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.

















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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.

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





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  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.



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     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



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  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.




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  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.









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  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.





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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.




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  [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.





























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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 --
  }





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  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: [email protected]


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

  EMail: [email protected]








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