serpent_set_key
serpent_set_key(ctx, keydata, keylen_in_bytes * 8);
serpent_set_key(&ctx->key1, keydata, (keylen_in_bytes / 2) * 8);
serpent_set_key(&ctx->key2, keydata + (keylen_in_bytes / 2),
void serpent_set_key(serpent_ctx *ctx, const u_int8_t in_key[], int key_len);