| Network Working Group | Y. Nir |
| Internet-Draft | Check Point |
| Intended status: Standards Track | T. Kivinen |
| Expires: August 12, 2016 | INSIDE Secure |
| P. Wouters | |
| Red Hat | |
| D. Migault | |
| Ericsson | |
| February 9, 2016 |
Algorithm Implementation Requirements and Usage Guidance for IKEv2
draft-ietf-ipsecme-rfc4307bis-03
The IPsec series of protocols makes use of various cryptographic algorithms in order to provide security services. The Internet Key Exchange protocol provides a mechanism to negotiate which algorithms should be used in any given Security Association. 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 will have available. This document defines the current algorithm implementation requirements and usage guidance of IKEv2. This document does not update the algorithms used for packet encryption using IPsec Encapsulated Security Payload (ESP)
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This Internet-Draft will expire on August 12, 2016.
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The Internet Key Exchange protocol [RFC7296] is used to negotiate the IPsec parameters, such as encryption algorithms and keys, for protected communications between two endpoints. The IKEv2 protocol itself is also protected by encryption, which is also negotiated between the two endpoints. Negotiation is performed by IKEv2 itself. This document describes the encryption parameters of the IKE protocol, not the encryption parameters of the ESP (IPsec) protocol. Different implementations of IKEv2 may negotiate different encryption algorithms based on their individual local policy. To ensure interoperability, a set of "mandatory-to-implement" IKEv2 encryption algorithms is defined.
The field of cryptography evolves continiously. 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 compromised. 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.
Ideally, the mandatory-to-implement algorithm of tomorrow should already be available in most implementations of IKE by the time it is made mandatory. To facilitate this, this document attempts to identify those algorithms for future mandatory-to-implement. There is no guarantee that the algorithms in use today may become mandatory in the future. Published algorithms are continiously 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 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 in order 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 constrainted devices and their choice of algorithms are motivated by minimizing the fooprint 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 needs to meet both high security expectations as well as high interoperability between various vendors and with different updates. 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 suites. On the other hand, comments and recommendations 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.
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-. |
| IoT | stands for Internet of Things. |
The algorithms in the below table are negotiated in the SA payload and used in the Encrypted Payload. References to the specifications 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. It is the only shared mandatory-to-implement algorithm with RFC4307 and as a result 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 and AES-GCM. It is also being standarized for IPsec for the same reasons. At the time of writing, there were not enough IKEv2 implementations of 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 as well as key longevity - compared to AES-CBC for example. This resulted in AES-GCM widely implemented for ESP. As the load of IKEv2 is expected to remain relatively small, many IKEv2 implementations do not include AES-GCM. In addition to its former MAY, this document does not promote AES-GCM to a greater status than SHOULD so to preserve interoperability between IKEv2 implementations. [PAUL: I dont follow the reasoning, as we could have AES and AES-GCM at MUST level] This document considers AES-GCM as mandatory to implement to promote the slightly more secure AEAD method over the traditional encrypt+auth method. Its status is expected to be raised once widely deployed.
ENCR_AES_CCM_8 was not considered in RFC4307. This document considers it 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 VPNs, its status is expected to remain to SHOULD. The 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-. All IKEv2 implementation already implement ENCR_AES_CBC, so there is no need to keep ENCR_3DES. In addition, ENCR_CHACHA20_POLY1305 provides a more modern alternative to AES. [PAUL: removed 'efficient' as we said above encryption efficiency at the IKE level hardly matters]
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.
In general, if you can trust an algorithm as INTEG algorithm, you can and should also use it as the PRF. When using an AEAD cipher, the choice is PRF is open, and picking one of the SHA2 variants is recommended.
| Name | Status | Comment |
|---|---|---|
| PRF_HMAC_SHA2_256 | MUST | |
| PRF_HMAC_SHA2_512 | SHOULD+ | |
| PRF_HMAC_SHA1 | MUST- | [1] |
| PRF_AES128_CBC | SHOULD | [IoT] |
[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 as a future replacement of SHA2_256 or when stronger security is required. PRF_HMAC_SHA2_512 is preferred over PRF_HMAC_SHA2_384, as the overhead of PRF_HMAC_SHA2_512 is negligible.
PRF_HMAC_SHA1_96 has been downgraded from MUST in RFC4307. There 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 wide VPN deployment, as it has not been widely adopted, it has been downgraded from SHOULD in RFC4307 to MAY.
The algorithms in the below table are negotiated in the SA payload and used in the ENCR payload. References to the specifications 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 | SHOULD | |
| AUTH_AES_XCBC_96 | SHOULD | [IoT] |
[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 as a future replacement of AUTH_HMAC_SHA2_256_128 or when stronger security is required. This value has been preferred to AUTH_HMAC_SHA2_384, as the overhead of AUTH_HMAC_SHA2_512 is negligible.
AUTH_HMAC_SHA1_96 has been downgraded from MUST in RFC4307. 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 wide VPN deployment, as it has not been widely adopted, it has been downgraded from SHOULD in RFC4307 to MAY.
There are several Modular Exponential (MODP) groups and several Elliptic Curve groups (ECC) that are defined for use in IKEv2. They are defined in both the [IKEv2] base document and in extensions documents. They are identified by group number.
| 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 | MUST NOT |
| 23 | 1024-bit MODP Group with 224-bit Prime Order Subgroup | MUST NOT |
| 24 | 1024-bit MODP Group with 256-bit Prime Order Subgroup | MUST 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. Group 19 is widely implemented and considered secure
Group 5 or 1536-bit MODP Group is downgrade from MUST- to SHOULD NOT. It was specified earlier, but 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 is downgrade from MUST- to SHOULD NOT. It was specified earlier, but now 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 can be broken within hours using cheap of-the-shelves hardware. It provides no security whatsoever.
Curve25519 has been designed with performance and security in mind and have been recommended by CFRG. At the time of writing, the IKEv2 specification is still at the draft status. This document specifies it as to encourage its implementation and deployment. If it gets widely implemented then it most likely will be upgraded to SHOULD or even MUST in the future.
Group 22-24 or 1024-bit MODP Group with 160-bit and 2048-bit MODP Group with 224-256-bit Prime Order Subgroup are exposed to synchronization or transcription attacks.
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 as what mandatory implementations are provided by the PKIX WG. This document is mostly concerned on signature verification and generation for the authentication.
| Number | Description | Status | Comment |
|---|---|---|---|
| 1 | RSA Digital Signature | MUST | With keys length 2048 |
| 1 | RSA Digital Signature | SHOULD | With keys length 3072/4096 |
| 1 | RSA Digital Signature | MUST NOT | With keys length lower than 2048 |
| 3 | DSS Digital Signature | MAY | |
| 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 mostly kept for interoperability. It is expected to be downgraded in the future as signatures are based on RSASSA-PKCS1-v1.5, not any more recommemded. Instead, more robust use of RSA is expected to be performed via the Digital Signature method.
ECDSA family are also expected to be downgraded as it does not provide hash function agility. Instead ECDSA is expected to be performed using the generic Digital Signature method.
DSS Digital Signature is bound to SHA-1 and thus is expected to be downgraded to MUST NOT in the future.
Digital Signature is expected to be promoted as it provides hash function, signature format and algorithm agility.
[MGLT: Do we have any recommendation for the authentication based on PSK?]
Here are the recommendations for the authentication methods.
| Number | Description | Status | Comment |
|---|---|---|---|
| OID | RSA | MUST | With keys length 2048 |
| OID | RSA | SHOULD | With keys length 3072/4096 |
| OID | RSA | MUST NOT | With keys length lower than 2048 |
| OID | ECDSA | SHOULD |
Here are the recommendations when a hash function is involved in a signature.
| Number | Description | Status | Comment |
|---|---|---|---|
| 1 | SHA1 | MUST | |
| 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, and RSASSA-PSS MUST be implemented.
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 Groups parameter is the most important one to choose conservatively. Any party capturing all traffic that can break the selected DH group can retroactively gain access to the symmetric keys used to encrypt all the IPsec data. However, specifying 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 thanks Paul Hoffman, Yaron Sheffer John Mattsson and Tommy Pauly for their valuable feed backs.
| [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. |
| [IKEV2-IANA] | , , "Internet Key Exchange Version 2 (IKEv2) Parameters" |