.6. Outputs
If certification path validation processing succeeds, return the
value of the subject_constraints, subject_default_attributes, and
excluded_content_types variables.
4. CMS Content Constraints Processing
CMS contents constraints processing is performed on a per-CMS-path
basis. The processing consists of traditional CMS processing
augmented by collection of information required to perform content
type and constraint checking. Content type and constraint checking
uses the collected information to build and validate a certification
path to each public key used to authenticate nodes in the CMS path
per the certification path processing steps described above.
Housley, et al. Standards Track PAGE 21
RFC 6010 CMS Content Constraints September 2010
4.1. CMS Processing and CCC Information Collection
Traditional CMS content processing is augmented by the following
three steps to support enforcement of CMS content constraints:
Collection of signer or originator keys
Collection of attributes
Leaf node classification
CMS processing and CCC information collection takes a CMS path as
input and returns a collection of public keys used to authenticate
protected content, a collection of authenticated attributes, and the
leaf node, as shown in the figure below.
Inputs
+ CMS path
|
|
_________V___________________
| |
| CMS processing and CCC |
| information collection |
|_____________________________|
|
|
V
Outputs upon success
+ Leaf node
+ Public keys used to authenticate content (cms_public_keys)
+ Authenticated attributes (cms_effective_attributes)
Figure 6. CMS Processing and CCC Information Collection
Processing is performed for each CMS path from the root node of a
CMS-protected content to a leaf node, proceeding from the root node
to the leaf node. Each path is processed independently of the other
paths. Thus, it is possible that some leaf nodes in a content
collection may be acceptable while other nodes are not acceptable.
The processing described in this section applies to CMS paths that
contain at least one SignedData, AuthEnvelopedData, or
AuthenticatedData node. Since countersignatures are defined as not
having a content, CMS content constraints are not used with
countersignatures.
Housley, et al. Standards Track PAGE 22
RFC 6010 CMS Content Constraints September 2010
Signer or originator public keys are collected when verifying
signatures or message authentication codes (MACs). These keys will
be used to determine the constraints of each signer or originator by
building and validating a certification path to the public key.
Public key values, public key certificates, or public key identifiers
are accumulated in a state variable named cms_public_keys, which is
either initialized to empty or to an application-provided set of keys
when processing begins. The variable will be updated each time a
SignedData, AuthEnvelopedData, or AuthenticatedData node is
encountered in the CMS path.
All authenticated attributes appearing in a CMS path are collected,
beginning with the attributes protected by the outermost SignedData,
AuthEnvelopedData, or AuthenticatedData and proceeding to the leaf
node. During processing, attributes collected from the nodes in the
CMS path are maintained in a state variable named
cms_effective_attributes, and default attributes derived from message
originator authorizations are collected in a state variable named
cms_default_attributes. A default attribute value comes from a
constraint that does not correspond to an attribute contained in the
CMS path and may be used during payload processing in lieu of an
explicitly included attribute. This prevents an originator from
avoiding a constraint through omission. When processing begins,
cms_effective_attributes and cms_default_attributes are initialized
to empty. Alternatively, cms_effective_attributes may be initialized
to an application-provided sequence of attributes. The
cms_effective_attributes value will be updated each time an attribute
set is encountered in a SignedData, AuthEnvelopedData,
AuthenticatedData, or (authenticated) ContentWithAttributes node
while processing a CMS path.
The output of content type and constraint checking always includes a
set of attributes collected from the various nodes in a CMS path.
When processing terminates at an encrypted node, the set of signer or
originator public keys is also returned. When processing terminates
at a leaf node, a set of default attribute values is also returned
along with a set of constraints that apply to the CMS-protected
content.
The output from CMS Content Constraints processing will depend on the
type of the leaf node that terminates the CMS path. Four different
output variables are possible. The conditions under which each is
returned is described in the following sections. The variables are:
Housley, et al. Standards Track PAGE 23
RFC 6010 CMS Content Constraints September 2010
cms_public_keys is a list of public key values, public key
certificates, or public key identifiers. Information maintained
in cms_public_keys will be used to perform the certification path
operations required to determine if a particular signer or
originator is authorized to produce a specific object.
cms_effective_attributes contains the attributes collected from the
nodes in a CMS path. cms_effective_attributes is a SEQUENCE OF
Attribute, which is the same as the AttrConstraintList structure
except that it may have zero entries in the sequence. An
attribute can occur multiple times in the cms_effective_attribute
set, potentially with different values.
cms_default_attributes contains default attributes derived from
message signer or originator authorizations. A default attribute
value is taken from a constraint that does not correspond to an
attribute contained in the CMS path. cms_default_attributes is a
SEQUENCE OF Attribute, which is the same as the AttrConstraintList
structure except that it may have zero entries in the sequence.
cms_constraints contains the constraints associated with the message
signer or originator for the content type of the leaf node.
cms_constraints is a SEQUENCE OF Attribute, which is the same as
the AttrConstraintList structure except that it may have zero
entries in the sequence.
4.1.1. Collection of Signer or Originator Information
Signer or originator constraints are identified using the public keys
to verify each SignedData, AuthEnvelopedData, or AuthenticatedData
layer encountered in a CMS path. The public key value, public key
certificate, or public key identifier of each signer or originator
are collected in a state variable named cms_public_keys. Constraints
are determined by building and validating a certification path for
each public key after the content type and attributes of the CMS-
protected object have been identified. If the CMS path has no
SignedData, AuthEnvelopedData, or AuthenticatedData nodes, CCC
processing succeeds and all output variables are set to empty.
The signature or MAC generated by the originator MUST be verified.
If signature or MAC verification fails, then the CMS path containing
the signature or MAC MUST be rejected. Signature and MAC
verification procedures are defined in [RFC 5652] [RFC 5083]. The
public key or public key certificate used to verify each signature or
MAC in a CMS path is added to the cms_public_keys state variable for
use in content type and constraint checking. Additional checks may
be performed during this step, such as timestamp verification
[RFC 3161] and ESSCertId [RFC 5035] processing.
Housley, et al. Standards Track PAGE 24
RFC 6010 CMS Content Constraints September 2010
4.1.1.1. Handling Multiple SignerInfo Elements
CMS content constraints MAY be applied to CMS-protected contents
featuring multiple parallel signers, i.e., SignedData contents
containing more than one SignerInfo. When multiple SignerInfo
elements are present, each may represent a distinct entity or each
may represent the same entity via different keys or certificates,
e.g., in the event of key rollover or when the entity has been issued
certificates from multiple organizations. For simplicity, signers
represented by multiple SignerInfos within a single SignedData are
not considered to be collaborating with regard to a particular
content, unlike signers represented in distinct SignedData contents.
Thus, for the purposes of CCC processing, each SignerInfo is treated
as if it were the only SignerInfo. A content is considered valid if
there is at least one valid CMS path employing one SignerInfo within
each SignedData content. Where collaboration is desired, usage of
multiple SignedData contents is RECOMMENDED.
Though not required by this specification, some applications may
require successful processing of all or multiple SignerInfo elements
within a single SignedData content. There are a number of potential
ways of treating the evaluation process, including the following two
possibilities:
- All signatures are meant to be collaborative: In this case, the
public keys associated with each SignerInfo are added to the
cms_public_keys variable, the attributes from each SignerInfo are
added to the cms_effective_attributes variable, and normal
processing is performed.
- All signatures are meant to be completely independent: In this
case, each of the SignerInfos is processed as if it were a fork in
the CMS path construction process. The application may require
more than one CMS path to be valid in order to accept a content.
The exact processing will be a matter of application and local
policy. See [RFC 5752] for an example of an attribute that requires
processing multiple SignerInfo elements within a SignedData content.
4.1.2. Collection of Attributes
Attributes are collected from all authenticated nodes in a CMS path.
That is, attributes are not collected from content types that are
unauthenticated, i.e., those that are not covered by a SignedData,
AuthEnvelopedData, or AuthenticatedData layer. Additionally, an
application MAY specify a set of attributes that it has
authenticated, perhaps from processing one or more content types that
encapsulate a CMS-protected content. Leaf node attributes MAY be
Housley, et al. Standards Track PAGE 25
RFC 6010 CMS Content Constraints September 2010
checked independent of the CCC processing, but such processing is not
addressed in this document. Applications are free to perform further
processing using all or some of the attributes returned from CCC
processing.
4.1.3. Leaf Node Classification
The type of leaf node that terminates a CMS path determines the types
of information that are returned and the type of processing that is
performed. There are two types of leaf nodes: encrypted leaf nodes
and payload leaf nodes.
A node in a CMS path is a leaf node if the content type of the node
is not one of the following content types:
id-signedData (SignedData),
id-digestedData (DigestedData),
id-ct-authData (AuthenticatedData),
id-ct-compressedData (CompressedData),
id-ct-contentCollection (ContentCollection), or
id-ct-contentWithAttrs (ContentWithAttributes).
A leaf node is an encrypted leaf node if the content type of the node
is one of the following content types:
id-encryptedData (EncryptedData),
id-envelopedData (EnvelopedData), or
id-ct-authEnvelopedData (AuthEnvelopedData).
All other leaf nodes are payload leaf nodes, since no further CMS
encapsulation can occur beyond that node. However, specifications
may define content types that provide protection similar to the CMS
content types, may augment the lists of possible leaf and encrypted
leaf nodes, or may define some encrypted types as payload leaf nodes.
When an encrypted leaf node is encountered, processing terminates and
returns information that may be used as input when processing the
decrypted contents. Content type and constraints checking are only
performed for payload leaf nodes. When an encrypted leaf node
terminates a CMS path, the attributes collected in
cms_effective_attributes are returned along with the public key
Housley, et al. Standards Track PAGE 26
RFC 6010 CMS Content Constraints September 2010
information collected in cms_public_keys. When a payload leaf node
terminates a CMS path, content type and constraint checking MUST be
performed, as described in the next section.
4.2. Content Type and Constraint Checking
Content type and constraint checking is performed when a payload leaf
node is encountered. This section does not apply to CMS paths that
are terminated by an encrypted leaf node nor to CMS paths that have
no SignedData, AuthEnvelopedData, or AuthenticatedData nodes.
4.2.1. Inputs
The inputs to content type and constraint checking are the values
collected in cms_public_keys and cms_effective_attributes from a CMS
path, along with the payload leaf node that terminates the CMS path,
as shown in the figure below.
Inputs
+ leaf node
+ cms_public_keys
+ cms_effective_attributes
|
|
________________V_________________________________________
| |
| Content type and constraint checking |
| (uses CCC-aware Certification Path Processor internally)|
|__________________________________________________________|
|
|
V
Outputs upon success
+ cms_constraints
+ cms_default_attributes
+ cms_effective_attributes
Figure 7. Content Type and Constraint Checking
4.2.2. Processing
When a payload leaf node is encountered in a CMS path and a signed or
authenticated content type is present in the CMS path, content type
and constraint checking MUST be performed. Content type and
constraint checking need not be performed for CMS paths that do not
contain at least one SignedData, AuthEnvelopedData, or
AuthenticatedData content type. The cms_effective_attributes and
cms_public_keys variables are used to perform constraint checking.
Housley, et al. Standards Track PAGE 27
RFC 6010 CMS Content Constraints September 2010
Two additional state variables are used during the processing:
cms_constraints and cms_default_attributes, both of which are
initialized to empty. The steps required to perform content type and
constraint checking are below.
For each public key in cms_public_keys, build and validate a
certification path from a trust anchor to the public key, providing
the content type of the payload leaf node and
cms_effective_attributes as input. Observe any limitations imposed
by intermediate layers. For example, if the SigningCertificateV2
[RFC 5035] attribute is used, the certificate identified by the
attribute is required to serve as the target certificate.
o If path validation is successful, add the contents of
subject_default_attributes to cms_default_attributes. The
subject_constraints variable returned from certification path
validation will contain a single entry. If the
subject_constraints entry is equal to the special value
anyContentType, content type and constraints checking succeeds.
If the subject_constraints entry is not equal to the special value
anyContentType, for each entry in the attrConstraints field of the
entry in subject_constraints,
* If there is an entry in cms_constraints with the same attrType
value, add the value from the attrValues entry to the entry in
cms_constraints if that value does not already appear.
* If there is no entry in cms_constraints with the same attrType
value, add a new entry to cms_constraints equal to the entry
from the attrConstraints field.
o If the value of the canSource field of the entry in the
subject_constraints variable for the public key used to verify the
signature or MAC closest to the payload leaf node is set to
cannotSource, constraints checking fails and the CMS path MUST be
rejected.
If no valid certification path can be found, constraints checking
fails and the CMS path MUST be rejected.
4.2.3. Outputs
When a payload leaf node is encountered and content type and
constraint checking succeeds, return cms_constraints,
cms_default_attributes, and cms_effective_attributes for use in leaf
node payload processing.
Housley, et al. Standards Track PAGE 28
RFC 6010 CMS Content Constraints September 2010
When an encrypted leaf node is encountered and constraint checking is
not performed, return cms_public_keys and cms_effective_attributes
for use in continued processing (as described in Section 4.2.1).
The cms_effective_attributes list may contain multiple instances of
the same attribute type. An instance of an attribute may contain
multiple values. Leaf node processing, which might take advantage of
these effective attributes, needs to describe the proper handling of
this situation. Leaf node processing is described in other
documents, and it is expected to be specific to a particular content
type.
The cms_default_attributes list may contain attributes with multiple
values. Payload processing, which might take advantage of these
default attributes, needs to describe the proper handling of this
situation. Payload processing is described in other documents, and
it is expected to be specific to a particular content type.
5. Subordination Processing in TAMP
TAMP [RFC 5934] does not define an authorization mechanism. CCC can
be used to authorize TAMP message signers and to delegate TAMP
message-signing authority. TAMP requires trust anchors managed by a
TAMP message signer to be subordinate to the signer. This section
describes subordination processing for CCC extensions of trust
anchors contained in a TrustAnchorUpdate message where CCC is used to
authorize TAMP messages.
For a Trust Anchor Update message that is not signed with the apex
trust anchor operational public key to be valid, the digital
signature MUST be validated using a management trust anchor
associated with the id-ct-TAMP-update content type, either directly
or via an X.509 certification path originating with an authorized
trust anchor. The following subordination checks MUST also be
performed as part of validation.
Each Trust Anchor Update message contains one or more individual
updates, each of which is used to add, modify, or remove a trust
anchor. For each individual update, the constraints of the TAMP
message signer MUST be greater than or equal to the constraints of
the trust anchor in the update. The constraints of the TAMP message
signer and the to-be-updated trust anchor are determined based on the
applicable CMS Content Constraints. Specifically, the constraints of
the TAMP message signer are determined as described in Section 3,
passing the special value id-ct-anyContentType and an empty set of
attributes as input; the constraints of the to-be-updated trust
anchor are determined as described below. If the constraints of a
trust anchor in an update exceed the constraints of the signer, that
Housley, et al. Standards Track PAGE 29
RFC 6010 CMS Content Constraints September 2010
update MUST be rejected. Each update is considered and accepted or
rejected individually without regard to other updates in the TAMP
message. The constraints of the to-be-updated trust anchors are
determined as follows:
o If the to-be-updated trust anchor is the subject of an add
operation, the constraints are read from the CMSContentConstraints
extension of the corresponding trust anchor in the update.
o If the to-be-updated trust anchor is the subject of a remove
operation, the trust anchor is located in the message recipient's
trust anchor store using the public key included in the update.
o If the to-be-updated trust anchor is the subject of a change
operation, the trust anchor has two distinct sets of constraints
that MUST be checked. The trust anchor's pre-change constraints
are determined by locating the trust anchor in the message
recipient's trust anchor store using the public key included in
the update and reading the constraints from the
CMSContentConstraints extension in the trust anchor. The trust
anchor's post-change constraints are read from the
CMSContentConstraints extension of the corresponding
TBSCertificateChangeInfo or the TrustAnchorChangeInfo in the
update. If the CMSContentConstraints extension is not present,
then the trust anchor's post-change constraints are equivalent to
the trust anchor's pre-change constraints.
The following steps can be used to determine if a Trust Anchor Update
message signer is authorized to manage each to-be-updated trust
anchor contained in a Trust Anchor Update message.
o The TAMP message signer's CMS Content Constraints are determined
as described in Section 3, passing the special value
id-ct-anyContentType and an empty set of attributes as input. The
message signer MUST be authorized for the Trust Anchor Update
message. This can be confirmed using the steps described in
Section 4.
o The constraints of each to-be-updated trust anchor in the TAMP
message MUST be checked against the message signer's constraints
(represented in the message signer's subject_constraints computed
above) using the following steps. For change operations, the
following steps MUST be performed for the trust anchor's pre-
change constraints and the trust anchor's post-change constraints.
* If the to-be-updated trust anchor is unconstrained, the message
signer MUST also be unconstrained, i.e., the message signer's
subject_constraints MUST be set to the special value
Housley, et al. Standards Track PAGE 30
RFC 6010 CMS Content Constraints September 2010
anyContentType. If the to-be-updated trust anchor is
unconstrained and the message signer is not, then the message
signer is not authorized to manage the trust anchor and the
update MUST be rejected.
* The message signer's authorization for each permitted content
type MUST be checked using the state variables and procedures
similar to those described in Sections 3.2 and 3.3. For each
permitted content type in the to-be-updated trust anchor's
constraints,
+ Set cms_effective_attributes equal to the value of the
attrConstraints field from the permitted content type.
+ If the content type does not match an entry in the message
signer's subject_constraints, the message signer is not
authorized to manage the trust anchor and the update MUST be
rejected. Note, the special value id-ct-anyContentType
produces a match for all content types that have the
resulting matching entry containing the content type,
canSource set to canSource, and attrConstraints absent.
+ If the content type matches an entry in the message signer's
subject_constraints, the canSource field of the entry is
cannotSource, and the canSource field in the to-be-updated
trust anchor's privilege is canSource, the message signer is
not authorized to manage the trust anchor and the update
MUST be rejected.
+ If the content type matches an entry in the message signer's
subject_constraints and the entry's attrConstraints field is
present, then constraints MUST be checked. For each
attrType in the entry's attrConstraints, a corresponding
attribute MUST be present in cms_effective_attributes
containing values from the entry's attrConstraints. If
values appear in the corresponding attribute that are not in
the entry's attrConstraints or if there is no corresponding
attribute, the message signer is not authorized to manage
the trust anchor and the update MUST be rejected.
Once these steps are completed, if the update has not been rejected,
then the message signer is authorized to manage the to-be-updated
trust anchor.
Housley, et al. Standards Track PAGE 31
RFC 6010 CMS Content Constraints September 2010
Note that a management trust anchor that has only the
id-ct-TAMP-update permitted content type is useful only for managing
identity trust anchors. It can sign a Trust Anchor Update message,
but it cannot impact a management trust anchor that is associated
with any other content type.
6. Security Considerations
For any given certificate, multiple certification paths may exist,
and each one can yield different results for CMS content constraints
processing. For example, default attributes can change when multiple
certification paths exist, as each path can potentially have
different attribute requirements or default values.
Compromise of a trust anchor private key permits unauthorized parties
to generate signed messages that will be acceptable to all
applications that use a trust anchor store containing the
corresponding management trust anchor. For example, if the trust
anchor is authorized to sign firmware packages, then the unauthorized
private key holder can generate firmware that may be successfully
installed and used by applications that trust the management trust
anchor.
For implementations that support validation of TAMP messages using
X.509 certificates, it is possible for the TAMP message signer to
have more than one possible certification path that will authorize it
to sign Trust Anchor Update messages, with each certification path
resulting in different CMS Content Constraints. The update is
authorized if the processing below succeeds for any one certification
path of the TAMP message signer. The resulting subject_constraints
variable is used to check each to-be-updated trust anchor contained
in the update message.
CMS does not provide a mechanism for indicating that an attribute
applies to a particular content within a ContentCollection or a set
CMS layers. For the sake of simplicity, this specification collects
all attributes that appear in a CMS path. These attributes are
processed as part of CCC processing and are made available for use in
processing leaf node contents. This can result in a collection of
attributes that have no relationship with the leaf node contents.
CMS does not provide a means for indicating what element within a CMS
message an attribute applies to. For example, a MessageDigest
attribute included in a SignedData signedAttributes collection
applies to a specific signature, but a Firmware Package Identifier
attribute appearing in the same list of attributes describes the
encapsulated content. As such, CCC treats all attributes as applying
to the encapsulated content type. Care should be taken to avoid
Housley, et al. Standards Track PAGE 32
RFC 6010 CMS Content Constraints September 2010
provisioning trust anchors or certificates that include constraints
on attribute types that are never used to describe a leaf content
type, such as a MessageDigest attribute.
The CMS Constraint Processing algorithm is designed to collect signer
information for processing when all information for a CMS path is
available. In cases where the certification path discovered during
SignedData layer processing is not acceptable, an alternative
certification path may be discovered that is acceptable. These
alternatives may include an alternative signer certificate. When the
ESSCertId attribute is used, alternative signer certificates are not
permitted. The certificate referenced by ESSCertId must be used,
possibly resulting in failure where alternative certificates would
yield success.
7. Acknowledgments
Thanks to Jim Schaad for thorough review and many suggestions.
8. References
8.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 3274] Gutmann, P., "Compressed Data Content Type for
Cryptographic Message Syntax (CMS)", RFC 3274, June 2002.
[RFC 4073] Housley, R., "Protecting Multiple Contents with the
Cryptographic Message Syntax (CMS)", RFC 4073, May 2005.
[RFC 5083] Housley, R., "Cryptographic Message Syntax (CMS)
Authenticated-Enveloped-Data Content Type", RFC 5083,
November 2007.
[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, May 2008.
[RFC 5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, September 2009.
[RFC 5911] Hoffman, P. and J. Schaad, "New ASN.1 Modules for
Cryptographic Message Syntax (CMS) and S/MIME", RFC 5911,
June 2010.
Housley, et al. Standards Track PAGE 33
RFC 6010 CMS Content Constraints September 2010
[RFC 5912] Hoffman, P. and J. Schaad, "New ASN.1 Modules for the
Public Key Infrastructure Using X.509 (PKIX)", RFC 5912,
June 2010.
[X.208] "ITU-T Recommendation X.208 - Specification of Abstract
Syntax Notation One (ASN.1)", 1988.
[X.501] ITU-T Recommendation X.501, "Information technology - Open
Systems Interconnection - The Directory: Models", ISO/
IEC 9594-2:2005, 2005.
[X.680] "ITU-T Recommendation X.680: Information Technology -
Abstract Syntax Notation One", 2002.
[X.690] "ITU-T Recommendation X.690 Information Technology - ASN.1
encoding rules: Specification of Basic Encoding Rules
(BER), Canonical Encoding Rules (CER) and Distinguished
Encoding Rules (DER)", 2002.
8.2. Informative References
[RFC 3161] Adams, C., Cain, P., Pinkas, D., and R. Zuccherato,
"Internet X.509 Public Key Infrastructure Time-Stamp
Protocol (TSP)", RFC 3161, August 2001.
[RFC 4108] Housley, R., "Using Cryptographic Message Syntax (CMS) to
Protect Firmware Packages", RFC 4108, August 2005.
[RFC 5035] Schaad, J., "Enhanced Security Services (ESS) Update:
Adding CertID Algorithm Agility", RFC 5035, August 2007.
[RFC 5272] Schaad, J. and M. Myers, "Certificate Management over CMS
(CMC)", RFC 5272, June 2008.
[RFC 5752] Schaad, J. and S. Turner, "Multiple Signatures in
Cryptographic Message Syntax (CMS)", December 2009.
[RFC 5914] Housley, R., Ashmore, S., and C. Wallace, "Trust Anchor
Format", RFC 5914, June 2010.
[RFC 5934] Housley, R., Ashmore, S., and C. Wallace, "Trust Anchor
Management Protocol (TAMP)", RFC 5934, August 2010.
Housley, et al. Standards Track PAGE 34
RFC 6010 CMS Content Constraints September 2010
Appendix A. ASN.1 Modules
Appendix A.1 provides the normative ASN.1 definitions for the
structures described in this specification using ASN.1 as defined in
[X.680]. Appendix A.2 provides a module using ASN.1 as defined in
[X.208]. The module in A.2 removes usage of newer ASN.1 features
that provide support for limiting the types of elements that may
appear in certain SEQUENCE and SET constructions. Otherwise, the
modules are compatible in terms of encoded representation, i.e., the
modules are bits-on-the-wire compatible aside from the limitations on
SEQUENCE and SET constituents. A.2 is included as a courtesy to
developers using ASN.1 compilers that do not support current ASN.1.
A.1 references an ASN.1 module from [RFC 5912] and [RFC 5911].
A.1. ASN.1 Module Using 1993 Syntax
CMSContentConstraintsCertExtn
{ iso(1) identified-organization(3) dod(6) internet(1) security(5)
mechanisms(5) pkix(7) id-mod(0) cmsContentConstr-93(42) }
DEFINITIONS IMPLICIT TAGS ::= BEGIN
IMPORTS
EXTENSION, ATTRIBUTE
FROM -- from [RFC 5912]
PKIX-CommonTypes-2009
{iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-pkixCommon-02(57)}
CONTENT-TYPE, ContentSet, SignedAttributesSet, ContentType
FROM -- from [RFC 5911]
CryptographicMessageSyntax-2009
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0)
id-mod-cms-2004-02(41) }
;
id-ct-anyContentType ContentType ::=
{ iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16)
ct(1) 0 }
ct-Any CONTENT-TYPE ::= {NULL IDENTIFIED BY id-ct-anyContentType }
Housley, et al. Standards Track PAGE 35
RFC 6010 CMS Content Constraints September 2010
--
-- Add this to CertExtensions in PKIX1Implicit-2009
--
ext-cmsContentConstraints EXTENSION ::= {
SYNTAX CMSContentConstraints
IDENTIFIED BY id-pe-cmsContentConstraints }
id-pe-cmsContentConstraints OBJECT IDENTIFIER ::=
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) pe(1) 18 }
CMSContentConstraints ::= SEQUENCE SIZE (1..MAX) OF
ContentTypeConstraint
ContentTypeGeneration ::= ENUMERATED {
canSource(0),
cannotSource(1)}
ContentTypeConstraint ::= SEQUENCE {
contentType CONTENT-TYPE.&id ({ContentSet|ct-Any,...}),
canSource ContentTypeGeneration DEFAULT canSource,
attrConstraints AttrConstraintList OPTIONAL }
Constraint { ATTRIBUTE:ConstraintList } ::= SEQUENCE {
attrType ATTRIBUTE.
&id({ConstraintList}),
attrValues SET SIZE (1..MAX) OF ATTRIBUTE.
&Type({ConstraintList}{@attrType}) }
SupportedConstraints ATTRIBUTE ::= {SignedAttributesSet, ... }
AttrConstraintList ::=
SEQUENCE SIZE (1..MAX) OF Constraint {{ SupportedConstraints }}
END
Housley, et al. Standards Track PAGE 36
RFC 6010 CMS Content Constraints September 2010
A.2. ASN.1 Module Using 1988 Syntax
CMSContentConstraintsCertExtn-88
{ iso(1) identified-organization(3) dod(6) internet(1) security(5)
mechanisms(5) pkix(7) id-mod(0) cmsContentConstr-88(41) }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
IMPORTS
AttributeType, AttributeValue
FROM PKIX1Explicit88 -- from [RFC 5280]
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-pkix1-explicit(18) } ;
id-ct-anyContentType OBJECT IDENTIFIER ::=
{ iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16)
ct(1) 0}
-- Extension object identifier
id-pe-cmsContentConstraints OBJECT IDENTIFIER ::=
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) pe(1) 18 }
-- CMS Content Constraints Extension
CMSContentConstraints ::= SEQUENCE SIZE (1..MAX) OF
ContentTypeConstraint
ContentTypeGeneration ::= ENUMERATED {
canSource(0),
cannotSource(1)}
ContentTypeConstraint ::= SEQUENCE {
contentType OBJECT IDENTIFIER,
canSource ContentTypeGeneration DEFAULT canSource,
attrConstraints AttrConstraintList OPTIONAL }
AttrConstraintList ::= SEQUENCE SIZE (1..MAX) OF AttrConstraint
AttrConstraint ::= SEQUENCE {
attrType AttributeType,
attrValues SET SIZE (1..MAX) OF AttributeValue }
END
Housley, et al. Standards Track PAGE 37
RFC 6010 CMS Content Constraints September 2010
Authors' Addresses
Russ Housley
Vigil Security, LLC
918 Spring Knoll Drive
Herndon, VA 20170
EMail: housley@vigilsec.com
Sam Ashmore
National Security Agency
Suite 6751
9800 Savage Road
Fort Meade, MD 20755
EMail: srashmo@radium.ncsc.mil
Carl Wallace
Cygnacom Solutions
Suite 5400
7925 Jones Branch Drive
McLean, VA 22102
EMail: cwallace@cygnacom.com
Housley, et al. Standards Track PAGE 38
Cryptographic Message Syntax (CMS) Content Constraints Extension
RFC TOTAL SIZE: 87495 bytes
PUBLICATION DATE: Friday, September 17th, 2010
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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