HMAC_PAD_BLOCKLEN
BUILD_BUG_ON(NPE_CTX_LEN < HMAC_PAD_BLOCKLEN);
memset(pad + key_len, 0, HMAC_PAD_BLOCKLEN - key_len);
for (i = 0; i < HMAC_PAD_BLOCKLEN; i++)
crypt->auth_len = HMAC_PAD_BLOCKLEN;
buf->buf_len = HMAC_PAD_BLOCKLEN;