Due to the nonce, the ESP key material is four bytes longer than needed for
the actual AES key. The crypto plugins, however, register their AES-CTR
implementations with the AES key length, so the lookup here failed.
For IKEv2 the key material is allocated after creating a crypter instance
with the negotiated AES key size. The length of the actual key material is
retrieved via get_key_size(), which adds the four bytes to the AES key length.
Fixes#1124.
Real AEADs directly provide a suitable IV generator, but traditional crypters
do not. For some (stream) ciphers, we should use sequential IVs, for which
we pass an appropriate generator to the AEAD wrapper.
The salt, or often called implicit nonce, varies between AEAD algorithms and
their use in protocols. For IKE and ESP, GCM uses 4 bytes, while CCM uses
3 bytes. With TLS, however, AEAD mode uses 4 bytes for both GCM and CCM.
Our GCM backends currently support 4 bytes and CCM 3 bytes only. This is fine
until we go for CCM mode support in TLS, which requires 4 byte nonces.
Tobias Brunner