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Internet Engineering Task Force (IETF)                            Y. NirRequest for Comments: 8247                                      Dell EMCObsoletes:4307                                               T. KivinenUpdates:7296Category: Standards Track                                     P. WoutersISSN: 2070-1721                                                  Red Hat                                                              D. Migault                                                                Ericsson                                                          September 2017Algorithm Implementation Requirements and Usage Guidancefor the Internet Key Exchange Protocol Version 2 (IKEv2)Abstract   The IPsec series of protocols makes use of various cryptographic   algorithms in order to provide security services.  The Internet Key   Exchange (IKE) protocol is used to negotiate the IPsec Security   Association (IPsec SA) parameters, such as which algorithms should be   used.  To ensure interoperability between different implementations,   it is necessary to specify a set of algorithm implementation   requirements and usage guidance to ensure that there is at least one   algorithm that all implementations support.  This document updatesRFC 7296 and obsoletesRFC 4307 in defining the current algorithm   implementation requirements and usage guidance for IKEv2, and does   minor cleaning up of the IKEv2 IANA registry.  This document does not   update the algorithms used for packet encryption using IPsec   Encapsulating Security Payload (ESP).Status of This Memo   This is an Internet Standards Track document.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Further information on   Internet Standards is available inSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttps://www.rfc-editor.org/info/rfc8247.Nir, et al.                  Standards Track                    [Page 1]

RFC 8247             IKEv2 Cryptographic Algorithms       September 2017Copyright Notice   Copyright (c) 2017 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   (https://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Table of Contents1. Introduction ....................................................21.1. Conventions Used in This Document ..........................3      1.2. Updating Algorithm Implementation Requirements and           Usage Guidance .............................................41.3. Updating Algorithm Requirement Levels ......................41.4. Document Audience ..........................................52. Algorithm Selection .............................................52.1. Type 1 - IKEv2 Encryption Algorithm Transforms .............52.2. Type 2 - IKEv2 Pseudorandom Function Transforms ............72.3. Type 3 - IKEv2 Integrity Algorithm Transforms ..............82.4. Type 4 - IKEv2 Diffie-Hellman Group Transforms .............92.5. Summary of Changes fromRFC 4307 ..........................113. IKEv2 Authentication ...........................................113.1. IKEv2 Authentication Method ...............................123.1.1. Recommendations for RSA Key Length .................133.2. Digital Signature Recommendations .........................134. Algorithms for Internet of Things ..............................145. Security Considerations ........................................156. IANA Considerations ............................................157. References .....................................................167.1. Normative References ......................................167.2. Informative References ....................................17   Acknowledgements ..................................................17   Authors' Addresses ................................................191.  Introduction   The Internet Key Exchange (IKE) protocol [RFC7296] is used to   negotiate the parameters of the IPsec SA, such as the encryption and   authentication algorithms and the keys for the protected   communications between the two endpoints.  The IKE protocol itself isNir, et al.                  Standards Track                    [Page 2]

RFC 8247             IKEv2 Cryptographic Algorithms       September 2017   also protected by cryptographic algorithms, which are negotiated   between the two endpoints using IKE.  Different implementations of   IKE may negotiate different algorithms based on their individual   local policy.  To ensure interoperability, a set of "mandatory-to-   implement" IKE cryptographic algorithms is defined.   This document describes the parameters of the IKE protocol and   updates the IKEv2 specification.  It changes the mandatory-to-   implement authentication algorithms inSection 4 of [RFC7296] by   saying that RSA key lengths of less than 2048 SHOULD NOT be used.  It   does not describe the cryptographic parameters of the Authentication   Header (AH) or ESP protocols.1.1.  Conventions Used in This Document   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and   "OPTIONAL" in this document are to be interpreted as described inBCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all   capitals, as shown here.   When used in the tables in this document, these terms indicate that   the listed algorithm MUST, MUST NOT, SHOULD, SHOULD NOT, or MAY be   implemented as part of an IKEv2 implementation.  Additional terms   used in this document are:   SHOULD+   This term means the same as SHOULD.  However, it is likely             that an algorithm marked as SHOULD+ will be promoted at             some future time to be a MUST.   SHOULD-   This term means the same as SHOULD.  However, an algorithm             marked as SHOULD- may be deprecated to a MAY in a future             version of this document.   MUST-     This term means the same as MUST.  However, it is expected             at some point that this algorithm will no longer be a MUST             in a future document.  Although its status will be             determined at a later time, it is reasonable to expect that             if a future revision of a document alters the status of a             MUST- algorithm, it will remain at least a SHOULD or a             SHOULD- level.   IoT       This abbreviation stands for "Internet of Things".Nir, et al.                  Standards Track                    [Page 3]

RFC 8247             IKEv2 Cryptographic Algorithms       September 20171.2.  Updating Algorithm Implementation Requirements and Usage Guidance   The field of cryptography evolves continuously.  New, stronger   algorithms appear and existing algorithms are found to be less secure   than originally thought.  Therefore, algorithm implementation   requirements and usage guidance need to be updated from time to time   to reflect the new reality.  The choices for algorithms must be   conservative to minimize the risk of algorithm compromise.   Algorithms need to be suitable for a wide variety of CPU   architectures and device deployments ranging from high-end bulk   encryption devices to small low-power IoT devices.   The algorithm implementation requirements and usage guidance may need   to change over time to adapt to the changing world.  For this reason,   the selection of mandatory-to-implement algorithms was removed from   the main IKEv2 specification and placed in this separate document.1.3.  Updating Algorithm Requirement Levels   The mandatory-to-implement algorithm of tomorrow should already be   available in most implementations of IKE by the time it is made   mandatory.  This document attempts to identify and introduce those   algorithms for future mandatory-to-implement status.  There is no   guarantee that the algorithms in use today may become mandatory in   the future.  Published algorithms are continuously subjected to   cryptographic attack and may become too weak or could become   completely broken before this document is updated.   This document provides updated recommendations for the mandatory-to-   implement algorithms.  As a result, any algorithm listed at the IKEv2   IANA registry not mentioned in this document MAY be implemented.  For   clarification and consistency with [RFC4307], an algorithm will be   denoted here as MAY only when it has been downgraded.   Although this document updates the algorithms to keep the IKEv2   communication secure over time, it also aims at providing   recommendations so that IKEv2 implementations remain interoperable.   IKEv2 interoperability is addressed by an incremental introduction or   deprecation of algorithms.  In addition, this document also considers   the new use cases for IKEv2 deployment, such as Internet of Things   (IoT).   It is expected that deprecation of an algorithm is performed   gradually.  This provides time for various implementations to update   their implemented algorithms while remaining interoperable.  Unless   there are strong security reasons, an algorithm is expected to be   downgraded from MUST to MUST- or SHOULD, instead of MUST NOT.Nir, et al.                  Standards Track                    [Page 4]

RFC 8247             IKEv2 Cryptographic Algorithms       September 2017   Similarly, an algorithm that has not been mentioned as mandatory-to-   implement is expected to be introduced with a SHOULD instead of a   MUST.   The current trend toward Internet of Things and its adoption of IKEv2   requires this specific use case to be taken into account as well.   IoT devices are resource-constrained devices and their choice of   algorithms are motivated by minimizing the footprint of the code, the   computation effort, and the size of the messages to send.  This   document indicates "(IoT)" when a specified algorithm is specifically   listed for IoT devices.  Requirement levels that are marked as "IoT"   apply to IoT devices and to server-side implementations that might   presumably need to interoperate with them, including any general-   purpose VPN gateways.1.4.  Document Audience   The recommendations of this document mostly target IKEv2 implementers   who need to create implementations that meet both high security   expectations as well as high interoperability between various vendors   and with different versions.  Interoperability requires a smooth move   to more secure cipher suites.  This may differ from a user point of   view that may deploy and configure IKEv2 with only the safest cipher   suite.   This document does not give any recommendations for the use of   algorithms, it only gives implementation recommendations regarding   implementations.  The use of algorithms by a specific user is   dictated by their own security policy requirements, which are outside   the scope of this document.   IKEv1 is out of scope of this document.  IKEv1 is deprecated and the   recommendations of this document must not be considered for IKEv1, as   most IKEv1 implementations have been "frozen" and will not be able to   update the list of mandatory-to-implement algorithms.2.  Algorithm Selection2.1.  Type 1 - IKEv2 Encryption Algorithm Transforms   The algorithms in the table below are negotiated in the Security   Association (SA) payload and used for the Encrypted Payload.   References to the specification defining these algorithms and the   ones in the following subsections are in the IANA registry   [IKEV2-IANA].  Some of these algorithms are Authenticated Encryption   with Associated Data (AEAD) [RFC5282].  Algorithms that are not AEAD   MUST be used in conjunction with one of the integrity algorithms inSection 2.3.Nir, et al.                  Standards Track                    [Page 5]

RFC 8247             IKEv2 Cryptographic Algorithms       September 2017          +------------------------+----------+-------+---------+          | Name                   | Status   | AEAD? | Comment |          +------------------------+----------+-------+---------+          | ENCR_AES_CBC           | MUST     | No    | (*)     |          | ENCR_CHACHA20_POLY1305 | SHOULD   | Yes   |         |          | ENCR_AES_GCM_16        | SHOULD   | Yes   | (*)     |          | ENCR_AES_CCM_8         | SHOULD   | Yes   | (IoT)   |          | ENCR_3DES              | MAY      | No    |         |          | ENCR_DES               | MUST NOT | No    |         |          | ENCR_NULL              | MUST NOT | No    |         |          +------------------------+----------+-------+---------+   (*)    This requirement level is for 128-bit and 256-bit keys.          192-bit keys remain at the MAY level.   (IoT)  This requirement is for interoperability with IoT.  Only          128-bit keys are at the SHOULD level. 192-bit and 256-bit          remain at the MAY level.   ENCR_AES_CBC is raised from SHOULD+ for 128-bit keys and MAY for   256-bit keys in [RFC4307] to MUST. 192-bit keys remain at the MAY   level.  ENCR_AES_CBC is the only shared mandatory-to-implement   algorithm withRFC 4307 and as a result, it is necessary for   interoperability with IKEv2 implementation compatible withRFC 4307.   ENCR_CHACHA20_POLY1305 was not ready to be considered at the time ofRFC 4307's publication.  It has been recommended by the Crypto Forum   Research Group (CFRG) of the IRTF as an alternative to AES-CBC and   AES-GCM.  It is also being standardized for IPsec for the same   reasons.  At the time of writing, there were not enough IKEv2   implementations supporting ENCR_CHACHA20_POLY1305 to be able to   introduce it at the SHOULD+ level.   ENCR_AES_GCM_16 was not considered inRFC 4307.  At the timeRFC 4307   was written, AES-GCM was not defined in an IETF document.  AES-GCM   was defined for ESP in [RFC4106] and later for IKEv2 in [RFC5282].   The main motivation for adopting AES-GCM for ESP is encryption   performance compared to AES-CBC.  This resulted in AES-GCM being   widely implemented for ESP.  As the computation load of IKEv2 is   relatively small compared to ESP, many IKEv2 implementations have not   implemented AES-GCM.  For this reason, AES-GCM is not promoted to a   greater status than SHOULD.  The reason for promotion from MAY to   SHOULD is to promote the slightly more secure AEAD method over the   traditional encrypt+auth method.  Its status is expected to be raised   once widely implemented.  As the advantage of the shorter (and   weaker) Integrity Check Values (ICVs) is minimal, the 8- and 12-octet   ICVs remain at the MAY level.Nir, et al.                  Standards Track                    [Page 6]

RFC 8247             IKEv2 Cryptographic Algorithms       September 2017   ENCR_AES_CCM_8 was not considered inRFC 4307.  This document   considers it as SHOULD be implemented in order to be able to interact   with IoT devices.  As this case is not a general use case for non-IoT   VPNs, its status is expected to remain as SHOULD.  The 8-octet size   of the ICV is expected to be sufficient for most use cases of IKEv2,   as far less packets are exchanged in those cases, and IoT devices   want to make packets as small as possible.  The SHOULD level is for   128-bit keys, 256-bit keys remains at MAY level.   ENCR_3DES has been downgraded fromRFC 4307 MUST- to MAY.  All IKEv2   implementations already implement ENCR_AES_CBC, so there is no need   to keep support for the much slower ENCR_3DES.  In addition,   ENCR_CHACHA20_POLY1305 provides a more modern alternative to AES.   ENCR_DES can be brute-forced using off-the-shelf hardware.  It   provides no meaningful security whatsoever and, therefore, MUST NOT   be implemented.   ENCR_NULL was incorrectly specified as MAY inRFC 4307, even when[RFC7296], Section 5 clearly states that it MUST NOT be used.  This   was fixed and this document now lists ENCR_NULL as MUST NOT.2.2.  Type 2 - IKEv2 Pseudorandom Function Transforms   Transform Type 2 algorithms are pseudorandom functions used to   generate pseudorandom values when needed.                +-------------------+----------+---------+                | Name              | Status   | Comment |                +-------------------+----------+---------+                | PRF_HMAC_SHA2_256 | MUST     |         |                | PRF_HMAC_SHA2_512 | SHOULD+  |         |                | PRF_HMAC_SHA1     | MUST-    |         |                | PRF_AES128_XCBC   | SHOULD   | (IoT)   |                | PRF_HMAC_MD5      | MUST NOT |         |                +-------------------+----------+---------+         (IoT) This requirement is for interoperability with IoT.   As no SHA2-based transforms were referenced inRFC 4307,   PRF_HMAC_SHA2_256 was not mentioned inRFC 4307.  PRF_HMAC_SHA2_256   MUST be implemented in order to replace SHA1 and PRF_HMAC_SHA1.   PRF_HMAC_SHA2_512 SHOULD be implemented as a future replacement for   PRF_HMAC_SHA2_256 or when stronger security is required.   PRF_HMAC_SHA2_512 is preferred over PRF_HMAC_SHA2_384 as the   additional overhead of PRF_HMAC_SHA2_512 is negligible.Nir, et al.                  Standards Track                    [Page 7]

RFC 8247             IKEv2 Cryptographic Algorithms       September 2017   PRF_HMAC_SHA1 has been downgraded from MUST inRFC 4307 to MUST-, as   cryptographic attacks against SHA1 are increasing, resulting in an   industry-wide trend to deprecate its usage.   PRF_AES128_XCBC is only recommended in the scope of IoT, as Internet   of Things deployments tend to prefer AES-based pseudorandom functions   in order to avoid implementing SHA2.  For the non-IoT VPN deployment,   it has been downgraded from SHOULD inRFC 4307 to MAY as it has not   seen wide adoption.   PRF_HMAC_MD5 has been downgraded from MAY inRFC 4307 to MUST NOT.   Cryptographic attacks against MD5, such as collision attacks   mentioned in [TRANSCRIPTION], are resulting in an industry-wide trend   to deprecate and remove MD5 (and thus HMAC-MD5) from cryptographic   libraries.2.3.  Type 3 - IKEv2 Integrity Algorithm Transforms   The algorithms in the table below are negotiated in the SA payload   and used for the Encrypted Payload.  References to the specification   defining these algorithms are in the IANA registry.  When an AEAD   algorithm (seeSection 2.1) is proposed, this algorithm transform   type is not in use.              +------------------------+----------+---------+              | Name                   | Status   | Comment |              +------------------------+----------+---------+              | AUTH_HMAC_SHA2_256_128 | MUST     |         |              | AUTH_HMAC_SHA2_512_256 | SHOULD   |         |              | AUTH_HMAC_SHA1_96      | MUST-    |         |              | AUTH_AES_XCBC_96       | SHOULD   | (IoT)   |              | AUTH_HMAC_MD5_96       | MUST NOT |         |              | AUTH_DES_MAC           | MUST NOT |         |              | AUTH_KPDK_MD5          | MUST NOT |         |              +------------------------+----------+---------+         (IoT) This requirement is for interoperability with IoT.   AUTH_HMAC_SHA2_256_128 was not mentioned inRFC 4307, as no   SHA2-based transforms were mentioned.  AUTH_HMAC_SHA2_256_128 MUST be   implemented in order to replace AUTH_HMAC_SHA1_96.   AUTH_HMAC_SHA2_512_256 SHOULD be implemented as a future replacement   of AUTH_HMAC_SHA2_256_128 or when stronger security is required.   This value has been preferred over AUTH_HMAC_SHA2_384, as the   additional overhead of AUTH_HMAC_SHA2_512 is negligible.Nir, et al.                  Standards Track                    [Page 8]

RFC 8247             IKEv2 Cryptographic Algorithms       September 2017   AUTH_HMAC_SHA1_96 has been downgraded from MUST inRFC 4307 to MUST-   as cryptographic attacks against SHA1 are increasing, resulting in an   industry-wide trend to deprecate its usage.   AUTH_AES_XCBC_96 is only recommended in the scope of IoT, as Internet   of Things deployments tend to prefer AES-based pseudorandom functions   in order to avoid implementing SHA2.  For the non-IoT VPN deployment,   it has been downgraded from SHOULD inRFC 4307 to MAY as it has not   been widely adopted.   AUTH_DES_MAC and AUTH_KPDK_MD5 were not mentioned inRFC 4307, so   their default statuses were MAY.  These have been downgraded to MUST   NOT.  AUTH_HMAC_MD5_96 is also demoted to MUST NOT.  This is because   there is an industry-wide trend to deprecate DES and MD5.  Note also   that MD5 support is being removed from cryptographic libraries in   general because its non-HMAC use is known to be subject to collision   attacks, for example, as mentioned in [TRANSCRIPTION].2.4.  Type 4 - IKEv2 Diffie-Hellman Group Transforms   There are several Modular Exponential (MODP) groups and several   Elliptic Curve Cryptography (ECC) groups that are defined for use in   IKEv2.  These groups are defined in both the base document [RFC7296]   and in extension documents and are identified by group number.  Note   that it is critical to enforce a secure Diffie-Hellman (DH) exchange   as this exchange provides keys for the session.  If an attacker can   retrieve one of the private numbers (a or b) and the complementary   public value (g**b or g**a), then the attacker can compute the secret   and the keys used and then decrypt the exchange and IPsec SA created   inside the IKEv2 SA.  Such an attack can be performed off-line on a   previously recorded communication, years after the communication   happened.  This differs from attacks that need to be executed during   the authentication that must be performed online and in near real   time.Nir, et al.                  Standards Track                    [Page 9]

RFC 8247             IKEv2 Cryptographic Algorithms       September 2017   +--------+---------------------------------------------+------------+   | Number | Description                                 | Status     |   +--------+---------------------------------------------+------------+   | 14     | 2048-bit MODP Group                         | MUST       |   | 19     | 256-bit random ECP group                    | SHOULD     |   | 5      | 1536-bit MODP Group                         | SHOULD NOT |   | 2      | 1024-bit MODP Group                         | SHOULD NOT |   | 1      | 768-bit MODP Group                          | MUST NOT   |   | 22     | 1024-bit MODP Group with 160-bit Prime      | MUST NOT   |   |        | Order Subgroup                              |            |   | 23     | 2048-bit MODP Group with 224-bit Prime      | SHOULD NOT |   |        | Order Subgroup                              |            |   | 24     | 2048-bit MODP Group with 256-bit Prime      | SHOULD NOT |   |        | Order Subgroup                              |            |   +--------+---------------------------------------------+------------+   Group 14 or the 2048-bit MODP Group is raised from SHOULD+ inRFC 4307 to MUST as a replacement for the 1024-bit MODP Group.  Group   14 is widely implemented and considered secure.   Group 19 or the 256-bit random ECP group was not specified inRFC 4307 as this group was not defined at that time.  Group 19 is   widely implemented and considered secure and, therefore, has been   promoted to the SHOULD level.   Group 5 or the 1536-bit MODP Group has been downgraded from MAY inRFC 4307 to SHOULD NOT.  It was specified earlier, but is now   considered to be vulnerable to being broken within the next few years   by a nation-state-level attack, so its security margin is considered   too narrow.   Group 2 or the 1024-bit MODP Group has been downgraded from MUST- inRFC 4307 to SHOULD NOT.  It is known to be weak against sufficiently   funded attackers using commercially available mass-computing   resources, so its security margin is considered too narrow.  It is   expected in the near future to be downgraded to MUST NOT.   Group 1 or the 768-bit MODP Group was not mentioned inRFC 4307 and   so its status was MAY.  It can be broken within hours using cheap   off-the-shelf hardware.  It provides no security whatsoever.  It has,   therefore, been downgraded to MUST NOT.   Groups 22, 23, and 24 are MODP groups with Prime Order Subgroups that   are not safe primes.  The seeds for these groups have not been   publicly released, resulting in reduced trust in these groups.  These   groups were proposed as alternatives for groups 2 and 14 but never   saw wide deployment.  It has been shown that group 22 with 1024-bit   MODP is too weak and academia have the resources to generateNir, et al.                  Standards Track                   [Page 10]

RFC 8247             IKEv2 Cryptographic Algorithms       September 2017   malicious values at this size.  This has resulted in group 22 to be   demoted to MUST NOT.  Groups 23 and 24 have been demoted to SHOULD   NOT and are expected to be further downgraded in the near future to   MUST NOT.  Since groups 23 and 24 have small subgroups, the checks   specified in the first bullet point ofSection 2.2 of "Additional   Diffie-Hellman Tests for the Internet Key Exchange Protocol Version 2   (IKEv2)" [RFC6989] MUST be done when these groups are used.2.5.  Summary of Changes fromRFC 4307   The following table summarizes the changes fromRFC 4307.      +---------------------+--------------------------+------------+      | Algorithm           |RFC 4307                 |RFC 8247   |      +---------------------+--------------------------+------------+      | ENCR_3DES           | MUST-                    | MAY        |      | ENCR_NULL           | MUST NOT (per [Err1937]) | MUST NOT   |      | ENCR_AES_CBC        | SHOULD+                  | MUST       |      | ENCR_AES_CTR        | SHOULD                   | MAY(*)     |      | PRF_HMAC_MD5        | MAY                      | MUST NOT   |      | PRF_HMAC_SHA1       | MUST                     | MUST-      |      | PRF_AES128_XCBC     | SHOULD+                  | SHOULD     |      | AUTH_HMAC_MD5_96    | MAY                      | MUST NOT   |      | AUTH_HMAC_SHA1_96   | MUST                     | MUST-      |      | AUTH_AES_XCBC_96    | SHOULD+                  | SHOULD     |      | Group 2 (1024-bit)  | MUST-                    | SHOULD NOT |      | Group 14 (2048-bit) | SHOULD+                  | MUST       |      +---------------------+--------------------------+------------+   (*)  This algorithm is not mentioned in the above sections, so it        defaults to MAY.3.  IKEv2 Authentication   IKEv2 authentication may involve a signatures verification.   Signatures may be used to validate a certificate or to check the   signature of the AUTH value.  Cryptographic recommendations regarding   certificate validation are out of scope of this document.  What is   mandatory to implement is provided by the PKIX community.  This   document is mostly concerned with signature verification and   generation for the authentication.Nir, et al.                  Standards Track                   [Page 11]

RFC 8247             IKEv2 Cryptographic Algorithms       September 20173.1.  IKEv2 Authentication Method      +--------+---------------------------------------+------------+      | Number | Description                           | Status     |      +--------+---------------------------------------+------------+      | 1      | RSA Digital Signature                 | MUST       |      | 2      | Shared Key Message Integrity Code     | MUST       |      | 3      | DSS Digital Signature                 | SHOULD NOT |      | 9      | ECDSA with SHA-256 on the P-256 curve | SHOULD     |      | 10     | ECDSA with SHA-384 on the P-384 curve | SHOULD     |      | 11     | ECDSA with SHA-512 on the P-521 curve | SHOULD     |      | 14     | Digital Signature                     | SHOULD     |      +--------+---------------------------------------+------------+   RSA Digital Signature is widely deployed and, therefore, kept for   interoperability.  It is expected to be downgraded in the future as   its signatures are based on the older RSASSA-PKCS1-v1.5, which is no   longer recommended.  RSA authentication, as well as other specific   authentication methods, are expected to be replaced with the generic   Digital Signature method of [RFC7427].   Shared Key Message Integrity Code is widely deployed and mandatory to   implement in the IKEv2 inRFC 7296.  The status remains MUST.   "DSS Digital Signature" (IANA value 3) signatures are bound to SHA-1   and have the same level of security as 1024-bit RSA.  They are   currently at SHOULD NOT and are expected to be downgraded to MUST NOT   in the future.   Authentication methods that are based on the Elliptic Curve Digital   Signature Algorithm (ECDSA) are also expected to be downgraded as   these do not provide hash function agility.  Instead, ECDSA (like   RSA) is expected to be performed using the generic Digital Signature   method.  Its status is SHOULD.   Digital Signature [RFC7427] is expected to be promoted as it provides   hash function, signature format, and algorithm agility.  Its current   status is SHOULD.Nir, et al.                  Standards Track                   [Page 12]

RFC 8247             IKEv2 Cryptographic Algorithms       September 20173.1.1.  Recommendations for RSA Key Length        +-------------------------------------------+------------+        | Description                               | Status     |        +-------------------------------------------+------------+        | RSA with key length 2048                  | MUST       |        | RSA with key length 3072 and 4096         | SHOULD     |        | RSA with key length between 2049 and 4095 | MAY        |        | RSA with key length smaller than 2048     | SHOULD NOT |        +-------------------------------------------+------------+   IKEv2 [RFC7296] mandates support for the RSA keys of the bit size   1024 or 2048, but key sizes less than 2048 are updated to SHOULD NOT   as there is an industry-wide trend to deprecate key lengths less than   2048 bits.  Since these signatures only have value in real time and   need no future protection, smaller keys were kept at SHOULD NOT   instead of MUST NOT.3.2.  Digital Signature Recommendations   When a Digital Signature authentication method is implemented, the   following recommendations are applied for hash functions:               +--------+-------------+----------+---------+               | Number | Description | Status   | Comment |               +--------+-------------+----------+---------+               | 1      | SHA1        | MUST NOT |         |               | 2      | SHA2-256    | MUST     |         |               | 3      | SHA2-384    | MAY      |         |               | 4      | SHA2-512    | SHOULD   |         |               +--------+-------------+----------+---------+   When the Digital Signature authentication method is used with RSA   signature algorithm, RSASSA-PSS MUST be supported and RSASSA-   PKCS1-v1.5 MAY be supported.Nir, et al.                  Standards Track                   [Page 13]

RFC 8247             IKEv2 Cryptographic Algorithms       September 2017   The following table lists recommendations for authentication methods   in [RFC7427] notation.  These recommendations are applied only if the   Digital Signature authentication method is implemented.        +------------------------------------+----------+---------+        | Description                        | Status   | Comment |        +------------------------------------+----------+---------+        | RSASSA-PSS with SHA-256            | MUST     |         |        | ecdsa-with-sha256                  | SHOULD   |         |        | sha1WithRSAEncryption              | MUST NOT |         |        | dsa-with-sha1                      | MUST NOT |         |        | ecdsa-with-sha1                    | MUST NOT |         |        | RSASSA-PSS with Empty Parameters   | MUST NOT | (*)     |        | RSASSA-PSS with Default Parameters | MUST NOT | (*)     |        +------------------------------------+----------+---------+   (*)  Empty or Default parameters means it is using SHA1, which is at        the MUST NOT level.4.  Algorithms for Internet of Things   Some algorithms in this document are marked for use with the Internet   of Things (IoT).  There are several reasons why IoT devices prefer a   different set of algorithms from regular IKEv2 clients.  IoT devices   are usually very constrained, meaning that the memory size and CPU   power is so limited that these clients only have resources to   implement and run one set of algorithms.  For example, instead of   implementing AES and SHA, these devices typically use AES_XCBC as an   integrity algorithm so SHA does not need to be implemented.   For example, IEEE Std 802.15.4 [IEEE-802-15-4] devices have a   mandatory-to-implement link-level security using AES-CCM with 128-bit   keys.  The "IEEE Recommended Practice for Transport of Key Management   Protocol (KMP) Datagrams" [IEEE-802-15-9] already provides a way to   use Minimal IKEv2 [RFC7815] over the 802.15.4 layer to provide link   keys for the 802.15.4 layer.   These devices might want to use AES-CCM as their IKEv2 algorithm, so   they can reuse the hardware implementing it.  They cannot use the   AES-CBC algorithm, as the hardware quite often does not include   support for the AES decryption needed to support the CBC mode.  So   despite the AES-CCM algorithm requiring AEAD [RFC5282] support, the   benefit of reusing the crypto hardware makes AES-CCM the preferred   algorithm.   Another important aspect of IoT devices is that their transfer rates   are usually quite low (in the order of tens of kbit/s), and each bit   they transmit has an energy consumption cost associated with it andNir, et al.                  Standards Track                   [Page 14]

RFC 8247             IKEv2 Cryptographic Algorithms       September 2017   shortens their battery life.  Therefore, shorter packets are   preferred.  This is the reason for recommending the 8-octet ICV over   the 16-octet ICV.   Because different IoT devices will have different constraints, this   document cannot specify the one mandatory profile for IoT.  Instead,   this document points out commonly used algorithms with IoT devices.5.  Security Considerations   The security of cryptographic-based systems depends on both the   strength of the cryptographic algorithms chosen and the strength of   the keys used with those algorithms.  The security also depends on   the engineering of the protocol used by the system to ensure that   there are no non-cryptographic ways to bypass the security of the   overall system.   The Diffie-Hellman Group parameter is the most important one to   choose conservatively.  Any party capturing all IKE and ESP traffic   that (even years later) can break the selected DH group in IKE, can   gain access to the symmetric keys used to encrypt all the ESP   traffic.  Therefore, these groups must be chosen very conservatively.   However, specifying an extremely large DH group also puts a   considerable load on the device, especially when this is a large VPN   gateway or an IoT-constrained device.   This document concerns itself with the selection of cryptographic   algorithms for the use of IKEv2, specifically with the selection of   "mandatory-to-implement" algorithms.  The algorithms identified in   this document as "MUST implement" or "SHOULD implement" are not known   to be broken at the current time, and cryptographic research so far   leads us to believe that they will likely remain secure into the   foreseeable future.  However, this isn't necessarily forever and it   is expected that new revisions of this document will be issued from   time to time to reflect the current best practice in this area.6.  IANA Considerations   This document renames some of the names in the "Transform Type 1 -   Encryption Algorithm Transform IDs" registry of the "Internet Key   Exchange Version 2 (IKEv2) Parameters".  All the other names have   ENCR_ prefix except 3, and all other entries use names in the format   of uppercase words separated with underscores except 6.  This   document changes those names to match others.Nir, et al.                  Standards Track                   [Page 15]

RFC 8247             IKEv2 Cryptographic Algorithms       September 2017   Per this document, IANA has renamed the following entries for the   AES-GCM cipher [RFC4106] and the Camellia cipher [RFC5529]:     +---------------------------------------+----------------------+     | Old name                              | New name             |     +---------------------------------------+----------------------+     | AES-GCM with a 8 octet ICV            | ENCR_AES_GCM_8       |     | AES-GCM with a 12 octet ICV           | ENCR_AES_GCM_12      |     | AES-GCM with a 16 octet ICV           | ENCR_AES_GCM_16      |     | ENCR_CAMELLIA_CCM with an 8-octet ICV | ENCR_CAMELLIA_CCM_8  |     | ENCR_CAMELLIA_CCM with a 12-octet ICV | ENCR_CAMELLIA_CCM_12 |     | ENCR_CAMELLIA_CCM with a 16-octet ICV | ENCR_CAMELLIA_CCM_16 |     +---------------------------------------+----------------------+   In addition, IANA has added this RFC as a reference to both the ESP   Reference and IKEv2 Reference columns for ENCR_AES_GCM entries, while   keeping the existing references there.  Also, IANA has added this RFC   as a reference to the ESP Reference column for ENCR_CAMELLIA_CCM   entries, while keeping the existing reference there.   The registry entries currently are:   Number Name                  ESP Reference       IKEv2 Reference   ...   18     ENCR_AES_GCM_8        [RFC4106][RFC8247]  [RFC5282][RFC8247]   19     ENCR_AES_GCM_12       [RFC4106][RFC8247]  [RFC5282][RFC8247]   20     ENCR_AES_GCM_16       [RFC4106][RFC8247]  [RFC5282][RFC8247]   ...   25     ENCR_CAMELLIA_CCM_8   [RFC5529][RFC8247]  -   26     ENCR_CAMELLIA_CCM_12  [RFC5529][RFC8247]  -   27     ENCR_CAMELLIA_CCM_16  [RFC5529][RFC8247]  -7.  References7.1.  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,              <https://www.rfc-editor.org/info/rfc2119>.   [RFC4106]  Viega, J. and D. McGrew, "The Use of Galois/Counter Mode              (GCM) in IPsec Encapsulating Security Payload (ESP)",RFC 4106, DOI 10.17487/RFC4106, June 2005,              <https://www.rfc-editor.org/info/rfc4106>.Nir, et al.                  Standards Track                   [Page 16]

RFC 8247             IKEv2 Cryptographic Algorithms       September 2017   [RFC4307]  Schiller, J., "Cryptographic Algorithms for Use in the              Internet Key Exchange Version 2 (IKEv2)",RFC 4307,              DOI 10.17487/RFC4307, December 2005,              <https://www.rfc-editor.org/info/rfc4307>.   [RFC5282]  Black, D. and D. McGrew, "Using Authenticated Encryption              Algorithms with the Encrypted Payload of the Internet Key              Exchange version 2 (IKEv2) Protocol",RFC 5282,              DOI 10.17487/RFC5282, August 2008,              <https://www.rfc-editor.org/info/rfc5282>.   [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, <https://www.rfc-editor.org/info/rfc7296>.   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase inRFC2119 Key Words",BCP 14,RFC 8174, DOI 10.17487/RFC8174,              May 2017, <https://www.rfc-editor.org/info/rfc8174>.7.2.  Informative References   [Err1937]  RFC Errata, Erratum ID 1937,RFC 4307,              <https://www.rfc-editor.org/errata/eid1937>.   [IEEE-802-15-4]              IEEE, "IEEE Standard for Low-Rate Wireless Personal Area              Networks (WPANs)", IEEE Standard 802.15.4,              DOI 10.1109/IEEESTD.2016.7460875, 2015,              <http://ieeexplore.ieee.org/document/7460875/>.   [IEEE-802-15-9]              IEEE, "IEEE Recommended Practice for Transport of Key              Management Protocol (KMP) Datagrams", IEEE Standard              802.15.9, DOI 10.1109/IEEESTD.2016.7544442, 2016,              <http://ieeexplore.ieee.org/document/7544442/>.   [IKEV2-IANA]              IANA, "Internet Key Exchange Version 2 (IKEv2)              Parameters",              <http://www.iana.org/assignments/ikev2-parameters>.   [RFC5529]  Kato, A., Kanda, M., and S. Kanno, "Modes of Operation for              Camellia for Use with IPsec",RFC 5529,              DOI 10.17487/RFC5529, April 2009,              <https://www.rfc-editor.org/info/rfc5529>.Nir, et al.                  Standards Track                   [Page 17]

RFC 8247             IKEv2 Cryptographic Algorithms       September 2017   [RFC6989]  Sheffer, Y. and S. Fluhrer, "Additional Diffie-Hellman              Tests for the Internet Key Exchange Protocol Version 2              (IKEv2)",RFC 6989, DOI 10.17487/RFC6989, July 2013,              <https://www.rfc-editor.org/info/rfc6989>.   [RFC7427]  Kivinen, T. and J. Snyder, "Signature Authentication in              the Internet Key Exchange Version 2 (IKEv2)",RFC 7427,              DOI 10.17487/RFC7427, January 2015,              <https://www.rfc-editor.org/info/rfc7427>.   [RFC7815]  Kivinen, T., "Minimal Internet Key Exchange Version 2              (IKEv2) Initiator Implementation",RFC 7815,              DOI 10.17487/RFC7815, March 2016,              <https://www.rfc-editor.org/info/rfc7815>.   [TRANSCRIPTION]              Bhargavan, K. and G. Leurent, "Transcript Collision              Attacks: Breaking Authentication in TLS, IKE, and SSH",              Network and Distributed System Security Symposium (NDSS),              DOI 10.14722/ndss.2016.23418, Feb 2016,              <https://hal.inria.fr/hal-01244855/>.AcknowledgementsRFC 4307 was authored by Jeffrey I. Schiller of the Massachusetts   Institute of Technology (MIT).  Much of the original text has been   copied verbatim.   We would like to thank Paul Hoffman, Yaron Sheffer, John Mattsson,   Tommy Pauly, Eric Rescorla, and Pete Resnick for their valuable   feedback and reviews.Nir, et al.                  Standards Track                   [Page 18]

RFC 8247             IKEv2 Cryptographic Algorithms       September 2017Authors' Addresses   Yoav Nir   Dell EMC   9 Andrei Sakharov Street   Haifa  3190500   Israel   Email: ynir.ietf@gmail.com   Tero Kivinen   Email: kivinen@iki.fi   Paul Wouters   Red Hat   Email: pwouters@redhat.com   Daniel Migault   Ericsson   8275 Trans Canada Route   Saint-Laurent, QC  H4S 0B6   Canada   Phone: +1 514-452-2160   Email: daniel.migault@ericsson.comNir, et al.                  Standards Track                   [Page 19]