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.
Turner & Housley Standards Track [Page 1]RFC 6032 CMS Encrypted Key Package Content Type December 2010
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(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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RFC 6032 CMS Encrypted Key Package Content Type December 2010
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.
Turner & Housley Standards Track [Page 3]RFC 6032 CMS Encrypted Key Package Content Type December 2010
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.
Turner & Housley Standards Track [Page 5]RFC 6032 CMS Encrypted Key Package Content Type December 2010
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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RFC 6032 CMS Encrypted Key Package Content Type December 2010
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.
Turner & Housley Standards Track [Page 7]RFC 6032 CMS Encrypted Key Package Content Type December 2010
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.
Turner & Housley Standards Track [Page 8]RFC 6032 CMS Encrypted Key Package Content Type December 2010
[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 --
}
Turner & Housley Standards Track [Page 10]RFC 6032 CMS Encrypted Key Package Content Type December 2010
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
Turner & Housley Standards Track [Page 11]
