Guidelines for Cryptographic Algorithm Agility and Selecting Mandatory-to-Implement Algorithms
RFC 7696

Document Type RFC - Best Current Practice (November 2015; No errata)
Also known as BCP 201
Was draft-iab-crypto-alg-agility (individual in sec area)
Last updated 2015-11-18
Replaces draft-housley-crypto-alg-agility
Stream IETF
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Stream WG state Submitted to IESG for Publication
Document shepherd Ted Hardie
Shepherd write-up Show (last changed 2015-08-17)
IESG IESG state RFC 7696 (Best Current Practice)
Consensus Boilerplate Yes
Telechat date
Responsible AD Stephen Farrell
Send notices to (None)
IANA IANA review state Version Changed - Review Needed
IANA action state No IC
Internet Engineering Task Force (IETF)                        R. Housley
Request for Comments: 7696                                Vigil Security
BCP: 201                                                   November 2015
Category: Best Current Practice
ISSN: 2070-1721

             Guidelines for Cryptographic Algorithm Agility
            and Selecting Mandatory-to-Implement Algorithms

Abstract

   Many IETF protocols use cryptographic algorithms to provide
   confidentiality, integrity, authentication, or digital signature.
   Communicating peers must support a common set of cryptographic
   algorithms for these mechanisms to work properly.  This memo provides
   guidelines to ensure that protocols have the ability to migrate from
   one mandatory-to-implement algorithm suite to another over time.

Status of This Memo

   This memo documents an Internet Best Current Practice.

   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
   BCPs 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/rfc7696.

Copyright Notice

   Copyright (c) 2015 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.

Housley                   Best Current Practice                 [Page 1]
RFC 7696        Guidelines for Cryptographic Alg Agility   November 2015

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Algorithm Agility Guidelines . . . . . . . . . . . . . . . . .  3
     2.1.  Algorithm Identifiers  . . . . . . . . . . . . . . . . . .  4
     2.2.  Mandatory-to-Implement Algorithms  . . . . . . . . . . . .  5
       2.2.1.  Platform Specifications  . . . . . . . . . . . . . . .  5
       2.2.2.  Cryptographic Key Size . . . . . . . . . . . . . . . .  5
       2.2.3.  Providing Notice of Expected Changes . . . . . . . . .  6
     2.3.  Transitioning from Weak Algorithms . . . . . . . . . . . .  6
     2.4.  Algorithm Transition Mechanisms  . . . . . . . . . . . . .  7
     2.5.  Cryptographic Key Management . . . . . . . . . . . . . . .  8
     2.6.  Preserving Interoperability  . . . . . . . . . . . . . . .  8
     2.7.  Balancing Security Strength  . . . . . . . . . . . . . . .  9
     2.8.  Balancing Protocol Complexity  . . . . . . . . . . . . . . 10
     2.9.  Opportunistic Security . . . . . . . . . . . . . . . . . . 10
   3.  Cryptographic Algorithm Specifications . . . . . . . . . . . . 11
     3.1.  Choosing Mandatory-to-Implement Algorithms . . . . . . . . 11
     3.2.  Too Many Choices Can Be Harmful  . . . . . . . . . . . . . 12
     3.3.  Picking One True Cipher Suite Can Be Harmful . . . . . . . 13
     3.4.  National Cipher Suites . . . . . . . . . . . . . . . . . . 14
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
   6.  Normative References . . . . . . . . . . . . . . . . . . . . . 16
   7.  Informative References . . . . . . . . . . . . . . . . . . . . 16
   Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . 19
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 19

1.  Introduction

   Many IETF protocols use cryptographic algorithms to provide
   confidentiality, integrity, authentication, or digital signature.
   For interoperability, communicating peers must support a common set
   of cryptographic algorithms.  In most cases, a combination of
   compatible cryptographic algorithms will be used to provide the
   desired security services.  The set of cryptographic algorithms being
   used at a particular time is often referred to as a cryptographic
   algorithm suite or cipher suite.  In a protocol, algorithm
   identifiers might name a single cryptographic algorithm or a full
   suite of algorithms.

   Cryptographic algorithms age; they become weaker with time.  As new
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