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Versions: (draft-housley-crypto-alg-agility)  00   Best Current Practice
          01 02 03 04 05 06 07 08 rfc7696                               
Internet-Draft                                                R. Housley
Intended Status: Best Current Practice                    Vigil Security
Expires: 6 December 2015                                     4 June 2015

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


   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 Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

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Copyright and License Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors. All rights reserved.

Housley                                                         [Page 1]

Guidelines for Cryptographic Algorithm Agility                 June 2015

   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.

0.  TO DO

   Three comments have not been resolved yet ...

   (1) There is a minor organizational issue.  It looks like part of
   Section 3 could be merged with Section 2.  (I got initially confused
   by some ambiguous advice in 2.1 that was cleared up in 3.1).  Some of
   Section 3 belongs with the text in Section 4.  Specifically, 3.3 is
   really tied quite closely to 4.2, 4.3, and 4.4.

   (2) Nowhere does the document discuss directly the problem of long-
   term signatures in certificates hindering the dropping of weak
   signature algorithms and key lengths, even though this is an example
   that many people are familiar with.  If a CA signed a long-term cert
   with a signature using RSA-MD5, you can't easily drop MD5 support,
   you have to try to use it only in the exactly right places, which
   leads to errors.

   (3) The note in Section 3.2 on Performance belongs elsewhere, likely
   in the section on strength.

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
   cryptanalysis techniques are developed and computing capabilities
   improve, the work factor to break a particular cryptographic
   algorithm will reduce or become more feasible for more attackers.

Housley                                                         [Page 2]

Guidelines for Cryptographic Algorithm Agility                 June 2015

   Algorithm agility is achieved when a protocol can easily migrate from
   one algorithm suite to another more desirable one, over time.  For
   the protocol implementer, this means that implementations should be
   modular to easily accommodate the insertion of new algorithms or
   suites of algorithms.  Ideally, implementations will also provide a
   way to measure when deployed implementations have shifted away from
   the old algorithms and to the better ones.  For the protocol
   designer, algorithm agility means that one or more algorithm
   identifier must be supported, the set of mandatory-to-implement
   algorithms will change over time, and an IANA registry of algorithm
   identifiers will be needed.

   Algorithm identifiers by themselves are not sufficient to ensure easy
   migration.  Action by people that maintain implementations and
   operate services is needed to develop, deploy, and adjust
   configuration settings to enable the new more desirable algorithms
   and to deprecate or disable older, less desirable ones.  For various
   reasons, most notably interoperability concerns, experience has shown
   that it has proven difficult for implementors and administrators to
   remove or disable weak algorithms.  Further, the inability of legacy
   systems and resource-constrained devices to support new algorithms
   adds to those concerns.  As a result, people live with weaker
   algorithms, sometimes seriously flawed ones, well after experts
   recommend migration.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  Algorithm Agility Guidelines

   These guidelines are for use by IETF working groups and protocol
   authors for IETF protocols that make use of cryptographic algorithms.

2.1.  Algorithm Identifiers

   IETF protocols that make use of cryptographic algorithms MUST support
   one or more algorithm or suite identifier.  The identifier might be
   explicitly carried in the protocol.  Alternatively, it can configured
   by a management mechanism.  For example, an entry in a key table that
   includes a key value and an algorithm identifier might be sufficient.

   Some approaches carry one identifier for each algorithm that is used.
   Other approaches carry one identifier for a full suite of algorithms.
   Both approaches are used in IETF protocols.  Designers are encouraged
   to pick one of these approaches and use it consistently throughout

Housley                                                         [Page 3]

Guidelines for Cryptographic Algorithm Agility                 June 2015

   the protocol or family of protocols.  Suite identifiers make it
   easier for the protocol designer to ensure that the algorithm
   selections are complete and compatible for future assignments.
   However, suite identifiers inherently face a combinatoric explosion
   as new algorithms are defined.  Algorithm identifiers, on the other
   hand, impose a burden on implementations by forcing a determination
   at run-time regarding which algorithm combinations are acceptable.

   Regardless of the approach used, protocols historically negotiate the
   symmetric cipher and cipher mode together to ensure that they are
   completely compatible.

   In the IPsec protocol suite, IKEv2 [RFC7296] carries the algorithm
   identifiers for AH [RFC4302] and ESP [RFC4303].  Such separation is a
   completely fine design choice.  In contrast, TLS [RFC5246] carries
   cipher suite identifiers, which is also a completely fine design

   An IANA registry SHOULD be used for these algorithm or suite
   identifiers.  Once an algorithm identifier is added to the registry,
   it should not be changed or removed.  However, it is desirable to
   mark a registry entry as deprecated when implementation is no longer

2.2.  Mandatory-to-Implement Algorithms

   For secure interoperability, BCP 61 [RFC3365] recognizes that
   communicating peers that use cryptographic mechanisms must support a
   common set of strong cryptographic algorithms.  For this reason, the
   protocol MUST specify one or more strong mandatory-to-implement
   algorithm or suite.  This does not require all deployments to use
   this algorithm or suite, but it does require that are available to
   all deployments.

   The IETF needs to be able to change the mandatory-to-implement
   algorithms over time.  It is highly desirable to make this change
   without updating the base protocol specification.  To achieve this
   goal, the base protocol specification includes a reference to a
   companion algorithms document, allowing the update of one document
   without necessarily requiring an update to the other.  This division
   also facilitates the advancement of the base protocol specification
   on the standards maturity ladder even if the algorithm document
   changes frequently.

   The IETF SHOULD keep the set of mandatory-to-implement algorithms
   small.  To do so, the set of algorithms will necessarily change over
   time, and the transition SHOULD happen before the algorithms in the
   current set have weakened to the breaking point.

Housley                                                         [Page 4]

Guidelines for Cryptographic Algorithm Agility                 June 2015

2.2.1.  Platform Specifications

   Note that mandatory-to-implement algorithms or suites are not
   specified for protocols that are embedded in other protocols; in
   these cases the system-level protocol specification identifies the
   mandatory-to-implement algorithm or suite.  For example, S/MIME
   [RFC5751] makes use of the cryptographic message Syntax (CMS)
   [RFC5652], and S/MIME specifies the mandatory-to-implement
   algorithms, not CMS.  This approach allows other protocols can make
   use of CMS and make different mandatory-to-implement algorithm

2.2.2.  Cryptographic Key Size

   Some cryptographic algorithms are inherently tied to a specific key
   size, but others allow many different key sizes.  Likewise, some
   algorithms support parameters of different sizes, such as integrity
   check values or nonces.  The algorithm specification MUST identify
   the specific key sizes and parameter sizes that are to be supported.
   When more than one key size is available, expect the mandatory-to-
   implement key size to increase over time.

   Guidance on cryptographic key size for asymmetric keys can be found
   in BCP 86 [RFC3766].

   Guidance on cryptographic key size for symmetric keys can be found in
   BCP 195 [RFC7525].

2.3.  Transition from Weak Algorithms

   Transition from an old algorithm that is found to be weak can be
   tricky.  It is of course straightforward to specify the use of a new,
   better algorithm.  And then, when the new algorithm is widely
   deployed, the old algorithm ought no longer be used.  However,
   knowledge about the implementation and deployment of the new
   algorithm will always be imperfect, so one cannot be completely
   assured of interoperability with the new algorithm.

   Algorithm transition is naturally facilitated as part of an algorithm
   selection or negotiation mechanism.  Protocols traditionally select
   the best algorithm or suite that is supported by all communicating
   peers and acceptable by their policies.  In addition, a mechanism is
   needed to determine whether the new algorithm has been deployed.  For
   example, SMIMECapabilities [RFC5751] allows S/MIME mail user agents
   to share the list of algorithms that they are willing to use in
   preference order.  For another example, the DNSSEC EDNS0 option
   [RFC6975] measures the acceptance and use of new digital signing

Housley                                                         [Page 5]

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   In the Resource Public Key Infrastructure (RPKI), a globally-
   recognized digital signature is needed.  BCP 182 [RFC6916] provides
   an approach to transition where a second signature algorithm is
   introduced and then the original one is phased out.

   In the worst case, the old algorithm may be found to be tragically
   flawed, permitting a casual attacker to download a simple script to
   break it.  Sadly, this has happened when a secure algorithm is used
   incorrectly or used with poor key management, resulting in a weak
   cryptographic algorithm suite.  In such situations, the protection
   offered by the algorithm is severely compromised, perhaps to the
   point that one wants to stop using the weak suite altogether,
   rejecting offers to use the weak suite well before the new suite is
   widely deployed.

   In any case, there comes a point in time where one refuses to use the
   old, weak algorithm or suite.  This can happen on a flag day, or each
   installation can select a date on their own.

2.4.  Balance Security Strength

   When selecting or negotiating a suite of cryptographic algorithms,
   the strength of each algorithm SHOULD be considered.  The algorithms
   in a suite SHOULD be roughly equal; however, the security service
   provided by each algorithm in a particular context needs to be
   considered in making the selection.  Algorithm strength needs to be
   considered at the time a protocol is designed.  It also needs to be
   considered at the time a protocol implementation is deployed and
   configured.  Advice from from experts is useful, but in reality, it
   is not often available to system administrators that are deploying
   and configuring a protocol implementation.  For this reason, protocol
   designers SHOULD provide clear guidance to implementors, leading to
   balanced options being available at the time of deployment and

   Some algorithms offer flexibility in their strength by adjusting the
   key size, number of rounds, authentication tag size, prime group
   size, and so on.  For example, cipher suites include Diffie-Hellman
   or RSA without specifying a particular public key length.  If the
   algorithm identifier or suite identifier named a particular public
   key length, migration to longer ones would be more difficult.  On the
   other hand, inclusion of a public key length would make it easier to
   migrate away from short ones when computational resources available
   to attacker dictate the need to do so.  Therefore, flexibility on
   asymmetric key length is both desirable and undesirable at the same

   In CMS [RFC5652], a previously distributed symmetric key-encryption

Housley                                                         [Page 6]

Guidelines for Cryptographic Algorithm Agility                 June 2015

   key can be used to encrypt a content-encryption key, which is in turn
   used to encrypt the content.  The key-encryption and content-
   encryption algorithms are often different.  If, for example, a
   message content is encrypted with 128-bit AES key and the content-
   encryption key is wrapped with a 256-bit AES key, then at most 128
   bits of protection is provided.  In this situation, the algorithm and
   key size selections should ensure that the key encryption is at least
   as strong as the content encryption.  In general, wrapping one key
   with another key of a different size yields the security strength of
   the shorter key.

2.5.  Opportunistic Security

   Despite the guidance in Section 2.4, opportunistic security [RFC7435]
   SHOULD also be considered, especially at the time a protocol
   implementation is deployed and configured.  Using algorithms that are
   weak against advanced attackers but sufficient against others is a
   way to make pervasive surveillance significantly more difficult.  As
   a result, algorithms that would not be acceptable in many negotiated
   situations are acceptable for opportunistic security.  Similarly,
   weaker algorithms and shorter key sizes are also acceptable for
   opportunistic security.  That said, the use of strong algorithms is
   always preferable.

3.  Algorithm Agility in Protocol Design

   Some attempts at algorithm agility have not been completely
   successful.  This section provides some of the insights based on
   protocol designs and deployments.

3.1.  Algorithm Identifiers

   If a protocol does not carry an algorithm identifier, then the
   protocol version number or some other major change is needed to
   transition from one algorithm to another.  The inclusion of an
   algorithm identifier is a minimal step toward cryptographic algorithm
   agility.  In addition, an IANA registry is needed to pair the
   identifier with an algorithm specification.

   Sometimes a combination of protocol version number and explicit
   algorithm or suite identifiers is appropriate.  For example, the TLS
   version number names the default key derivation function and the
   cipher suite identifier names the rest of the needed algorithms.

   Sometimes application layer protocols can make use of transport layer
   security protocols, such as TLS or DTLS.  This insulates the
   application layer protocol from the details of cryptography, but it
   is likely to still be necessary to handle the transition from

Housley                                                         [Page 7]

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   unprotected traffic to protected traffic in the application layer
   protocol.  In addition, the application layer protocol may need to
   handle the downgrade from encrypted communication to plaintext

3.2.  Migration Mechanisms

   Cryptographic algorithm selection or negotiation SHOULD be integrity
   protected.  If selection is not integrity protected, then the
   protocol will be subject to a downgrade attack.  Without integrity
   protection of algorithm or suite selection, the attempt to transition
   to a new algorithm or suite may introduce new opportunities for
   downgrade attack.

   If a protocol specifies a single mandatory-to-implement integrity
   algorithm, eventually that algorithm will be found to be weak.

   Extra care is needed when a mandatory-to-implement algorithm is used
   to provide integrity protection for the negotiation of other
   cryptographic algorithms.  In this situation, a flaw in the
   mandatory-to-implement algorithm may allow an attacker to influence
   the choices of the other algorithms.

   Performance is always a factor is selecting cryptographic algorithms.
   In many algorithms, shorter keys offer higher performance, but less
   security.  Performance and security need to be balanced.  Yet, all
   algorithms age, and the advances in computing power available to the
   attacker will eventually make any algorithm obsolete.  For this
   reason, protocols need mechanisms to migrate from one algorithm suite
   to another over time, including the algorithm used to provide
   integrity protection for algorithm negotiation.

3.3.  Preserving Interoperability

   Cryptographic algorithm deprecation is very hard.  People do not like
   to introduce interoperability problems, even to preserve security.
   As a result, flawed algorithms are supported for far too long.  The
   impacts of legacy software an long support tails on security can be
   reduced by making it easy to rollover from old algorithms and suites
   to new ones.

   Without clear mechanisms for algorithm and suite rollover, preserving
   interoperability becomes a difficult social problem.  For example,
   consider web browsers.  Dropping support for an algorithm suite can
   break connectivity to some web sites because, and the browser will
   lose users by doing so.  This situation creates incentives to support
   algorithm suites that would otherwise be deprecated, but preserving

Housley                                                         [Page 8]

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   Institutions, being large or dominate users within a large user base,
   can assist by coordinating the demise of an algorithm suite, making
   the rollover easier for their own users as well as others.

3.4.  Cryptographic Key Management

   Traditionally, protocol designers have avoided more than one approach
   to key management because it makes the security analysis of the
   overall protocol more difficult.  When frameworks such as EAP and
   GSSAPI are employed, the key management is very flexible, often
   hiding many of the details from the application.  This results in
   protocols that support multiple key management approaches.  In fact,
   the key management approach itself may be negotiable, which creates a
   design challenge to protect the negotiation of the key management
   approach before it is used to produce cryptographic keys.

   Protocols can negotiate a key management approach, derive an initial
   cryptographic key, and then authenticate the negotiation.  However,
   if the authentication fails, the only recourse is to start the
   negotiation over from the beginning.

   Some environments will restrict the key management approaches by
   policy.  Such policies tend to improve interoperability within a
   particular environment, but they cause problems for individuals that
   need to work in multiple incompatible environments.

4.  Cryptographic Algorithm Specifications

   There are tradeoffs between the number of cryptographic algorithms
   that are supported, time to deploy a new algorithm, and protocol
   complexity.  This section provides some of the insights about the
   tradeoff faced by protocol designers.

   Ideally, two independent sets of mandatory-to-implement algorithms
   will be specified, allowing for a primary suite and a secondary
   suite.  This approach ensures that the secondary suite is widely
   deployed if a flaw is found in the primary one.

4.1.  Choosing Mandatory-to-Implement Algorithms

   It seems like the ability to use an algorithm of one's own choosing
   is very desirable; however, the selection is often better left to
   experts.  Further, any and all cryptographic algorithm choices ought
   not be available in every implementation.  Mandatory-to-implement
   algorithms ought to have a public stable specification and public
   documentation that it has been well studied, giving rise to
   significant confidence.  The IETF has alway had a preference for
   unencumbered algorithms.  There are significant benefits in selecting

Housley                                                         [Page 9]

Guidelines for Cryptographic Algorithm Agility                 June 2015

   algorithms and suites that are widely deployed.  The selected
   algorithms need to be resistant to side-channel attacks as well as
   meeting the performance, power, and code size requirements on a wide
   variety of platforms.  In addition, inclusion of too many
   alternatives may add complexity to algorithm selection or

   Section 4.1 neglects to mention the obvious benefit in selecting
   something that lots of other people are using too.  That's a non-
   trivial advantage over and above the reasons cited (though many of
   them might be considered different ways of saying the same thing).

   Sometime more than one mandatory-to-implement algorithm is needed to
   increase the likelihood of interoperability among a diverse
   population.  For example, authenticated encryption is provided by
   AES-CCM [RFC3610] and AES-GCM [GCM].  Both of these algorithms are
   considered to be secure.  AES-CCM is available in hardware used by
   many small devices, and AES-GCM is parallelizable and well suited
   high-speed devices.  Therefore an application needing authenticated
   encryption might specify one of these algorithms or both of these
   algorithms, depending of the population.

4.2.  Too Many Choices Can Be Harmful

   It is fairly easy to specify the use of any arbitrary cryptographic
   algorithm, and once the specification is available, the algorithm
   gets implemented and deployed.  Some people say that the freedom to
   specify algorithms independently from the rest of the protocol has
   lead to the specification of too many cryptographic algorithms.  Once
   deployed, even with moderate uptake, it is quite difficult to remove
   algorithms because interoperability with some party will be impacted.
   As a result, weaker ciphers stick around far too long.  Sometimes
   implementors are forced to maintain cryptographic algorithm
   implementations well beyond their useful lifetime.

   In order to manage the proliferation of algorithm choices and provide
   an expectation of interoperability, many protocols specify mandatory-
   to-implement algorithms or suites.  All implementors are expected to
   support the mandatory-to-implement cryptographic algorithm, and they
   can include any others algorithms that they desire.  The mandatory-
   to-implement algorithms are chosen to be highly secure and follow the
   guidance in RFC 1984 [RFC1984].  Of course, many other factors,
   including intellectual property rights, have an impact on the
   cryptographic algorithms that are selected by the community.
   Generally, the mandatory-to-implement algorithms ought to be
   preferred, and the other algorithms ought to be selected only in
   special situations.  However, it can be very difficult for a skilled
   system administrator to determine the proper configuration to achieve

Housley                                                        [Page 10]

Guidelines for Cryptographic Algorithm Agility                 June 2015

   these preferences.

   In some cases, more than one mandatory-to-implement cryptographic
   algorithm has been specified.  This is intended to ensure that at
   least one secure cryptographic algorithm will be available, even if
   other mandatory-to-implement algorithms are broken.  To achieve this
   goal, the selected algorithms must be diverse, so that a
   cryptoanalytic advance against one of the algorithms does not also
   impact the other selected algorithms.  The idea is to have an
   implemented and deployed algorithm as a fallback.  However, all of
   the selected algorithms need to be routinely exercised to ensure
   quality implementation.  This is not always easy to do, especially if
   the various selected algorithms require different credentials.
   Obtaining multiple credentials for the same installation is an
   unacceptable burden on system administrators.  Also, the manner by
   which system administrators are advised to switch algorithms or
   suites is at best ad hoc, and at worst entirely absent.

4.3.  Picking One True Cipher Suite Can Be Harmful

   In the past, protocol designers have chosen one cryptographic
   algorithm or suite, and then tied many protocol details to that
   selection.  Plan for algorithm transition, either because a mistake
   is made in the initial selection or because the protocol is
   successfully used for a long time and the algorithm becomes week with
   age.  Either way, the design should enable transition.

   Protocol designers are sometimes mislead by the simplicity that
   results from selecting one true algorithm or suite.  Since algorithms
   age, the selection cannot be stable forever.  Even the most simple
   protocol needs a version number to signal which algorithm that is
   being used.  This approach has at least two desirable consequences.
   First, the protocol is simpler because there is no need for algorithm
   negotiation.  Second, system administrators do not need to make any
   algorithm-related configuration decisions.  However, the only way to
   respond to news that the an algorithm that is part of the one true
   cipher suite has been broken is to update the protocol specification
   to the next version, implement the new specification, and then get it

   The first IEEE 802.11 [WiFi] specification included the Wired
   Equivalent Privacy (WEP) as the only encryption technique.  Many of
   the protocol details were driven by the selected algorithm.  WEP was
   found to be quite weak [WEP], and a very large effort was needed to
   specify, implement, and deploy the alternative encryption techniques.
   This effort was made even harder by the protocol design choices that
   were tied to the initial algorithm selection and the desire for
   backward compatibility.

Housley                                                        [Page 11]

Guidelines for Cryptographic Algorithm Agility                 June 2015

   Experience with the transition from SHA-1 to SHA-256 indicates that
   the time from protocol specificate to widespread use takes more than
   five years.  In this case, the protocol specifications and
   implementation were straightforward and fairly prompt.  In many
   software products, the new algorithm was not considered an update to
   existing release, so the roll out of the next release, subsequent
   deployment, and finally adjustment of the configuration by system
   administrators took many years.  In many consumer hardware products,
   firmware to implement the new algorithm were difficult to locate and
   install, or the were simply not available.  Further, infrastructure
   providers were unwilling to make the transition until all of their
   potential clients were able to use the new algorithm.

4.4.  National Cipher Suites

   Some nations specify cryptographic algorithms, and then require their
   use through legislation or regulations.  These algorithms may not
   have wide public review, and they can have limited reach of
   deployments.  Yet, the legislative or regulatory mandate creates a
   captive market.  As a result, the use of such algorithms get
   specified, implemented, and deployed.  The default server or
   responder configuration SHOULD disable such algorithms; in this way,
   explicit action by the system administrator is needed to enable them
   where they are actually required.  For tiny devices with no user
   interface, an administrator action may only be possible at the time
   the device is purchased.

   National algorithms can force an implementer to produce several
   incompatible product releases for a different countries or regions,
   which has significantly greater cost over development of a product
   using a globally-acceptable algorithm.  This situation could be even
   worse if the various national algorithms impose different
   requirements on the protocol, its key management, or its use of
   random values.

4.5.  Balance Protocol Complexity

   Protocol designers MUST be prepared for the supported cryptographic
   algorithm set to change over time.  As shown by the discussion in the
   previous two sections, there is a spectrum of ways to enable the

   Keep implementations as simple as possible.  Complex protocol
   negotiation provides opportunities for attack, such as downgrade
   attacks.  Support for many algorithm alternatives is also harmful, as
   discussed in Section 4.1.  Both of these can lead to portions of the
   implementation that are rarely used, increasing the opportunity for
   undiscovered exploitable implementation bugs.

Housley                                                        [Page 12]

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4.6.  Providing Notice

   Fortunately, catastrophic algorithm failures without warning are
   rare.  More often, algorithm transition is the result of age.  For
   example, the transition from DES to Triple-DES to AES took place over
   decades, causing a shift in symmetric block cipher strength from 56
   bits to 112 bits to 128 bits.  Where possible, authors SHOULD provide
   notice to implementers about expected algorithm transitions.  One
   approach that was first used in RFC 4307 [RFC4307] is to use SHOULD+,
   SHOULD-, and MUST- in the specification of algorithms.

      SHOULD+  This term means the same as SHOULD.  However, it is
               likely that an algorithm marked as SHOULD+ will be
               promoted to a MUST in the future.

      SHOULD-  This term means the same as SHOULD.  However, it is
               likely that an algorithm marked as SHOULD- will be
               deprecated to a MAY or worse in the future.

      MUST-    This term means the same as MUST.  However, it is
               expected that an algorithm marked as MUST- will be
               downgraded in the future.  Although the status of the
               algorithm will be determined at a later time, it is
               reasonable to expect that a the status of a MUST-
               algorithm will remain at least a SHOULD or a SHOULD-.

5.  Security Considerations

   This document provides guidance to working groups and protocol
   designers.  The security of the Internet is improved when broken or
   weak cryptographic algorithms can be easily replaced with strong

   From a software development and maintenance perspective,
   cryptographic algorithms can often be added and removed without
   making changes to surrounding data structures, protocol parsing
   routines, or state machines.  This approach separates the
   cryptographic algorithm implementation from the rest of the code,
   which makes it easier to tackle special security concerns such as key
   exposure and constant-time execution.

   The situation is different for hardware, for both tiny devices and
   very high-end data center equipment.  Many tiny devices do not
   include the ability to update the firmware at all.  Even if the
   firmware can be updated, tiny devices are often deployed in places
   that make it very inconvenient to do so.  High-end data center
   equipment may use special-purpose chips to achieve very high
   performance, which means that board-level replacement may be needed

Housley                                                        [Page 13]

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   to change the algorithm.  Cost and down-time are both factors in such
   an upgrade.

   In most cases, the cryptographic algorithm remains strong, but an
   attack is found against the way that the strong algorithm is used in
   a particular protocol.  In these cases, a protocol change will
   probably be needed.  For example, the order of cryptographic
   operations in the TLS protocol has evolved as various attacks have
   been discovered.  Originally, TLS performed encryption after
   computation of the message authentication code (MAC).  This order of
   operations is called MAC-then-encrypt, which actually involves MAC
   computation, padding, and then encryption.  This is no longer
   considered secure [BN][K].  As a result, a mechanism was specified to
   use encrypt-then-MAC instead [RFC7366].  Future versions of TLS are
   expected to use exclusively authenticated encryption algorithms
   [RFC5166], which should resolve the ordering discussion altogether.
   After discovery of such attacks, updating the cryptographic
   algorithms is not likely to be sufficient to thwart the new attack.
   It may necessary to make significant changes to the protocol.

   Some protocols are used to protected stored data.  For example,
   S/MIME [RFC5751] can protect a message kept in a mailbox.  To recover
   the protected stored data, protocol implementations need to support
   older algorithms, even when they no longer use the older algorithms
   for the protection of new stored data.

   Support for too many algorithms can lead to implementation
   vulnerabilities.  When many algorithms are supported, some of them
   will be rarely used.  Any code that is rarely used can contain
   undetected bugs, and algorithm implementations are no different.
   Measurements SHOULD be used to determine whether implemented
   algorithms are actually being used, and if they are not, future
   releases should remove them.  In addition, unused algorithms or
   suites SHOULD be marked as deprecated in the IANA registry.  In
   short, eliminate the cruft.

   Section 2.3 talks about algorithm transition without considering any
   other aspects of the protocol design.  In practice, there are
   dependencies between the cryptographic algorithm and other aspects of
   the protocol.  For example, the BEAST attack [BEAST] against TLS
   [RFC5246] caused many sites to turn off modern cryptographic
   algorithms in favor of older and clearly weaker algorithms.

   Mechanisms for timely update of devices are needed to deploy a
   replacement algorithm or suite.  It takes a long time to specify,
   implement, and deploy a replacement, therefore the transition process
   needs to begin when practically exploitable flaws become known.  The
   update processes on some devices involve certification, which further

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Guidelines for Cryptographic Algorithm Agility                 June 2015

   increases the time to deploy a replacement.  For example, devices
   that are part of health or safety systems often require certification
   before deployment.  Embedded systems and SCADA systems often have
   upgrade cycles stretching over many years, leading to similar time to
   deployment issues.  Prompt action is needed if a replacement has any
   hope of being deployed before exploitation techniques become widely
   available exploits.

6.  IANA Considerations

   This document does not establish any new IANA registries, nor does it
   add any entries to existing registries.

   This document does RECOMMEND a convention for new registries for
   cryptographic algorithm or suite identifiers.  Once an algorithm or
   suite identifier is added to the registry, it SHOULD NOT be changed
   or removed.  However, it is desirable to include a means of marking a
   registry entry as deprecated when implementation is no longer

7.  Normative References

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For Public
             Keys Used For Exchanging Symmetric Keys", BCP 86, RFC 3766,
             April 2004.

8.  Informative References

   [BEAST]   http://en.wikipedia.org/wiki/

   [BN]      Bellare, M. and C. Namprempre, "Authenticated Encryption:
             Relations among notions and analysis of the generic
             composition paradigm", Proceedings of AsiaCrypt '00,
             Springer-Verlag LNCS No. 1976, p. 531, December 2000.

   [GCM]     Dworkin, M, "Recommendation for Block Cipher Modes of
             Operation: Galois/Counter Mode (GCM) and GMAC", NIST
             Special Publication 800-30D, November 2007.

   [K]       Krawczyk, H., "The Order of Encryption and Authentication
             for Protecting Communications (or: How Secure Is SSL?)",
             Proceedings of Crypto '01, Springer-Verlag LNCS No. 2139,
             p. 310, August 2001.

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Guidelines for Cryptographic Algorithm Agility                 June 2015

   [RFC1984] IAB and IESG, "IAB and IESG Statement on Cryptographic
             Technology and the Internet", RFC 1984, August 1996.

   [RFC3365] Schiller, J., "Strong Security Requirements for Internet
             Engineering Task Force Standard Protocols", BCP 61, RFC
             3365, August 2002.

   [RFC3610]  Whiting, D., Housley, R., and N. Ferguson, "Counter with
             CBC-MAC (CCM)", RFC 3610, September 2003.

   [RFC4302] Kent, S., "IP Authentication Header", RFC 4302, December

   [RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
             RFC 4303, December 2005.

   [RFC4307] Schiller, J., "Cryptographic Algorithms for Use in the
             Internet Key Exchange Version 2 (IKEv2)", RFC 4307,
             December 2005.

   [RFC5166] Floyd, S., Ed., "Metrics for the Evaluation of Congestion
             Control Mechanisms", RFC 5166, March 2008.

   [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
             (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
             RFC 5652, September 2009.

   [RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
             Mail Extensions (S/MIME) Version 3.2 Message
             Specification", RFC 5751, January 2010.

   [RFC6916] Gagliano, R., Kent, S., and S. Turner, "Algorithm Agility
             Procedure for the Resource Public Key Infrastructure
             (RPKI)", BCP 182, RFC 6916, April 2013.

   [RFC6975] Crocker, S. and S. Rose, "Signaling Cryptographic Algorithm
             Understanding in DNS Security Extensions (DNSSEC)",
             RFC 6975, July 2013.

   [RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
             Kivinen, "Internet Key Exchange Protocol Version 2
             (IKEv2)", STD 79, RFC 7296, October 2014.

   [RFC7366] Gutmann, P., "Encrypt-then-MAC for Transport Layer Security
             (TLS) and Datagram Transport Layer Security (DTLS)",
             RFC 7366, September 2014.

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Guidelines for Cryptographic Algorithm Agility                 June 2015

   [RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection Most
             of the Time", RFC 7435, December 2014.

   [RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre, "Recommendations
             for Secure Use of Transport Layer Security (TLS) and
             Datagram Transport Layer Security (DTLS)", RFC 7525,
             BCP 195, May 2015.

   [WEP]     http://en.wikipedia.org/wiki/Wired_Equivalent_Privacy

   [WiFi]    IEEE , "Wireless LAN Medium Access Control (MAC) And
             Physical Layer (PHY) Specifications, IEEE Std 802.11-1997,


   Thanks to Bernard Aboba, Derek Atkins, David Black, Randy Bush, Jon
   Callas, Andrew Chi, Steve Crocker, Viktor Dukhovni, Stephen Farrell,
   Tony Finch, Ian Grigg, Peter Gutmann, Wes Hardaker, Joe Hildebrand,
   Paul Hoffman, Phillip Hallam-Baker, Christian Huitema, Watson Ladd,
   Paul Lambert, Ben Laurie, Eliot Lear, Nikos Mavrogiannopoulos, Yoav
   Nir, Rich Salz, Rene Struik, Kristof Teichel, Martin Thompson,
   Jeffrey Walton, Nico Williams, and Peter Yee for their review and
   insightful comments.  While some of these people do not agree with
   some aspects of this document, the discussion that resulted for their
   comments has certainly resulted in a better document.

Author's Address

   Russ Housley
   Vigil Security, LLC
   918 Spring Knoll Drive
   Herndon, VA 20170
   EMail: housley@vigilsec.com

Housley                                                        [Page 17]