Cryptographic Algorithm Implementation Requirements and Usage Guidance for Encapsulating Security Payload (ESP) and Authentication Header (AH)

The Encapsulating Security Payload (ESP) and the Authentication Header (AH) are the mechanisms for applying cryptographic protection to data being sent over an IPsec Security Association (SA) . This document provides guidance and recommendations so that ESP and AH can be used with a cryptographic algorithms that are up to date. The challenge of such document is to make sure that over the time IPsec implementations can use secure and up-to-date cryptographic algorithms while keeping IPsec interoperable.

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 a separate document.

The mandatory-to-implement algorithm of tomorrow should already be available in most implementations of AH/ESP 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 and algorithms too weak that are recommended not to be implemented. As a result, any algorithm listed at the IPsec IANA registry not mentioned in this document MAY be implemented. As omitted most of the algorithms mentioned by the IPsec IANA repository, which makes it difficult to define whether non mentioned algorithms are optional to implement or must not be implemented as they are too weak. This document provides explicit guidance for all of them. It is expected that this document will be updated over time and next versions will only mention algorithms which status has evolved. For clarification when an algorithm has been mentioned in , this document states explicitly the update of the status. Although this document updates the algorithms to keep the AH/ESP communication secure over time, it also aims at providing recommendations so that AH/ESP implementations remain interoperable. AH/ESP interoperability is addressed by an incremental introduction or deprecation of algorithms. In addition, this document also considers the new use cases for AH/ESP 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 AH/ESP 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.

The recommendations of this document mostly target AH/ESP 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 AH/ESP with only the safest cipher suite. This document does not give any recommendations for the use of algorithms, it only gives implementation recommendations for implementations. The use of algorithms by users is dictated by the security policy requirements for that specific user, and are outside the scope of this document. The algorithms considered here are listed by the IANA as part of the IKEv2 parameters. IKEv1 is out of scope of this document. IKEv1 is deprecated and the recommendations of this document must not be considered for IKEv1, nor IKEv1 parameters be considered by this document. The IANA registry for Internet Key Exchange Version 2 (IKEv2) Parameters contains some entries that are not for use with ESP or AH. This document does not modify the status of those algorithms.

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 . Following , we define some additional key words: This term means the same as MUST. However, we expect that at some point in the future this algorithm will no longer be a MUST. 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.

Manual Keying is not to be used as it is inherently dangerous. Without any keying protocol, it does not offer Perfect Forward Secrecy ("PFS") protection. Deployments tend to never be reconfigured with fresh session keys. It also fails to scale and keeping SPI's unique amongst many servers is impractical. This document was written for deploying ESP/AH using IKE (RFC7298) and assumes that keying happens using IKEv2. If manual keying is used anyway, ENCR_AES_CBC MUST be used, and ENCR_AES_CCM, ENCR_AES_GCM and ENCR_CHACHA20_POLY1305 MUST NOT be used as these algorithms require IKE.

Name Status AEAD Comment ENCR_DES_IV64 MUST NOTNoUNSPECIFIED ENCR_DES MUST NOTNo ENCR_3DES SHOULD NOTNo ENCR_BLOWFISH MUST NOTNo ENCR_3IDEA MUST NOTNoUNSPECIFIED ENCR_DES_IV32 MUST NOTNoUNSPECIFIED ENCR_NULL MUSTNo ENCR_AES_CBC MUSTNo[1] ENCR_AES_CCM_8 SHOULDYesIoT] ENCR_AES_GCM_16 MUSTYes[1] ENCR_CHACHA20_POLY1305 SHOULD Yes [1] - This requirement level is for 128-bit and 256-bit keys. 192-bit keys remain at MAY level. [IoT] - This requirement is for interoperability with IoT. Only 128-bit keys are at MUST level. 192-bit and 256-bit keys are at the MAY level. IPsec sessions may have very long life time, and carry multiple packets, so there is a need to move to 256-bit keys in the long term. For that purpose the requirement level for 128 bit keys and 256 bit keys are at MUST (when applicable). In that sense 256 bit keys status has been raised from MAY in RFC7321 to MUST. IANA has allocated codes for cryptographic algorithms that have not been specified by the IETF. Such algorithms are noted as UNSPECIFIED. Usually, the use of theses algorithms is limited to specific cases, and the absence of specification makes interoperability difficult for IPsec communications. These algorithms were not been mentioned in and this document clarify that such algorithms MUST NOT be implemented for IPsec communications. Similarly IANA also allocated code points for algorithms that are not expected to be used to secure IPsec communications. Such algorithms are noted as Non IPsec. As a result, these algorithms MUST NOT be implemented. Various older and not well tested and never widely implemented ciphers have been changed to MUST NOT. ENCR_3DES status has been downgraded from MAY in RFC7321 to SHOULD NOT. ENCR_CHACHA20_POLY1305 is a more modern approach alternative for ENCR_3DES than ENCR_AES_CBC and so it expected to be favored to replace ENCR_3DES. ENCR_BLOWFISH has been downgraded to MUST NOT as it has been deprecated for years by TWOFISH, which is not standarized for ESP and therefore not listed in this document. Some implementations support TWOFISH using a private range number. ENCR_NULL status was set to MUST in and remains a MUST to enable the use of ESP with only authentication which is preferred over AH due to NAT traversal. ENCR_NULL is expected to remain MUST by protocol requirements. ENCR_AES_CBC status remains at MUST. ENCR_AES_CBC MUST be implemented in order to enable interoperability between implementations that followed RFC7321. However, there is a trend for the industry to move to AEAD encryption, and the overhead of ENCR_AES_CBC remains quite large so it is expected to be replaced by AEAD algorithms in the long term. ENCR_AES_CCM_8 status was set to MAY in and has been raised from MAY to SHOULD in order to interact with Internet of Things devices. As this case is not a general use case for VPNs, its status is expected to remain as SHOULD. ENCR_AES_GCM_16 status has been updated from SHOULD+ to MUST in order to favor the use of authenticated encryption and AEAD algorithms. ENCR_AES_GCM_16 has been widely implemented for ESP due to its increased performance and key longevity compared to ENCR_AES_CBC. ENCR_CHACHA20_POLY1305 was not ready to be considered at the time of RFC7321. It has been recommended by the CRFG and others as an alternative to ENCR_AES_XCBC and ENCR_AES_GCM_*. It is also being standardized for ESP for the same reasons. At the time of writing, there are not enough ESP implementations of ENCR_CHACHA20_POLY1305 to be able to introduce it at the SHOULD+ level. Its status has been set to SHOULD and is expected to become MUST in the long term.

Encryption without authentication MUST NOT be used. As a result, authentication algorithm recommendations in this section are targeting two types of communications: Firstly authenticated only communications without encryption. Such communications can be ESP with NULL encryption or AH communications. Secondly, communications that are encrypted with non AEAD encryption algorithms mentioned above. In this case, they MUST be combined with an authentication algorithm. Name Status Comment AUTH_NONE MUST / MUST NOT AEAD AUTH_HMAC_MD5_96 MUST NOT AUTH_HMAC_SHA1_96 MUST- AUTH_DES_MAC MUST NOT [UNSPECIFIED] AUTH_KPDK_MD5 MUST NOT [UNSPECIFIED] AUTH_AES_XCBC_96 SHOULD [IoT] AUTH_AES_128_GMAC MAY AUTH_AES_256_GMAC MAY AUTH_HMAC_SHA2_256_128 MUST AUTH_HMAC_SHA2_512_256 SHOULD [IoT] - This requirement is for interoperability with IoT AUTH_NONE has been downgraded from MAY in RFC7321 to MUST NOT. The only reason NULL is acceptable is when authenticated encryption algorithms are selected from . In all other cases, NULL MUST NOT be selected. As ESP and AH both provides authentication, one may be tempted to combine these protocols to provide authentication. As mentioned by RFC7321, it is NOT RECOMMENDED to use ESP with NULL authentication - with non authenticated encryption - in conjunction with AH; some configurations of this combination of services have been shown to be insecure . In addition, NULL authentication cannot be combined with ESP NULL encryption. AUTH_HMAC_MD5_96 and AUTH_KPDK_MD5 were not mentioned in RFC7321. As MD5 is known to be vulnerable to collisions, these algorithms MUST NOT be used. AUTH_HMAC_SHA1_96 has been downgraded from MUST in RFC7321 to MUST- as there is an industry-wide trend to deprecate its usage. AUTH_DES_MAC was not mentioned in RFC7321. As DES is known to be vulnerable, it MUST NOT be used. AUTH_AES_XCBC_96 is set as SHOULD only in the scope of IoT, as Internet of Things deployments tend to prefer AES based HMAC 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 to MAY. AUTH_AES_128_GMAC status has been downgraded from SHOULD+ to MAY. Along with AUTH_AES_192_GMAC and AUTH_AES_256_GMAC, these algorithms should only be used for AH and not for ESP. If using ENCR_NULL, AUTH_HMAC_SHA2_256_128 is recommended for integrity. If using AES-GMAC in ESP without authentication, ENCR_NULL_AUTH_AES_GMAC is recommended. Therefore, these ciphers are kept at MAY. AUTH_HMAC_SHA2_256_128 was not mentioned in RFC7321, as no SHA2 based authentication was mentioned. AUTH_HMAC_SHA2_256_128 MUST be implemented in order to replace AUTH_HMAC_SHA1_96. Note that due to a long standing common implementation bug of this algorithm that truncates the hash at 96-bits instead of 128-bits, it is recommended that implementations prefer AUTH_HMAC_SHA2_512_256 over AUTH_HMAC_SHA2_256_128 if they implement AUTH_HMAC_SHA2_512_256. 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 to AUTH_HMAC_SHA2_384, as the additional overhead of AUTH_HMAC_SHA2_512 is negligible.

Name Status Comment IPCOMP_OUIMUST NOTUNSPECIFIED IPCOMP_DEFLATEMAY IPCOMP_LZSMAY IPCOMP_LZJHMAY [IoT] - This requirement is for interoperability with IoT Compression was not mentioned in RFC7321. As it is not widely deployed, it remains optional and at the MAY-level.

The following table summarizes the changes from RFC 7321. RFC EDITOR: PLEASE REMOVE THIS PARAGRAPH AND REPLACE XXXX IN THE TABLE BELOW WITH THE NUMBER OF THIS RFC Algorithm RFC 7321 RFC XXXX ENCR_AES_GCM_16SHOULD+MUST ENCR_AES_CCM_8MAYSHOULD ENCR_AES_CTRMAY(*) ENCR_3DESMAYSHOULD NOT AUTH_HMAC_SHA1_96MUSTMUST- AUTH_AES_128_GMACSHOULD+MAY AUTH_NONEMAYMUST / MUST NOT (*) This algorithm is not mentioned in the above sections, so it defaults to MAY.

Some of the wording in this document was adapted from , the document that this one obsoletes, which was written by D. McGrew and P. Hoffman.

This document has no IANA actions.

The security of a system that uses cryptography 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 and administration of the protocol used by the system to ensure that there are no non-cryptographic ways to bypass the security of the overall system. This document concerns itself with the selection of cryptographic algorithms for the use of ESP and AH, 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 to date leads us to believe that they will likely remain secure into the foreseeable future. However, this is not necessarily forever. Therefore, we expect that revisions of that document will be issued from time to time to reflect the current best practice in this area.