ceph_crypto_key_decode
ret = ceph_crypto_key_decode(&new_session_key, &dp, dend);
ret = ceph_crypto_key_decode(key, &p, p + blen);
ret = ceph_crypto_key_decode(ckey, &p, (char*)prep->data+datalen);
int ceph_crypto_key_decode(struct ceph_crypto_key *key, void **p, void *end);