Network Working Group                                             Y. Nir
Internet-Draft                                               Check Point
Obsoletes: 4307 (if approved)                                 T. Kivinen
Updates: 7296 (if approved)                                INSIDE Secure
Intended status: Standards Track                              P. Wouters
Expires: October 09, November 12, 2016                                       Red Hat
                                                              D. Migault
                                                                Ericsson
                                                          April 07,
                                                            May 11, 2016

   Algorithm Implementation Requirements and Usage Guidance for IKEv2
                    draft-ietf-ipsecme-rfc4307bis-07
                    draft-ietf-ipsecme-rfc4307bis-08

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 defines
   the current algorithm implementation requirements and usage guidance
   for IKEv2.  This document does not update the algorithms used for
   packet encryption using IPsec Encapsulated Security Payload (ESP).

Status of This Memo

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   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on October 09, November 12, 2016.

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   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Updating Algorithm Implementation Requirements and Usage
           Guidance  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Updating Algorithm Requirement Levels . . . . . . . . . .   3
     1.3.  Document Audience . . . . . . . . . . . . . . . . . . . .   4
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   4
   3.  Algorithm Selection . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Type 1 - IKEv2 Encryption Algorithm Transforms  . . . . .   5
     3.2.  Type 2 - IKEv2 Pseudo-random Function Transforms  . . . .   7   6
     3.3.  Type 3 - IKEv2 Integrity Algorithm Transforms . . . . . .   8   7
     3.4.  Type 4 - IKEv2 Diffie-Hellman Group Transforms  . . . . .   9   8
   4.  IKEv2 Authentication  . . . . . . . . . . . . . . . . . . . .  10
     4.1.  IKEv2 Authentication Method . . . . . . . . . . . . . . .  10
       4.1.1.  Recommendations for RSA key length  . . . . . . . . .  11
     4.2.  Digital Signature Recommendations . . . . . . . . . . . .  11
   5.  Algorithms for Internet of Things . . . . . . . . . . . . . .  12
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  14  13
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  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 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 because it changes the mandatory to
   implement authentication algorithms of the section 4 of the RFC7296
   by saying RSA key lengths of less than 2048 are SHOULD NOT.  It does
   not describe the cryptographic parameters of the AH or ESP protocols.

1.1.  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
   then 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 a separate document.

1.2.  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 only provides recommendations for the mandatory-to-
   implement algorithms or algorithms too weak that are recommended not
   to be implemented.  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 set
   to
   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.

1.3.  Document Audience

   The recommendations of this document mostly target IKEv2 implementers
   as implementations need to 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.  On
   the other hand, comments and recommendations from this document are also expected to be
   useful for such users.

   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.  Conventions Used in This Document

   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].

   We define some additional terms here:

   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, we expect 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       stands for Internet of Things.

                                  Table 1

3.  Algorithm Selection

3.1.  Type 1 - IKEv2 Encryption Algorithm Transforms

   The algorithms in the below table are negotiated in the 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 an
   integrity algorithms in Section 3.3.

       +-----------------------------+----------+-------+----------+
       | Name                        | Status   | AEAD? | Comment  |
       +-----------------------------+----------+-------+----------+
       | ENCR_AES_CBC                | MUST-    | No    | [1]      |
       | ENCR_CHACHA20_POLY1305      | SHOULD   | Yes   |          |
       | AES-GCM with a 16 octet ICV | SHOULD   | Yes   | [1]      |
       | ENCR_AES_CCM_8              | SHOULD   | Yes   | [1][IoT] |
       | ENCR_3DES                   | MAY      | No    |          |
       | ENCR_DES                    | MUST NOT | No    |          |
       +-----------------------------+----------+-------+----------+

   [1] - This requirement level is for 128-bit keys. 256-bit keys are at
   SHOULD. 192-bit keys can safely be ignored.  [IoT] - This requirement
                     is for interoperability with IoT.

                                  Table 2

   ENCR_AES_CBC is raised from SHOULD+ in [RFC4307] to MUST.  It is the
   only shared mandatory-to-implement algorithm with RFC4307 and as a
   result it is necessary for interoperability with IKEv2 implementation
   compatible with RFC4307.

   ENCR_CHACHA20_POLY1305 was not ready to be considered at the time of
   RFC4307.  It has been recommended by the CRFG and others 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.

   AES-GCM with a 16 octet ICV was not considered in RFC4307.  At the
   time RFC4307 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 and key longevity 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) ICVs is minimal, the 8 and 12
   octet ICV's remain at the MAY level.

   ENCR_AES_CCM_8 was not considered in RFC4307.  This document
   considers it as SHOULD be implemented in order to be able to interact
   with Internet of Things 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 on those cases, and
   IoT devices want to make packets as small as possible.  When
   implemented, ENCR_AES_CCM_8 MUST be implemented for key length 128
   and MAY be implemented for key length 256.

   ENCR_3DES has been downgraded from RFC4307 MUST- to SHOULD NOT.  All
   IKEv2 implementation 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 of-the-shelves hardware.  It
   provides no meaningful security whatsoever and therefor MUST NOT be
   implemented.

3.2.  Type 2 - IKEv2 Pseudo-random Function Transforms

   Transform Type 2 algorithms are pseudo-random functions used to
   generate pseudo-random values when needed.

   If an algorithm is selected as the integrity algorithm, it SHOULD
   also be used as the PRF.  When using an AEAD cipher, a choice of PRF
   needs to be made.  The table below lists the recommended algorithms.

                +-------------------+----------+---------+
                | 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

                                  Table 3

   PRF_HMAC_SHA2_256 was not mentioned in RFC4307, as no SHA2 based
   transforms were mentioned.  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 RFC4307 to MUST- as
   their is 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 pseudo-random
   functions in order to avoid implementing SHA2.  For the non-IoT VPN
   deployment it has been downgraded from SHOULD in RFC4307 to MAY as it
   has not seen wide adoption.

   PRF_HMAC_MD5 has been downgraded from MAY in RFC4307 to MUST NOT.
   There is an industry-wide trend to deprecate its usage as 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].

3.3.  Type 3 - IKEv2 Integrity Algorithm Transforms

   The algorithms in the below table 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 (see Section 3.1) 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

                                  Table 4

   AUTH_HMAC_SHA2_256_128 was not mentioned in RFC4307, 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 RFC4307 to MUST-
   as there is an industry-wide trend to deprecate its usage.

   AUTH_AES-XCBC is only recommended in the scope of IoT, as Internet of
   Things deployments tend to prefer AES based pseudo-random functions
   in order to avoid implementing SHA2.  For the non-IoT VPN deployment,
   it has been downgraded from SHOULD in RFC4307 to MAY as it has not
   been widely adopted.

   AUTH_DES_MAC, AUTH_HMAC_MD5_96, AUTH_DES_MAC and AUTH_KPDK_MD5 were not mentioned
   in RFC4307 so its their default status was ware MAY.  It has  They have been
   downgraded to MUST NOT.  There is an industry-wide trend to deprecate its usage.
   DES and MD5.  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].

3.4.  Type 4 - IKEv2 Diffie-Hellman Group Transforms

   There are several Modular Exponential (MODP) groups and several
   Elliptic Curve groups (ECC) that are defined for use in IKEv2.  These
   groups are defined in both the [IKEv2] base document and in
   extensions documents and are identified by group number.  Note that
   it is critical to enforce a secure Diffie-Hellman exchange as this
   exchange provides keys for the session.  If an attacker can retrieve
   the private numbers (a, or b) and the public values (g**a, and g**b),
   then the attacker can compute the secret and the keys used and
   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 which
   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      | SHOULD 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                              |            |
   +------------+----------------------------------------+-------------+

                                  Table 5
   +--------+---------------------------------------------+------------+

   Group 14 or 2048-bit MODP Group is raised from SHOULD+ in RFC4307 as
   a replacement for 1024-bit MODP Group.  Group 14 is widely
   implemented and considered secure.

   Group 19 or 256-bit random ECP group was not specified in RFC4307, as
   this group were not specified at that time.  Group 19 is widely
   implemented and considered secure.

   Group 5 or 1536-bit MODP Group has been downgraded from MAY in
   RFC4307 to SHOULD NOT.  It was specified earlier, but is now
   considered to be vulnerable to be broken within the next few years by
   a nation state level attack, so its security margin is considered too
   narrow.

   Group 2 or 1024-bit MODP Group has been downgraded from MUST- in
   RFC4307 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 768-bit MODP Group was not mentioned in RFC4307 and so its
   status was MAY.  It can be broken within hours using cheap of-the-
   shelves hardware.  It provides no security whatsoever.

   Group 22, 23 and 24 or 1024-bit MODP Group with 160-bit, and 2048-bit
   MODP Group with 224-bit and 256-bit Prime Order Subgroup have small
   subgroups, which means that checks specified in the "Additional
   Diffie-Hellman Test for the IKEv2" [RFC6989] section 2.2 first bullet
   point MUST be done when these groups are used.  These groups are also
   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 group 2 and 14 but never saw wide
   deployment.  It is expected in the near future to be further
   downgraded to MUST NOT.

4.  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 on signature verification and generation
   for the authentication.

4.1.  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     |
      +--------+---------------------------------------+------------+
                                  Table 6

   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].  RSA Digital Signature is not
   recommended for keys smaller then 2048, but since these signatures
   only have value in real-time, and need no future protection, smaller
   keys was kept at SHOULD NOT instead of MUST NOT.

   Shared Key Message Integrity Code is widely deployed and mandatory to
   implement in the IKEv2 in the RFC7296.

   ECDSA based Authentication Methods are also expected to be downgraded
   as it does not provide hash function agility.  Instead, ECDSA (like
   RSA) is expected to be performed using the generic Digital Signature
   method.

   DSS Digital Signature is bound to SHA-1 and has the same level of
   security as 1024-bit RSA.  It is expected to be downgraded to MUST
   NOT in the future.

   Digital Signature [RFC7427] is expected to be promoted as it provides
   hash function, signature format and algorithm agility.

4.1.1.  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 |
        +-------------------------------------------+------------+

                                  Table 7

   The IKEv2 RFC7296 mandates support for the RSA keys of size 1024 or
   2048 bits, but here we make key sizes less than 2048 SHOULD NOT as
   there is industry-wide trend to deprecate key lengths less than 2048
   bits.

4.2.  Digital Signature Recommendations

   Recommendations

   When Digital Signature authentication method is implemented, then the
   following recommendations are applied for when a hash function is involved in a signature: functions:

              +--------+-------------+------------+---------+
              | Number | Description | Status     | Comment |
              +--------+-------------+------------+---------+
              | 1      | SHA1        | SHOULD NOT |         |
              | 2      | SHA2-256    | MUST       |         |
              | 3      | SHA2-384    | MAY        |         |
              | 4      | SHA2-512    | SHOULD     |         |
              +--------+-------------+------------+---------+

                                  Table 8

   With the use of

   When Digital Signature, RSASSA-PKCS1-v1.5 MAY be
   implemented. Signature authentication method is used with RSA
   signature algorithm, then RSASSA-PSS MUST be implemented.

   Recommendation of Authentication Method described supported and RSASSA-
   PKCS1-v1.5 MAY be supported.

   The following table lists recommendations for authentication methods
   in RFC7427 [RFC7427]
   notation: notation.  These recommendations are applied
   only if Digital Signature authentication method is implemented.

       +------------------------------------+------------+---------+
       | Description                        | Status     | Comment |
       +------------------------------------+------------+---------+
       | RSASSA-PSS with SHA-256            | SHOULD MUST       |         |
       | ecdsa-with-sha256                  | SHOULD     |         |
       | sha1WithRSAEncryption              | SHOULD NOT |         |
       | dsa-with-sha1                      | SHOULD NOT |         |
       | ecdsa-with-sha1                    | SHOULD NOT |         |
       | RSASSA-PSS with Empty Parameters   | SHOULD NOT |         |
       | RSASSA-PSS with Default Parameters | SHOULD NOT |         |
       +------------------------------------+------------+---------+

                                  Table 9

5.  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 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 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 provide a way to use
   Minimal IKEv2 [RFC7815] over 802.15.4 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 do not include support
   for 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 order of tens of kbits/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.

6.  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.

7.  IANA Considerations

   This document makes no requests of IANA.

8.  Acknowledgements

   The first version of this document was RFC 4307 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 and
   Tommy Pauly for their valuable feedback.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/
              RFC2119, 10.17487/RFC2119, March 1997,
              <http://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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc4307>.

   [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>.

   [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,
              <http://www.rfc-editor.org/info/rfc5282>.

9.2.  Informative References

   [RFC7427]  Kivinen, T. and J. Snyder, "Signature Authentication in
              the Internet Key Exchange Version 2 (IKEv2)", RFC 7427,
              DOI 10.17487/RFC7427, January 2015,
              <http://www.rfc-editor.org/info/rfc7427>.

   [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,
              <http://www.rfc-editor.org/info/rfc6989>.

   [RFC7815]  Kivinen, T., "Minimal Internet Key Exchange Version 2
              (IKEv2) Initiator Implementation", RFC 7815,
              DOI 10.17487/
              RFC7815, 10.17487/RFC7815, March 2016,
              <http://www.rfc-editor.org/info/rfc7815>.

   [IKEV2-IANA]
              ,
              "Internet Key Exchange Version 2 (IKEv2) Parameters", ,
              <http://www.iana.org/assignments/ikev2-parameters>.

   [TRANSCRIPTION]
              Bhargavan, K. and G. Leurent, "Transcript Collision
              Attacks: Breaking Authentication in TLS, IKE, and SSH",
              NDSS , feb 2016.

   [IEEE-802-15-4]
              ,
              "IEEE Standard for Low-Rate Wireless Personal Area
              Networks (WPANs)", IEEE Standard 802.15.4, 2015.

   [IEEE-802-15-9]
              ,
              "IEEE Recommended Practice for Transport of Key Management
              Protocol (KMP) Datagrams", IEEE Standard 802.15.9, 2016.

Authors' Addresses

   Yoav Nir
   Check Point Software Technologies Ltd.
   5 Hasolelim st.
   Tel Aviv  6789735
   Israel

   EMail: ynir.ietf@gmail.com

   Tero Kivinen
   INSIDE Secure
   Eerikinkatu 28
   HELSINKI  FI-00180
   FI

   EMail: kivinen@iki.fi

   Paul Wouters
   Red Hat

   EMail: pwouters@redhat.com
   Daniel Migault
   Ericsson
   8400 boulevard Decarie
   Montreal, QC   H4P 2N2
   Canada

   Phone: +1 514-452-2160
   EMail: daniel.migault@ericsson.com