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RFC 6010

Cryptographic Message Syntax (CMS) Content Constraints Extension

Pages: 38
Proposed Standard
Errata

Top   ToC   RFC6010 - Page 1
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/rfc6010.
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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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.
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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
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1. Introduction

The Cryptographic Message Syntax (CMS) SignedData [RFC5652] construct is used to sign many things, including cryptographic module firmware packages [RFC4108] and certificate management messages [RFC5272]. 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 [RFC5914], and constraints are optionally included in public key certificates [RFC5280]. 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
Top   ToC   RFC6010 - Page 5
   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 [RFC5934].

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 [RFC5652] and ContentCollection [RFC4073] are examples of intermediate content types. FirmwarePkgData [RFC4108] and TSTInfo [RFC3161] 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 [RFC5652]: Cryptographic Message Syntax (CMS) - SignedData - EnvelopedData - EncryptedData - DigestedData - AuthenticatedData RFC 5083 [RFC5083]: The Cryptographic Message Syntax (CMS) AuthEnvelopedData Content Type - AuthEnvelopedData
Top   ToC   RFC6010 - Page 6
   RFC 4073 [RFC4073]: Protecting Multiple Contents with the
   Cryptographic Message Syntax (CMS)

      - ContentCollection

      - ContentWithAttributes

   RFC 3274 [RFC3274]: 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 [RFC4108] 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.
Top   ToC   RFC6010 - Page 7
         +---------------------------------------------------------+
         | 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
Top   ToC   RFC6010 - Page 8
         +---------------------------------------------------------+
         | ContentInfo                                             |
         |                                                         |
         | +-----------------------------------------------------+ |
         | | SignedData                                          | |
         | |                                                     | |
         | | +-------------------------------------------------+ | |
         | | | ContentCollection                               | | |
         | | |                                                 | | |
         | | | +----------------------+ +--------------------+ | | |
         | | | | SignedData           | | SignedData         | | | |
         | | | |                      | |                    | | | |
         | | | | +------------------+ | | +----------------+ | | | |
         | | | | | EncryptedData    | | | | Firmware       | | | | |
         | | | | |                  | | | | Package        | | | | |
         | | | | | +--------------+ | | | |                | | | | |
         | | | | | | Firmware     | | | | +----------------+ | | | |
         | | | | | | Package      | | | +--------------------+ | | |
         | | | | | |              | | |                        | | |
         | | | | | +--------------+ | |                        | | |
         | | | | +------------------+ |                        | | |
         | | | +----------------------+                        | | |
         | | +-------------------------------------------------+ | |
         | +-----------------------------------------------------+ |
         +---------------------------------------------------------+

         Figure 3.  Example of Two Firmware Packages in a Collection
Top   ToC   RFC6010 - Page 9
         +---------------------------------------------------------+
         |                                                         |
         |     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
Top   ToC   RFC6010 - Page 10
   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 [RFC5280] 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
Top   ToC   RFC6010 - Page 11
   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.
Top   ToC   RFC6010 - Page 12
   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.
Top   ToC   RFC6010 - Page 13
      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 [RFC5280] extensions are defined using ASN.1 [X.680][X.690]. CMS content types [RFC5652] 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 [RFC2119].

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 }
Top   ToC   RFC6010 - Page 14
   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.
Top   ToC   RFC6010 - Page 15
   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 [RFC4108].  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).
Top   ToC   RFC6010 - Page 16
   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 [RFC5280], as shown in the figure below. Alternative implementations are allowed but MUST yield the same results as described below.
Top   ToC   RFC6010 - Page 17
   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.
Top   ToC   RFC6010 - Page 18
   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 [RFC5280] 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.
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   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.
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      -- 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 [RFC5280].

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.
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   -  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.
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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.
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   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:
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   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 [RFC5652] [RFC5083]. 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 [RFC3161] and ESSCertId [RFC5035] processing.
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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 [RFC5752] 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
Top   ToC   RFC6010 - Page 26
   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
Top   ToC   RFC6010 - Page 27
   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.
Top   ToC   RFC6010 - Page 28
   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
   [RFC5035] 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.
Top   ToC   RFC6010 - Page 29
   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 [RFC5934] 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
Top   ToC   RFC6010 - Page 30
   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
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         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.
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   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
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   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

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3274] Gutmann, P., "Compressed Data Content Type for Cryptographic Message Syntax (CMS)", RFC 3274, June 2002. [RFC4073] Housley, R., "Protecting Multiple Contents with the Cryptographic Message Syntax (CMS)", RFC 4073, May 2005. [RFC5083] Housley, R., "Cryptographic Message Syntax (CMS) Authenticated-Enveloped-Data Content Type", RFC 5083, November 2007. [RFC5280] 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. [RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, RFC 5652, September 2009. [RFC5911] Hoffman, P. and J. Schaad, "New ASN.1 Modules for Cryptographic Message Syntax (CMS) and S/MIME", RFC 5911, June 2010.
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   [RFC5912]  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

[RFC3161] Adams, C., Cain, P., Pinkas, D., and R. Zuccherato, "Internet X.509 Public Key Infrastructure Time-Stamp Protocol (TSP)", RFC 3161, August 2001. [RFC4108] Housley, R., "Using Cryptographic Message Syntax (CMS) to Protect Firmware Packages", RFC 4108, August 2005. [RFC5035] Schaad, J., "Enhanced Security Services (ESS) Update: Adding CertID Algorithm Agility", RFC 5035, August 2007. [RFC5272] Schaad, J. and M. Myers, "Certificate Management over CMS (CMC)", RFC 5272, June 2008. [RFC5752] Schaad, J. and S. Turner, "Multiple Signatures in Cryptographic Message Syntax (CMS)", December 2009. [RFC5914] Housley, R., Ashmore, S., and C. Wallace, "Trust Anchor Format", RFC 5914, June 2010. [RFC5934] Housley, R., Ashmore, S., and C. Wallace, "Trust Anchor Management Protocol (TAMP)", RFC 5934, August 2010.
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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 [RFC5912] and [RFC5911].

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 [RFC5912] 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 [RFC5911] 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 }
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   --
   --  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
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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 [RFC5280] { 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
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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