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Internet Engineering Task Force (IETF)                        R. HousleyRequest for Comments: 7696                                Vigil SecurityBCP: 201                                                   November 2015Category: Best Current PracticeISSN: 2070-1721Guidelines for Cryptographic Algorithm Agilityand Selecting Mandatory-to-Implement AlgorithmsAbstract   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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://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 toBCP 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 2015Table of Contents1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .21.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .32.  Algorithm Agility Guidelines . . . . . . . . . . . . . . . . .32.1.  Algorithm Identifiers  . . . . . . . . . . . . . . . . . .42.2.  Mandatory-to-Implement Algorithms  . . . . . . . . . . . .52.2.1.  Platform Specifications  . . . . . . . . . . . . . . .52.2.2.  Cryptographic Key Size . . . . . . . . . . . . . . . .52.2.3.  Providing Notice of Expected Changes . . . . . . . . .62.3.  Transitioning from Weak Algorithms . . . . . . . . . . . .62.4.  Algorithm Transition Mechanisms  . . . . . . . . . . . . .72.5.  Cryptographic Key Management . . . . . . . . . . . . . . .82.6.  Preserving Interoperability  . . . . . . . . . . . . . . .82.7.  Balancing Security Strength  . . . . . . . . . . . . . . .92.8.  Balancing Protocol Complexity  . . . . . . . . . . . . . .102.9.  Opportunistic Security . . . . . . . . . . . . . . . . . .103.  Cryptographic Algorithm Specifications . . . . . . . . . . . .113.1.  Choosing Mandatory-to-Implement Algorithms . . . . . . . .113.2.  Too Many Choices Can Be Harmful  . . . . . . . . . . . . .123.3.  Picking One True Cipher Suite Can Be Harmful . . . . . . .133.4.  National Cipher Suites . . . . . . . . . . . . . . . . . .144.  Security Considerations  . . . . . . . . . . . . . . . . . . .145.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .166.  Normative References . . . . . . . . . . . . . . . . . . . . .167.  Informative References . . . . . . . . . . . . . . . . . . . .16   Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . .19   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . .191.  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 required to break a particular cryptographic   algorithm will reduce, making an attack on the algorithm more   feasible for more attackers.  While it is unknown how cryptoanalytic   attacks will evolve, it is certain that they will get better.  It isHousley                   Best Current Practice                 [Page 2]

RFC 7696        Guidelines for Cryptographic Alg Agility   November 2015   unknown how much better they will become or when the advances will   happen.  Protocol designers need to assume that advances in computing   power or advances in cryptoanalytic techniques will eventually make   any algorithm obsolete.  For this reason, protocols need mechanisms   to migrate from one algorithm suite to another over time.   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 or suite   identifiers 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 implementers 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",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   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.   Past attempts at algorithm agility have not been completely   successful, and this section provides some insights from those   experiences.Housley                   Best Current Practice                 [Page 3]

RFC 7696        Guidelines for Cryptographic Alg Agility   November 20152.1.  Algorithm Identifiers   IETF protocols that make use of cryptographic algorithms MUST support   one or more algorithms or suites.  The protocol MUST include a   mechanism to identify the algorithm or suite that is being used.  An   algorithm identifier might be explicitly carried in the protocol.   Alternatively, a management mechanism can be used to identify the   algorithm.  For example, an entry in a key table that includes a key   value and an algorithm identifier might be sufficient.   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.   Sometimes a combination of protocol version number and explicit   algorithm or suite identifiers is appropriate.  For example, the   Transport Layer Security (TLS) [RFC5246] version number names the   default key derivation function, and the cipher suite identifier   names the rest of the needed algorithms.   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   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   compatible.   In the IPsec protocol suite, the Internet Key Exchange Protocol   version 2 (IKEv2) [RFC7296] carries the algorithm identifiers for the   Authentication Header (AH) [RFC4302] and the Encapsulating Security   Payload (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 choice.Housley                   Best Current Practice                 [Page 4]

RFC 7696        Guidelines for Cryptographic Alg Agility   November 2015   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   advisable.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, IETF   protocols that employ cryptography MUST specify one or more strong   mandatory-to-implement algorithms or suites.  This does not require   all deployments to use this algorithm or suite, but it does require   that it be 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, it is RECOMMENDED that 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.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 to make   use of CMS and make different mandatory-to-implement algorithm   choices.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 integrityHousley                   Best Current Practice                 [Page 5]

RFC 7696        Guidelines for Cryptographic Alg Agility   November 2015   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   inBCP 86 [RFC3766].   Guidance on cryptographic key size for symmetric keys can be found inBCP 195 [RFC7525].2.2.3.  Providing Notice of Expected Changes   Fortunately, 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 inRFC 4307 [RFC4307] is to use SHOULD+, SHOULD-,   and MUST- in the specification of algorithms.  The definitions below   are slightly modified from those inRFC 4307.      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-.2.3.  Transitioning 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.Housley                   Best Current Practice                 [Page 6]

RFC 7696        Guidelines for Cryptographic Alg Agility   November 2015   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   algorithms.   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.  Algorithm Transition 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 attacks.   Transition mechanisms need to consider the algorithm that is used to   provide integrity protection for algorithm negotiation itself.   If a protocol specifies a single mandatory-to-implement integrity   algorithm, eventually that algorithm will be found to be weak.Housley                   Best Current Practice                 [Page 7]

RFC 7696        Guidelines for Cryptographic Alg Agility   November 2015   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.2.5.  Cryptographic Key Establishment   Traditionally, protocol designers have avoided more than one approach   to exchanges that establish cryptographic keys because it makes the   security analysis of the overall protocol more difficult.  When   frameworks such as the Extensible Authentication Protocol (EAP)   [RFC3748] and Simple Authentication and Security Layer (SASL)   [RFC4422] are employed, key establishment is very flexible, often   hiding many of the details from the application.  This results in   protocols that support multiple key establishment approaches.  In   fact, the key establishment approach itself is negotiable, which   creates a design challenge to protect the negotiation of the key   establishment approach before it is used to produce cryptographic   keys.   Protocols can negotiate a key establishment 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 establishment 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.2.6.  Preserving Interoperability   Cryptographic algorithm deprecation is very difficult.  People do not   like to introduce interoperability problems, even to preserve   security.  As a result, flawed algorithms are supported for far too   long.  The impact of legacy software and long support tails on   security can be reduced by making it easy to transition from old   algorithms and suites to new ones.  Social pressure is often needed   to cause the transition to happen.   Implementers have been reluctant to remove deprecated algorithms or   suites from server software, and server administrators have been   reluctant to disable them over concerns that some party will no   longer have the ability to connect to their server.  Implementers and   administrators want to improve security by using the best supported   algorithms, but their actions are tempered by the desire to preserve   connectivity.  Recently, some browser vendors have started to provideHousley                   Best Current Practice                 [Page 8]

RFC 7696        Guidelines for Cryptographic Alg Agility   November 2015   visual warnings when a deprecated algorithm or suite is used.  These   visual warnings provide a new incentive to transition away from   deprecated algorithms and suites, prompting customers to ask for   improved security.   Transition in Internet infrastructure is particularly difficult.  The   digital signature on the certificate for an intermediate   certification authority (CA) [RFC5280] is often expected to last   decades, which hinders the transition away from a weak signature   algorithm or short key length.  Once a long-lived certificate is   issued with a particular signature algorithm, that algorithm will be   used by many relying parties, and none of them can stop supporting it   without invalidating all of the subordinate certificates.  In a   hierarchical system, many subordinate certificates could be impacted   by the decision to drop support for a weak signature algorithm or an   associated hash function.   Organizations that have a significant influence can assist by   coordinating the demise of an algorithm suite, making the transition   easier for their own users as well as others.2.7.  Balancing 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 by providing comparable best-known   attack work factors.  However, the security service provided by each   algorithm in a particular context needs to be considered when 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   experts is useful, but, in reality, such advice is often unavailable   to system administrators that are deploying a protocol   implementation.  For this reason, protocol designers SHOULD provide   clear guidance to implementers, leading to balanced options being   available at the time of deployment.   Performance is always a factor is selecting cryptographic algorithms.   Performance and security need to be balanced.  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, TLS 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.  The flexibility on asymmetric key lengthHousley                   Best Current Practice                 [Page 9]

RFC 7696        Guidelines for Cryptographic Alg Agility   November 2015   has led to interoperability problems, and to avoid these problems in   the future any aspect of the algorithm not specified by the algorithm   identifiers need to be negotiated, including key size and parameters.   In CMS [RFC5652], a previously distributed symmetric key-encryption   key can be used to encrypt a content-encryption key, which in turn is   used to encrypt the content.  The key-encryption and content-   encryption algorithms are often different.  If, for example, a   message content is encrypted with a 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.2.8.  Balancing Protocol Complexity   Protocol designs need to anticipate changes in the supported   cryptographic algorithm set over time.  There are a number of ways to   enable the transition, andSection 3 discusses some of the related   issues.   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.   Both of these can lead to portions of the implementation that are   rarely used, increasing the opportunity for undiscovered exploitable   implementation bugs.2.9.  Opportunistic Security   Despite the guidance inSection 2.4, opportunistic security [RFC7435]   also deserves consideration, especially at the time a protocol   implementation is deployed and configured.  Opportunistic security,   like other reasons for encrypting traffic, needs to make use of the   strongest encryption algorithms that are implemented and allowed by   policy.  When communicating parties do not have strong algorithms in   common, using algorithms that are weak against advanced attackers but   sufficient against others is one way to make pervasive surveillance   significantly more difficult.  As a result, when communicating   parties do not have strong algorithms in common, algorithms that   would not be acceptable in many negotiated situations are acceptable   for opportunistic security when legacy systems are in use for   unauthenticated encrypted sessions (as discussed inSection 3 of   [RFC7435]) as long as their use does not facilitate downgrade   attacks.  Similarly, weaker algorithms and shorter key sizes are also   acceptable for opportunistic security with the same constraints.Housley                   Best Current Practice                [Page 10]

RFC 7696        Guidelines for Cryptographic Alg Agility   November 2015   That said, the use of strong algorithms is always preferable.3.  Cryptographic Algorithm Specifications   There are tradeoffs between the number of cryptographic algorithms   that are supported and the time to deploy a new algorithm.  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.3.1.  Choosing Mandatory-to-Implement Algorithms   It may seem as if the ability to use an algorithm of one's own   choosing is very desirable; however, the selection is often better   left to experts.  When there are choices, end-users might select   between configuration profiles that have been defined by experts.   Further, experts need not specify each and every cryptographic   algorithm alternative.  Specifying all possible choices will not lead   to them all being available in every implementation.  Mandatory-to-   implement algorithms MUST have a stable public specification and   public documentation that has been well studied, giving rise to   significant confidence.  The IETF has always had a preference for   unencumbered algorithms.  There are significant benefits in selecting   algorithms and suites that are widely deployed.  The selected   algorithms need to be resistant to side-channel attacks and also meet   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 negotiation.  Specification   of too many alternatives will likely hamper interoperability and may   hamper security as well.  When specifying new algorithms or suites,   protocol designers would be prudent to consider whether existing ones   can be deprecated.   There is significant benefit in selecting the same algorithms and   suites for different protocols.  Using the same algorithms can   simplify implementation when more than one of the protocols is used   in the same device or system.   Sometimes 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 toHousley                   Best Current Practice                [Page 11]

RFC 7696        Guidelines for Cryptographic Alg Agility   November 2015   high-speed devices.  Therefore, an application needing authenticated   encryption might specify one of these algorithms or both of these   algorithms, depending on the population.3.2.  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   led 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   implementers 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 implementers 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 inRFC 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   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.Housley                   Best Current Practice                [Page 12]

RFC 7696        Guidelines for Cryptographic Alg Agility   November 20153.3.  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 weak with   age.  Either way, the design should enable transition.   Protocol designers are sometimes misled 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 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 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   deployed.   The first IEEE 802.11 [WiFi] specification included 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.   Experience with the transition from SHA-1 to SHA-256 indicates that   the time from protocol specification 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   the 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 was difficult to locate and   install, or it was 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.Housley                   Best Current Practice                [Page 13]

RFC 7696        Guidelines for Cryptographic Alg Agility   November 20153.4.  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 geographic scope   in their deployment.  Yet, the legislative or regulatory mandate   creates a captive market.  As a result, such algorithms will 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 different countries or regions;   this 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.  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   ones.   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.   Sometimes application-layer protocols can make use of transport-layer   security protocols, such as TLS [RFC5246] or Datagram TLS (DTLS)   [RFC6347].  This insulates the application-layer protocol from the   details of cryptography, but it is likely to still be necessary to   handle the transition from 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 communication.Housley                   Best Current Practice                [Page 14]

RFC 7696        Guidelines for Cryptographic Alg Agility   November 2015   Hardware offers challenges in the transition of algorithms, 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   to change the algorithm.  Cost and downtime 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   [RFC5116], 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 protect 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 ofHousley                   Best Current Practice                [Page 15]

RFC 7696        Guidelines for Cryptographic Alg Agility   November 2015   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 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   (supervisory control and data acquisition) 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.5.  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   advisable.6.  Normative References   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate             Requirement Levels",BCP 14,RFC 2119,             DOI 10.17487/RFC2119, March 1997,             <http://www.rfc-editor.org/info/rfc2119>.   [RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For Public             Keys Used For Exchanging Symmetric Keys",BCP 86,RFC 3766,             DOI 10.17487/RFC3766, April 2004,             <http://www.rfc-editor.org/info/rfc3766>.7.  Informative References   [BEAST]   Wikipedia, "BEAST attack" under "Transport Layer Security",             November 2015, <https://en.wikipedia.org/w/index.php?title=Transport_Layer_Security&oldid=689441642#BEAST_attack>.Housley                   Best Current Practice                [Page 16]

RFC 7696        Guidelines for Cryptographic Alg Agility   November 2015   [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,             DOI 10.1007/3-540-44448-3_41, 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, DOI 10.1007/3-540-44647-8_19, August 2001.   [RFC1984] IAB and IESG, "IAB and IESG Statement on Cryptographic             Technology and the Internet",BCP 200,RFC 1984,             DOI 10.17487/RFC1984, August 1996,             <http://www.rfc-editor.org/info/rfc1984>.   [RFC3365] Schiller, J., "Strong Security Requirements for Internet             Engineering Task Force Standard Protocols",BCP 61,RFC 3365, DOI 10.17487/RFC3365, August 2002,             <http://www.rfc-editor.org/info/rfc3365>.   [RFC3610] Whiting, D., Housley, R., and N. Ferguson, "Counter with             CBC-MAC (CCM)",RFC 3610, DOI 10.17487/RFC3610, September             2003, <http://www.rfc-editor.org/info/rfc3610>.   [RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.             Levkowetz, Ed., "Extensible Authentication Protocol (EAP)",RFC 3748, DOI 10.17487/RFC3748, June 2004,             <http://www.rfc-editor.org/info/rfc3748>.   [RFC4302] Kent, S., "IP Authentication Header",RFC 4302,             DOI 10.17487/RFC4302, December 2005,             <http://www.rfc-editor.org/info/rfc4302>.   [RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",RFC 4303, DOI 10.17487/RFC4303, December 2005,             <http://www.rfc-editor.org/info/rfc4303>.   [RFC4307] Schiller, J., "Cryptographic Algorithms for Use in the             Internet Key Exchange Version 2 (IKEv2)",RFC 4307,             DOI 10.17487/RFC4307, December 2005,             <http://www.rfc-editor.org/info/rfc4307>.Housley                   Best Current Practice                [Page 17]

RFC 7696        Guidelines for Cryptographic Alg Agility   November 2015   [RFC4422] Melnikov, A., Ed., and K. Zeilenga, Ed., "Simple             Authentication and Security Layer (SASL)",RFC 4422,             DOI 10.17487/RFC4422, June 2006,             <http://www.rfc-editor.org/info/rfc4422>.   [RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated             Encryption",RFC 5116, DOI 10.17487/RFC5116, January 2008,             <http://www.rfc-editor.org/info/rfc5116>.   [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security             (TLS) Protocol Version 1.2",RFC 5246,             DOI 10.17487/RFC5246, August 2008,             <http://www.rfc-editor.org/info/rfc5246>.   [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, DOI 10.17487/RFC5280, May 2008,             <http://www.rfc-editor.org/info/rfc5280>.   [RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,RFC 5652, DOI 10.17487/RFC5652, September 2009,             <http://www.rfc-editor.org/info/rfc5652>.   [RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet             Mail Extensions (S/MIME) Version 3.2 Message             Specification",RFC 5751, DOI 10.17487/RFC5751, January             2010, <http://www.rfc-editor.org/info/rfc5751>.   [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer             Security Version 1.2",RFC 6347, DOI 10.17487/RFC6347,             January 2012, <http://www.rfc-editor.org/info/rfc6347>.   [RFC6916] Gagliano, R., Kent, S., and S. Turner, "Algorithm Agility             Procedure for the Resource Public Key Infrastructure             (RPKI)",BCP 182,RFC 6916, DOI 10.17487/RFC6916, April             2013, <http://www.rfc-editor.org/info/rfc6916>.   [RFC6975] Crocker, S. and S. Rose, "Signaling Cryptographic Algorithm             Understanding in DNS Security Extensions (DNSSEC)",RFC 6975, DOI 10.17487/RFC6975, July 2013,             <http://www.rfc-editor.org/info/rfc6975>.   [RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.             Kivinen, "Internet Key Exchange Protocol Version 2             (IKEv2)", STD 79,RFC 7296, DOI 10.17487/RFC7296, October             2014, <http://www.rfc-editor.org/info/rfc7296>.Housley                   Best Current Practice                [Page 18]

RFC 7696        Guidelines for Cryptographic Alg Agility   November 2015   [RFC7366] Gutmann, P., "Encrypt-then-MAC for Transport Layer Security             (TLS) and Datagram Transport Layer Security (DTLS)",RFC 7366, DOI 10.17487/RFC7366, September 2014,             <http://www.rfc-editor.org/info/rfc7366>.   [RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection Most             of the Time",RFC 7435, DOI 10.17487/RFC7435, December             2014, <http://www.rfc-editor.org/info/rfc7435>.   [RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre, "Recommendations             for Secure Use of Transport Layer Security (TLS) and             Datagram Transport Layer Security (DTLS)",BCP 195,RFC 7525, DOI 10.17487/RFC7525, May 2015,             <http://www.rfc-editor.org/info/rfc7525>.   [WEP]     Wikipedia, "Wired Equivalent Privacy", November 2015,             <https://en.wikipedia.org/w/index.php?title=Wired_Equivalent_Privacy&oldid=688848497>.   [WiFi]    IEEE, "Wireless LAN Medium Access Control (MAC) And             Physical Layer (PHY) Specifications", IEEE Std 802.11-1997,             1997.Acknowledgements   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, Phillip Hallam-Baker, Wes   Hardaker, Joe Hildebrand, Paul Hoffman, Christian Huitema, Leif   Johansson, Suresh Krishnan, Watson Ladd, Paul Lambert, Ben Laurie,   Eliot Lear, Nikos Mavrogiannopoulos, Kathleen Moriarty, Yoav Nir,   Kenny Paterson, Rich Salz, Wendy Seltzer, Joel Sing, 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   United States   Email: housley@vigilsec.comHousley                   Best Current Practice                [Page 19]