Support for algorithms and ciphers tries to narrowly follow two RFCs:

• RFC 8221: Cryptographic Algorithm Implementation Requirements and Usage Guidance for Encapsulating Security Payload (ESP) and Authentication Header (AH)

• RFC 8247: Algorithm Implementation Requirements and Usage Guidance for the Internet Key Exchange Protocol Version 2 (IKEv2)

Integrity and PRF

In both IKEv1 and IKEv2 there is a PRF and an INTEG algorithm. Libreswan only supports scenario’s where the PRF and INTEG are the same. The reason is, if the algorithm is good enough for PRF, it is goof enough for INTEG. If it is not good enough for one of the two, it is also not good enough for the other. But when the IKEv2 RFCs were written, some people insisted on being able to have different values for these. Libreswan in fact cannot pass one TAHI suite test case because libreswan do not support such strange (ut RFC compliant) configurations.

There is one special case. While classic encryption/integrity uses two separate algorithms (eg AES_CBC with HMAC-SHA2, or 3DES with HMAC-SHA1), the newer AEAD algorithms work differently. These algorithms only require one pass of the data to do both encryption/decryption and integrity in one step. Examples of such algorithms are AES_GCM, AES_CCM and CHACHA20-POLY1305. IKE negotiations (for IKE as well as IPsec) send proposals listing encryption and integrity (and prf) values. Instead of adding a new type for AEADs, they decided to list these AEAD algorithms as “encryption algorithm” types. These AEAD encryption types are not allowed to specify an “integrity” algorithm (or specify integrity algorithm “none”).

Since there the integrity algorithm is none, we don’t know what to use for PRF. We cannot fill it in from the integrity, so you have to specify which prf you want to use. Hence you end up with a syntax like:

        ike=aes_gcm-sha2

Which means ENCR=aes-gcm, INTEG=null/none and PRF=sha2

The PRF is used to generate key material, which is used to create keys for IPsec SA. The IPsec SA themselves do not have a “prf”. So, to specify aes_gcm for IPsec/ESP, you would use:

        phase2alg=aes_gcm-null

And as of libreswan 3.22, you can leave out the -null for the phase2alg entry, as this is assumed to be the case. Any other integrity function will be rejected.