Internet Engineering Task Force (IETF)                         S. Turner
Request for Comments: 6032                                          IECA
Category: Standards Track                                     R. Housley
ISSN: 2070-1721                                           Vigil Security
                                                           December 2010


                   Cryptographic Message Syntax (CMS)
                   Encrypted Key Package Content Type

Abstract

   This document defines the Cryptographic Message Syntax (CMS)
   encrypted key package content type, which can be used to encrypt a
   content that includes a key package, such as a symmetric key package
   or an asymmetric key package.  It is transport independent.  CMS can
   be used to digitally sign, digest, authenticate, or further encrypt
   this content type.  It is designed to be used with the CMS Content
   Constraints (CCC) extension, which does not constrain the
   EncryptedData, EnvelopedData, and AuthEnvelopedData.

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/rfc6032.

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.

1.  Introduction

   The Cryptographic Message Syntax (CMS) specification [RFC5652]
   defines mechanisms to digitally sign, digest, authenticate, or
   encrypt arbitrary message content.  Many specifications define
   content types intended for use with CMS.  [RFC6031] and [RFC5958]
   define symmetric key package and asymmetric key package content types
   that can be signed or encrypted using CMS.  CMS allows the
   composition of complex messages with an arbitrary number of layers.
   CMS has been augmented by several specifications ([RFC3274],
   [RFC4073], and [RFC5083]) that define additional mechanisms to enable
   creation of messages of arbitrary depth and breadth using a variety
   of authentication, encryption, and compression techniques.

   The CMS Content Constraints (CCC) certificate extension [RFC6010]
   defines an authorization mechanism that allows recipients to
   determine whether the originator of an authenticated CMS content type
   is authorized to produce messages of that type.  CCC is used to
   authorize CMS-protected content.  CCC cannot be used to constrain the
   following structures that are used to provide layers of protection:
   SignedData, EnvelopedData, EncryptedData, DigestData, CompressedData,
   AuthenticatedData, ContentCollection, ContentWithAttributes, or
   AuthEnvelopedData.

   Using the existing CMS mechanisms, producers of authenticated
   plaintext key packages can be authorized by including a CCC extension
   containing the appropriate content type in the producer's
   certificate.  However, these mechanisms cannot be used to authorize
   the producers of encrypted key material.  In some key management
   systems, encrypted key packages are exchanged between entities that
   cannot decrypt the key package.  The encrypted key package itself may

   be authenticated and passed to another entity.  In these cases,
   checking the authorization of the producer of the encrypted key
   package may be desired at the intermediate points.

   This document defines the encrypted key package content type, which
   can be used to encrypt a content that includes a key package, such as
   a symmetric key package [RFC6031] or an asymmetric key package
   [RFC5958].  It is transport independent.  The Cryptographic Message
   Syntax (CMS) [RFC5652] can be used to digitally sign, digest,
   authenticate, or further encrypt this content type.

   The encrypted key package content type is designed for use with
   [RFC6010].  To authorize an originator's public key to originate an
   encrypted key package, the object identifier associated with the
   encrypted key package content type is included in the originator's
   public key certificate CCC certificate extension.  For CCC to
   function, originators encapsulate the encrypted key package in a
   SignedData, EnvelopedData, or AuthEnvelopedData; then, during
   certificate path validation, the recipient determines whether the
   originator is authorized to originate the encrypted key package.

   In [RFC6010] terminology, the encrypted key package is a leaf node.
   Additional authorization checks may be required once the key package
   is decrypted.  For example, the key package shown below consists of a
   SignedData layer that encapsulates an encrypted key package that
   encapsulates a SignedData layer containing a symmetric key package.
   A recipient capable of decrypting the key package would perform the
   following steps prior to accepting the encapsulated symmetric key
   material:

      o Verify the signature on the outer SignedData layer per
        [RFC5652].

      o Build and validate a certification path of the outer signer and
        confirm the outer signer is authorized to produce the encrypted
        key package per [RFC5280] and [RFC6010].

      o Decrypt the encrypted key package.

      o Verify the signature on the inner SignedData layer per
        [RFC5652].

      o Build and validate a certification path to the signer of the
        inner SignedData and confirm the inner signer is authorized to
        produce the symmetric key package per [RFC5280] and [RFC6010].
        As specified in [RFC6010], the validator may use the attributes
        and public keys returned from the second step as inputs for this
        CMS content constraints processing.

      o Use the symmetric key material.

            +--------------------------------------+
            | ContentInfo                          |
            |                                      |
            | +----------------------------------+ |
            | | SignedData                       | |
            | |                                  | |
            | | +------------------------------+ | |
            | | | EncryptedKeyPackage          | | |
            | | |   (encrypted)                | | |
            | | |                              | | |
            | | | +-------------------------+  | | |
            | | | | SignedData              |  | | |
            | | | |                         |  | | |
            | | | | +---------------------+ |  | | |
            | | | | | SymmetricKeyPackage | |  | | |
            | | | | +---------------------+ |  | | |
            | | | +-------------------------+  | | |
            | | +------------------------------+ | |
            | +----------------------------------+ |
            +--------------------------------------+

   In the example, authorization of the SymmetricKeyPackage originator
   need not require an intermediate SignedData layer.  For example, if
   the AuthEnvelopedData option within an EncryptedKeyPackage were used,
   the second authorization check would be performed beginning with the
   authEnveloped field.

   This document also defines an unprotected attribute, Content
   Decryption Key Identifier, for use with EncryptedData.

1.1.  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 [RFC2119].

1.2.  ASN.1 Syntax Notation

   The key package is defined using the ASN.1 [X.680], [X.681], [X.682],
   and [X.683].

2.  Encrypted Key Package

   The encrypted key package content type is used to encrypt a content
   that includes a key package.  This content type is usually used to
   provide encryption of a key package or a signed key package.  This

   content type makes use of the CMS EncryptedData content type
   [RFC5652], the CMS EnvelopedData content type [RFC5652], or the CMS
   AuthEnvelopedData content type [RFC5083] depending on the fields that
   are needed for key management.  The difference between the encrypted
   key package content type and these three protecting content types is
   the object identifier and one tag; otherwise, the encrypted key
   package content type is the same as the selected protecting content
   type, which is either EncryptedData, EnvelopedData, or
   AuthEnvelopedData.

   The encrypted key package content type has the following syntax:

      ct-encrypted-key-package CONTENT-TYPE ::=
        { TYPE EncryptedKeyPackage
          IDENTIFIED BY id-ct-KP-encryptedKeyPkg }

      id-ct-KP-encryptedKeyPkg OBJECT IDENTIFIER ::=
        { joint-iso-itu-t(2) country(16) us(840) organization(1)
          gov(101) dod(2) infosec(1) formats(2)
          key-package-content-types(78) 2 }

      EncryptedKeyPackage ::= CHOICE {
        encrypted        EncryptedData,
        enveloped        [0] EnvelopedData,
        authEnveloped    [1] AuthEnvelopedData }

   The EncryptedData structure is used for simple symmetric encryption,
   where the sender and the receiver already share the necessary
   encryption key.  The EncryptedData structure carries an encryption
   algorithm identifier, and an unprotected attribute can be used to
   carry an encryption key identifier if one is needed (see Section 3).
   See [RFC5652] for further discussion of the EncryptedData fields.

   The EnvelopedData structure is used for encryption, where transferred
   key management information enables decryption by the receiver.
   Encryption details depend on the key management algorithm used.  In
   addition to the key management information, the EnvelopedData
   structure carries an encryption algorithm identifier.  See [RFC5652]
   for further discussion of the EnvelopedData fields.

   The AuthEnvelopedData structure is used for authenticated encryption,
   and it includes key management information in a manner similar to
   EnvelopedData.  Encryption details depend on the key management
   algorithm used.  In addition to the key management information, the
   AuthEnvelopedData structure carries a message authentication code
   that covers the content as well as authenticated attributes.  See
   [RFC5083] for further discussion of the AuthEnvelopedData fields.

   Implementations of this document MUST support the EnvelopedData
   choice, SHOULD support the EncryptedData choice, and MAY support the
   AuthEnvelopedData.

   Implementations that support EnvelopedData and EncryptedData to
   encapsulate with this content type MUST support an
   EncryptedKeyPackage that encapsulates either a SignedData [RFC5652]
   that further encapsulates a SymmetricKeyPackage [RFC6031] or a
   SignedData that further encapsulates an AsymmetricKeyPackage
   [RFC5958].  Implementations that support AuthEnvelopedData to
   encapsulate with this content type MUST support an
   EncryptedKeyPackage that encapsulates either a SymmetricKeyPackage
   [RFC6031] or an AsymmetricKeyPackage [RFC5958].  It is OPTIONAL for
   implementations that support AuthEnvelopedData to encapsulate with
   this content type to support an EncryptedKeyPackage that encapsulates
   either a SignedData [RFC5652] that further encapsulates a
   SymmetricKeyPackage [RFC6031] or a SignedData that further
   encapsulates an AsymmetricKeyPackage [RFC5958].  Likewise,
   implementations that process this content type to decrypt the
   encapsulated data MUST support an EncryptedKeyPackage that
   encapsulates either a SignedData that further encapsulates a
   SymmetricKeyPackage or a SignedData that further encapsulates an
   AsymmetricKeyPackage.  An EncryptedKeyPackage content type MUST
   contain at least one SymmetricKeyPackage or AsymmetricKeyPackage.
   Implementations MAY support additional encapsulating layers.

   Note that interoperability between an originator and a recipient that
   do not support the same innermost content (e.g., originator supports
   AsymmetricKeyPackage while recipient supports SymmetricKeyPackage) is
   not a concern as originators should be aware of the recipient's
   capabilities; however, the mechanism for the exchange of the
   recipient's capabilities is beyond the scope of this document.

3.  Content Decryption Key Identifier

   The content-decryption-key-identifier attribute can be used to
   identify the symmetric keying material that is needed for decryption
   of the EncryptedData content if there is any ambiguity.  The
   ATTRIBUTE definition is taken from [RFC5912].  There MUST be only one
   instance of the content-decryption-key-identifier attribute and there
   MUST be only one value for the content-decryption-key-identifier
   attribute.

   The content-decryption-key-identifier attribute has the following
   syntax:

      aa-content-decrypt-key-identifier ATTRIBUTE ::= {
        TYPE          ContentDecryptKeyID
        IDENTIFIED BY id-aa-KP-contentDecryptKeyID }

      id-aa-KP-contentDecryptKeyID OBJECT IDENTIFIER ::= {
        joint-iso-itu-t(2) country(16) us(840) organization(1) gov(101)
        dod(2) infosec(1) attributes(5) 66 }

      ContentDecryptKeyID ::= OCTET STRING

   The content decryption key identifier contains an OCTET STRING, and
   this syntax does not impose any particular structure on the
   identifier value.

   Due to multiple layers of encryption, the content-decryption-key-
   identifier attribute can appear in more than one location in the
   overall key package.  When there are multiple occurrences of the
   content-decryption-key-identifier attribute, each occurrence is
   evaluated independently.  Each one is used to identify the needed
   keying material for that layer of encryption.

4.  Security Considerations

   Implementers of this protocol are strongly encouraged to consider
   generally accepted principles of secure key management when
   integrating this capability within an overall security architecture.

   The security considerations from [RFC5083], [RFC5652], [RFC5911],
   [RFC5912], [RFC5958], and [RFC6031] apply.  If the CCC extension is
   used as an authorization mechanism, then the security considerations
   from [RFC6010] also apply.

   The encrypted key package content type might not provide proof of
   origin if the content encryption algorithm does not support
   authenticated key exchange.  To provide proof of origin for this
   content, another security protocol needs to be used.  This is the
   reason that support for encapsulating the SymmetricKeyPackage and
   AsymmetricKeyPackage with a SignedData content type from [RFC5652] is
   required for the EnvelopedData and EncryptedData choices.

   When this content type is used the CMS SignedData [RFC5652]
   validation rules MUST be used.  The PKCS #7 [RFC2315] validation
   rules MUST NOT be used.

5.  IANA Considerations

   This document makes use of object identifiers to identify a CMS
   content type, a CMS attribute, and the ASN.1 module; all found in
   Appendix A.  All OIDs are registered in an arc delegated by RSADSI to
   the SMIME Working Group.

6.  References

6.1.  Normative References

   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

   [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.

   [RFC5912]   Hoffman, P. and J. Schaad, "New ASN.1 Modules for the
               Public Key Infrastructure Using X.509 (PKIX)", RFC 5912,
               June 2010.

   [RFC5958]   Turner, S., "Asymmetric Key Packages", RFC 5958, August
               2010.

   [RFC6010]   Housley, R., Ashmore, S., and C. Wallace, "Cryptographic
               Message Syntax (CMS) Content Constraints Extension", RFC
               6010, September 2010.

   [RFC6031]   Turner, S. and R. Housley, "Cryptographic Message Syntax
               (CMS) Symmetric Key Package Content Type", RFC 6031,
               December 2010.

   [X.680]     ITU-T Recommendation X.680 (2002) | ISO/IEC 8824-1:2002.
               Information Technology - Abstract Syntax Notation One.

   [X.681]     ITU-T Recommendation X.681 (2002) | ISO/IEC 8824-2:2002.
               Information Technology - Abstract Syntax Notation One:
               Information Object Specification.

   [X.682]     ITU-T Recommendation X.682 (2002) | ISO/IEC 8824-3:2002.
               Information Technology - Abstract Syntax Notation One:
               Constraint Specification.

   [X.683]     ITU-T Recommendation X.683 (2002) | ISO/IEC 8824-4:2002.
               Information Technology - Abstract Syntax Notation One:
               Parameterization of ASN.1 Specifications.

6.2.  Informative References

   [RFC2315]   Kaliski, B., "PKCS #7: Cryptographic Message Syntax
               Version 1.5", RFC 2315, March 1998.

   [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.

Appendix A.  ASN.1 Module

   This appendix provides the normative ASN.1 [X.680] definitions for
   the structures described in this specification using ASN.1, as
   defined in [X.680] through [X.683].

   EncryptedKeyPackageModuleV1
     { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
       smime(16) modules(0) id-mod-encryptedKeyPkgV1(51) }

   DEFINITIONS IMPLICIT TAGS ::=

   BEGIN

   -- EXPORTS ALL --

   IMPORTS

   -- From New SMIME ASN.1 [RFC5911]

   EncryptedData, EnvelopedData, CONTENT-TYPE
     FROM CryptographicMessageSyntax-2009
       { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
         smime(16) modules(0) cms-2004-02(41) }

   -- From New SMIME ASN.1 [RFC5911]

   AuthEnvelopedData
     FROM CMS-AuthEnvelopedData-2009
        { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
          pkcs-9(9) smime(16) modules(0) cms-authEnvelopedData-02(43) }

   -- From New PKIX ASN.1 [RFC5912]

   ATTRIBUTE
     FROM 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) }

   ;

   ContentSet CONTENT-TYPE ::= {
     ct-encrypted-key-package,
     ... -- Expect additional content types --
   }

   ct-encrypted-key-package CONTENT-TYPE ::=
       { TYPE EncryptedKeyPackage
         IDENTIFIED BY id-ct-KP-encryptedKeyPkg }

   id-ct-KP-encryptedKeyPkg OBJECT IDENTIFIER ::=
     { joint-iso-itu-t(2) country(16) us(840) organization(1) gov(101)
       dod(2) infosec(1) formats(2) key-package-content-types(78) 2 }

   EncryptedKeyPackage ::= CHOICE {
       encrypted        EncryptedData,
       enveloped        [0] EnvelopedData,
       authEnveloped    [1] AuthEnvelopedData }

   aa-content-decrypt-key-identifier ATTRIBUTE ::= {
       TYPE          ContentDecryptKeyID
       IDENTIFIED BY id-aa-KP-contentDecryptKeyID }

   id-aa-KP-contentDecryptKeyID OBJECT IDENTIFIER ::= {
     joint-iso-itu-t(2) country(16) us(840) organization(1) gov(101)
     dod(2) infosec(1) attributes(5) 66 }

   ContentDecryptKeyID ::= OCTET STRING

   END

Authors' Addresses

   Sean Turner
   IECA, Inc.
   3057 Nutley Street, Suite 106
   Fairfax, VA 22031
   USA

   EMail: turners@ieca.com


   Russell Housley
   Vigil Security, LLC
   918 Spring Knoll Drive
   Herndon, VA 20170
   USA

   EMail: housley@vigilsec.com