RFC6033
Internet Engineering Task Force (IETF) S. Turner Request for Comments: 6033 IECA Category: Standards Track December 2010 ISSN: 2070-1721
Algorithms for Cryptographic Message Syntax (CMS)
Encrypted Key Package Content Type
Abstract
This document describes the conventions for using several cryptographic algorithms with the Cryptographic Message Syntax (CMS) encrypted key package content type. Specifically, it includes conventions necessary to implement EnvelopedData, EncryptedData, 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/rfc6033.
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.
Contents
Introduction
This document describes the conventions for using several cryptographic algorithms with the Cryptographic Message Syntax (CMS) encrypted key package content type RFC6032. Specifically, it includes conventions necessary to implement the following CMS content types: EnvelopedData RFC5652, EncryptedData RFC5652, and AuthEnvelopedData RFC5083.
This document does not define any new algorithms; instead, it refers to previously defined algorithms.
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.
EnvelopedData
EnvelopedData RFC5652 supports a number of key management techniques. Implementations that claim conformance to this document MUST support the key transport mechanisms and SHOULD support the key agreement mechanisms as defined below. Other techniques MAY be supported.
When key transport is used, RSA encryption RFC3370 MUST be supported and RSA Encryption Scheme - Optimal Asymmetric Encryption Padding (RSAES-OAEP) RFC3560 SHOULD be supported.
When key agreement is used, Ephemeral-Static Diffie-Hellman (DH) RFC3370 MUST be supported.
Since the content type is used to carry a cryptographic key and its attributes, an algorithm that is traditionally used to encrypt one key with another is employed. Regardless of the key management technique choice, implementations MUST support AES-128 Key Wrap with Padding RFC5649 as the content-encryption algorithm. Implementations SHOULD support AES-256 Key Wrap with Padding RFC5649 as the content-encryption algorithm.
When key agreement is used, a key wrap algorithm is also specified to wrap the content-encryption key. If the content-encryption algorithm is AES-128 Key Wrap with Padding, then the key wrap algorithm MUST be AES-128 Key Wrap with Padding RFC5649. If the content-encryption algorithm is AES-256 Key Wrap with Padding, then the key wrap algorithm MUST be AES-256 Key Wrap with Padding RFC5649.
EncryptedData
EncryptedData RFC5652 requires that keys be managed by other means; therefore, the only algorithm specified is the content-encryption algorithm. Since the content type is used to carry a cryptographic key and its attributes, an algorithm that is traditionally used to encrypt one key with another is employed. Implementations MUST support AES-128 Key Wrap with Padding RFC5649. Implementations SHOULD support AES-256 Key Wrap with Padding RFC5649.
AuthEnvelopedData
AuthEnvelopedData RFC5083, like EnvelopedData, supports a number of key management techniques. The key management requirements for AuthEnvelopedData are the same as for EnvelopedData. The difference is the content-encryption algorithm. Implementations MUST support 128-bit AES-Galois/Counter Mode (AES-GCM) RFC5084 and SHOULD support 256-bit AES-GCM RFC5084. Implementations MAY also support AES-Counter with CBC-MAC (AES-CCM) RFC5084.
Signed Data
Implementations of SignedData RFC5652 MUST support the signature scheme RSA RFC3370 RFC5754 and SHOULD support the signature schemes RSA Probabilistic Signature Scheme (RSASSA-PSS) RFC4056 and Digital Signature Algorithm (DSA) RFC3370 RFC5754. Additionally, implementations MUST support in concert with these signature schemes the hash function SHA-256 RFC5754 and it SHOULD support the hash function SHA-1 RFC3370.
Public Key Sizes
The easiest way to implement SignedData, EnvelopedData, and AuthEnvelopedData is with public key certificates RFC5280. If an implementation supports RSA, RSAES-OAEP, DH, RSASSA-PSS, or DSA, then it MUST support key lengths from 1024 bits to 2048 bits, inclusive.
Security Considerations
The security considerations from RFC3370, RFC3560, RFC4056, RFC5083, RFC5084, RFC5649, RFC5652, RFC5754, and RFC6032 apply.
The choice of content-encryption algorithms for this document was based on RFC5649: "In the design of some high assurance cryptographic modules, it is desirable to segregate cryptographic keying material from other data. The use of a specific cryptographic mechanism solely for the protection of cryptographic keying material
can assist in this goal". Unfortunately, there is no AES-GCM or AES-CCM mode that provides the same properties. If an AES-GCM and AES-CCM mode that provides the same properties is defined, then this document will be updated to adopt that algorithm.
[SP800-57] provides comparable bits of security for some algorithms and key sizes. [SP800-57] also provides time frames during which certain numbers of bits of security are appropriate, and some environments may find these time frames useful.
References
Normative References
RFC2119 Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
RFC3370 Housley, R., "Cryptographic Message Syntax (CMS)
Algorithms", RFC 3370, August 2002.
RFC3560 Housley, R., "Use of the RSAES-OAEP Key Transport
Algorithm in Cryptographic Message Syntax (CMS)", RFC
3560, July 2003.
RFC4056 Schaad, J., "Use of the RSASSA-PSS Signature Algorithm in
Cryptographic Message Syntax (CMS)", RFC 4056, June 2005.
RFC5083 Housley, R., "Cryptographic Message Syntax (CMS)
Authenticated-Enveloped-Data Content Type", RFC 5083, November 2007.
RFC5084 Housley, R., "Using AES-CCM and AES-GCM Authenticated
Encryption in the Cryptographic Message Syntax (CMS)",
RFC 5084, 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.
RFC5649 Housley, R. and M. Dworkin, "Advanced Encryption Standard
(AES) Key Wrap with Padding Algorithm", RFC 5649, September 2009.
RFC5652 Housley, R., "Cryptographic Message Syntax (CMS)", STD
70, RFC 5652, September 2009.
RFC5754 Turner, S., "Using SHA2 Algorithms with Cryptographic
Message Syntax", RFC 5754, January 2010.
RFC6032 Turner, S. and R. Housley, "Cryptographic Message Syntax
(CMS) Encrypted Key Package Content Type", RFC 6032, December 2010.
Informative References
[SP800-57] National Institute of Standards and Technology (NIST),
Special Publication 800-57: Recommendation for Key
Management - Part 1 (Revised), March 2007.
Author's Address
Sean Turner IECA, Inc. 3057 Nutley Street, Suite 106 Fairfax, VA 22031 USA
EMail: [email protected]
