Internet Engineering Task Force (IETF) Y. Nir Request for Comments: 8247 Dell EMC Obsoletes: 4307 T. Kivinen Updates: 7296 Category: Standards Track P. Wouters ISSN: 2070-1721 Red Hat D. Migault Ericsson September 2017 Algorithm Implementation Requirements and Usage Guidance for the Internet Key Exchange Protocol Version 2 (IKEv2)
AbstractThe 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 updates RFC 7296 and obsoletes RFC 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 in Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8247.
Copyright Notice Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (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. 1. Introduction ....................................................2 1.1. Conventions Used in This Document ..........................3 1.2. Updating Algorithm Implementation Requirements and Usage Guidance .............................................4 1.3. Updating Algorithm Requirement Levels ......................4 1.4. Document Audience ..........................................5 2. Algorithm Selection .............................................5 2.1. Type 1 - IKEv2 Encryption Algorithm Transforms .............5 2.2. Type 2 - IKEv2 Pseudorandom Function Transforms ............7 2.3. Type 3 - IKEv2 Integrity Algorithm Transforms ..............8 2.4. Type 4 - IKEv2 Diffie-Hellman Group Transforms .............9 2.5. Summary of Changes from RFC 4307 ..........................11 3. IKEv2 Authentication ...........................................11 3.1. IKEv2 Authentication Method ...............................12 3.1.1. Recommendations for RSA Key Length .................13 3.2. Digital Signature Recommendations .........................13 4. Algorithms for Internet of Things ..............................14 5. Security Considerations ........................................15 6. IANA Considerations ............................................15 7. References .....................................................16 7.1. Normative References ......................................16 7.2. Informative References ....................................17 Acknowledgements ..................................................17 Authors' Addresses ................................................19 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 is
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 in Section 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. 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".
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.
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. 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 in Section 2.3.
+------------------------+----------+-------+---------+ | 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 with RFC 4307 and as a result, it is necessary for interoperability with IKEv2 implementation compatible with RFC 4307. ENCR_CHACHA20_POLY1305 was not ready to be considered at the time of RFC 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 in RFC 4307. At the time RFC 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.
ENCR_AES_CCM_8 was not considered in RFC 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 from RFC 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 in RFC 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. RFC 4307, PRF_HMAC_SHA2_256 was not mentioned in RFC 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.
PRF_HMAC_SHA1 has been downgraded from MUST in RFC 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 in RFC 4307 to MAY as it has not seen wide adoption. PRF_HMAC_MD5 has been downgraded from MAY in RFC 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. Section 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 in RFC 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.
AUTH_HMAC_SHA1_96 has been downgraded from MUST in RFC 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 in RFC 4307 to MAY as it has not been widely adopted. AUTH_DES_MAC and AUTH_KPDK_MD5 were not mentioned in RFC 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]. 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.
+--------+---------------------------------------------+------------+ | 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+ in RFC 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 in RFC 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 in RFC 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- in RFC 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 in RFC 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 generate
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 of Section 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. RFC 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.
RFC7427]. Shared Key Message Integrity Code is widely deployed and mandatory to implement in the IKEv2 in RFC 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.
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.
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. 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 and
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.
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] - [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>.
[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 in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>. [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>.
[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/>. RFC 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.