BoringSSL only supports a limited list of (hard-coded) algorithms via
EVP_get_cipherbyname(), which does not include AES-GCM. While BoringSSL
deprecated these functions they are also supported by OpenSSL (in BoringSSL
a completely new interface for AEADs was added, which OpenSSL currently does
not support).
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