Using Message Authentication Code (MAC) Encryption in the Cryptographic Message Syntax (CMS)
RFC 6476

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Internet Engineering Task Force (IETF)                        P. Gutmann
Request for Comments: 6476                        University of Auckland
Category: Standards Track                                   January 2012
ISSN: 2070-1721

           Using Message Authentication Code (MAC) Encryption
               in the Cryptographic Message Syntax (CMS)

Abstract

   This document specifies the conventions for using Message
   Authentication Code (MAC) encryption with the Cryptographic Message
   Syntax (CMS) authenticated-enveloped-data content type.  This mirrors
   the use of a MAC combined with an encryption algorithm that's already
   employed in IPsec, Secure Socket Layer / Transport Layer Security
   (SSL/TLS) and Secure SHell (SSH), which is widely supported in
   existing crypto libraries and hardware and has been extensively
   analysed by the crypto community.

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/rfc6476.

Copyright Notice

   Copyright (c) 2012 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.

Gutmann                      Standards Track                    [Page 1]
RFC 6476                  MAC Encryption in CMS             January 2012

Table of Contents

   1. Introduction ....................................................2
      1.1. Conventions Used in This Document ..........................2
   2. Background ......................................................2
   3. CMS Encrypt-and-Authenticate Overview ...........................3
      3.1. Rationale ..................................................3
   4. CMS Encrypt-and-Authenticate ....................................4
      4.1. Encrypt-and-Authenticate Message Processing ................5
      4.2. Rationale ..................................................6
      4.3. Test Vectors ...............................................8
   5. SMIMECapabilities Attribute ....................................12
   6. Security Considerations ........................................12
   7. IANA Considerations ............................................13
   8. Acknowledgements ...............................................14
   9. References .....................................................14
      9.1. Normative References ......................................14
      9.2. Informative References ....................................14

1.  Introduction

   This document specifies the conventions for using MAC-authenticated
   encryption with the Cryptographic Message Syntax (CMS) authenticated-
   enveloped-data content type.  This mirrors the use of a MAC combined
   with an encryption algorithm that's already employed in IPsec, SSL/
   TLS and SSH, which is widely supported in existing crypto libraries
   and hardware and has been extensively analysed by the crypto
   community.

1.1.  Conventions Used in This Document

   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].

2.  Background

   Integrity-protected encryption is a standard feature of session-
   oriented security protocols like [IPsec], [SSH], and [TLS].  Until
   recently, however, integrity-protected encryption wasn't available
   for message-based security protocols like CMS, although [OpenPGP]
   added a form of integrity protection by encrypting a SHA-1 hash of
   the message alongside the message contents to provide authenticate-
   and-encrypt protection.  Usability studies have shown that users
   expect encryption to provide integrity protection [Garfinkel],
   creating cognitive dissonance problems when the security mechanisms
   don't in fact provide this assurance.

Gutmann                      Standards Track                    [Page 2]
RFC 6476                  MAC Encryption in CMS             January 2012