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)
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 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.
Nir, et al. Standards Track [Page 1]RFC 8247 IKEv2 Cryptographic Algorithms September 2017
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.
Table of Contents
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
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
Nir, 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 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.
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 in
BCP 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 2017
1.2. 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 Selection
2.1. 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 in
Section 2.3.
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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 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.
Nir, et al. Standards Track [Page 6]RFC 8247 IKEv2 Cryptographic Algorithms September 2017
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.
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 in 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.
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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.
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 (see 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.
Nir, et al. Standards Track [Page 8]RFC 8247 IKEv2 Cryptographic Algorithms September 2017
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].
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+ 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
Nir, 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 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.
2.5. Summary of Changes from RFC 4307
The following table summarizes the changes from 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.
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.
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3.1. 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 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.
Nir, et al. Standards Track [Page 12]RFC 8247 IKEv2 Cryptographic Algorithms September 2017
3.1.1. 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 and
Nir, 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. References
7.1. 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 in RFC
2119 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/>.
Acknowledgements
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.
Nir, et al. Standards Track [Page 18]RFC 8247 IKEv2 Cryptographic Algorithms September 2017
Authors' 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.com
Nir, et al. Standards Track [Page 19]
