| Network Working Group | Y. Nir |
| Internet-Draft | Check Point |
| Intended status: Standards Track | T. Kivinen |
| Expires: September 17, 2016 | INSIDE Secure |
| P. Wouters | |
| Red Hat | |
| D. Migault | |
| Ericsson | |
| March 16, 2016 |
Algorithm Implementation Requirements and Usage Guidance for IKEv2
draft-ietf-ipsecme-rfc4307bis-04
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)
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Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 17, 2016.
Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved.
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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 encryption which is 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 encryption algorithms is defined.
This document describes the parameters of the IKE protocol. It does not describe the cryptographic parameters of the AH or ESP protocols.
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 this document.
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 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.
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.
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. |
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 MAY. 192-bit keys can safely be ignored. [IoT] - This requirement is for interoperability with IoT.
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 as 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 the IKE SA compared to the IPsec SA. 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.
Transform Type 2 Algorithms are pseudo-random functions used to generate 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_CBC | SHOULD | [IoT] |
| PRF_HMAC_MD5 | MUST NOT |
[IoT] - This requirement is for interoperability with IoT
PRF_HMAC_SHA2_256 was not mentioned in RFC4307, as no SHA2 based authentication was 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 for 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_CBC 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].
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
AUTH_HMAC_SHA2_256_128 was not mentioned in RFC4307, as no SHA2 based authentication was 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 for 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_HMAC_MD5_96, AUTH_DES_MAC and AUTH_KPDK_MD5 were not mentioned in RFC4307 so its default status was MAY. It has been downgraded to MUST NOT. There is an industry-wide trend to deprecate its usage. 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].
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 encryption for the session. If an attacker can retrieve the private numbers (for example a, b) (and? or?) the public values (for example g**a, g**b), then the attacker can compute the secret and the keys used and decrypt the exchange. 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 Order Subgroup | SHOULD NOT |
| 23 | 2048-bit MODP Group with 224-bit Prime Order Subgroup | SHOULD NOT |
| 24 | 2048-bit MODP Group with 256-bit Prime Order Subgroup | SHOULD NOT |
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.
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.
| Number | Description | Status |
|---|---|---|
| 1 | RSA Digital Signature | 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 therefor 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.
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.
| 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 smaler than 2048 | SHOULD NOT |
Recommendations for when a hash function is involved in a signature:
| Number | Description | Status | Comment |
|---|---|---|---|
| 1 | SHA1 | SHOULD NOT | |
| 2 | SHA2-256 | MUST | |
| 3 | SHA2-384 | MAY | |
| 4 | SHA2-512 | SHOULD |
With the use of Digital Signature, RSASSA-PKCS1-v1.5 MAY be implemented. RSASSA-PSS MUST be implemented.
Recommendation of Authentication Method described in [RFC7427] notation:
| Description | Status | Comment |
|---|---|---|
| RSASSA-PSS with SHA-256 | SHOULD | |
| 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 | |
| sha256WithRSAEncryption | MAY | |
| sha384WithRSAEncryption | MAY | |
| sha512WithRSAEncryption | MAY | |
| sha512WithRSAEncryption | MAY | |
| dsa-with-sha256 | MAY | |
| ecdsa-with-sha384 | MAY | |
| ecdsa-with-sha512 | MAY | ?SHOULD |
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.
This document makes no requests of IANA.
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.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [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. |
| [RFC4307] | Schiller, J., "Cryptographic Algorithms for Use in the Internet Key Exchange Version 2 (IKEv2)", RFC 4307, DOI 10.17487/RFC4307, December 2005. |
| [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. |
| [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. |
| [RFC7427] | Kivinen, T. and J. Snyder, "Signature Authentication in the Internet Key Exchange Version 2 (IKEv2)", RFC 7427, DOI 10.17487/RFC7427, January 2015. |
| [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. |
| [IKEV2-IANA] | , , "Internet Key Exchange Version 2 (IKEv2) Parameters" |
| [TRANSCRIPTION] | Bhargavan, K. and G. Leurent, "Transcript Collision Attacks: Breaking Authentication in TLS, IKE, and SSH", NDSS , feb 2016. |