Cryptographic Message Syntax (CMS) Algorithm Identifier Protection Attribute
RFC 6211

Document Type RFC - Proposed Standard (April 2011; No errata)
Was draft-schaad-smime-algorithm-attribute (individual in sec area)
Last updated 2013-03-02
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Internet Engineering Task Force (IETF)                         J. Schaad
Request for Comments: 6211                       Soaring Hawk Consulting
Category: Standards Track                                     April 2011
ISSN: 2070-1721

                   Cryptographic Message Syntax (CMS)
               Algorithm Identifier Protection Attribute

Abstract

   The Cryptographic Message Syntax (CMS), unlike X.509/PKIX
   certificates, is vulnerable to algorithm substitution attacks.  In an
   algorithm substitution attack, the attacker changes either the
   algorithm being used or the parameters of the algorithm in order to
   change the result of a signature verification process.  In X.509
   certificates, the signature algorithm is protected because it is
   duplicated in the TBSCertificate.signature field with the proviso
   that the validator is to compare both fields as part of the signature
   validation process.  This document defines a new attribute that
   contains a copy of the relevant algorithm identifiers so that they
   are protected by the signature or authentication process.

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/rfc6211.

Schaad                       Standards Track                    [Page 1]
RFC 6211                 CMS Algorithm Attribute              April 2011

Copyright Notice

   Copyright (c) 2011 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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.1.  Notation  . . . . . . . . . . . . . . . . . . . . . . . . . 5
   2.  Attribute Structure . . . . . . . . . . . . . . . . . . . . . . 5
   3.  Verification Process  . . . . . . . . . . . . . . . . . . . . . 7
     3.1.  Signed Data Verification Changes  . . . . . . . . . . . . . 7
     3.2.  Authenticated Data Verification Changes . . . . . . . . . . 7
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 8
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
     6.1.  Normative References  . . . . . . . . . . . . . . . . . . . 8
     6.2.  Informational References  . . . . . . . . . . . . . . . . . 9
   Appendix A.  2008 ASN.1 Module  . . . . . . . . . . . . . . . . .  10

Schaad                       Standards Track                    [Page 2]
RFC 6211                 CMS Algorithm Attribute              April 2011

1.  Introduction

   The Cryptographic Message Syntax [CMS], unlike X.509/PKIX
   certificates [RFC5280], is vulnerable to algorithm substitution
   attacks.  In an algorithm substitution attack, the attacker changes
   either the algorithm being used or the parameters of the algorithm in
   order to change the result of a signature verification process.  In
   X.509 certificates, the signature algorithm is protected because it
   is duplicated in the TBSCertificate.signature field with the proviso
   that the validator is to compare both fields as part of the signature
   validation process.  This document defines a new attribute that
   contains a copy of the relevant algorithm identifiers so that they
   are protected by the signature or authentication process.

   In an algorithm substitution attack, the attacker looks for a
   different algorithm that produces the same result as the algorithm
   used by the signer.  As an example, if the creator of the message
   used SHA-1 as the digest algorithm to hash the message content, then
   the attacker looks for a different hash algorithm that produces a
   result that is of the same length, but with which it is easier to
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