core-lightning/lightningd/key_derive.c

#include <bitcoin/privkey.h>
#include <bitcoin/pubkey.h>
#include <ccan/crypto/sha256/sha256.h>
#include <lightningd/key_derive.h>
#include <utils.h>

/* BOLT #3:
 *
 * ### `localkey`, `remotekey`, `local-delayedkey` and `remote-delayedkey` Derivation
 *
 * These keys are simply generated by addition from their base points:
 *
 * 	pubkey = basepoint + SHA256(per-commitment-point || basepoint)*G
 *
 * The `localkey` uses the local node's `payment-basepoint`, `remotekey`
 * uses the remote node's `payment-basepoint`, the `local-delayedkey`
 * uses the local node's `delayed-payment-basepoint`, and the
 * `remote-delayedkey` uses the remote node's
 * `delayed-payment-basepoint`.
 */
bool derive_simple_key(const struct pubkey *basepoint,
		    const struct pubkey *per_commitment_point,
		    struct pubkey *key)
{
	struct sha256 sha;
	unsigned char der_keys[PUBKEY_DER_LEN * 2];

	pubkey_to_der(der_keys, per_commitment_point);
	pubkey_to_der(der_keys + PUBKEY_DER_LEN, basepoint);
	sha256(&sha, der_keys, sizeof(der_keys));
#ifdef SUPERVERBOSE
	printf("# SHA256(per-commitment-point || basepoint)\n");
	printf("# => SHA256(0x%s || 0x%s)\n",
	       tal_hexstr(tmpctx, der_keys, PUBKEY_DER_LEN),
	       tal_hexstr(tmpctx, der_keys + PUBKEY_DER_LEN, PUBKEY_DER_LEN));
	printf("# = 0x%s\n",
	       tal_hexstr(tmpctx, &sha, sizeof(sha)));
#endif

	*key = *basepoint;
	if (secp256k1_ec_pubkey_tweak_add(secp256k1_ctx,
					  &key->pubkey, sha.u.u8) != 1)
		return false;
#ifdef SUPERVERBOSE
	printf("# + basepoint (0x%s)\n",
	       type_to_string(tmpctx, struct pubkey, basepoint));
	printf("# = 0x%s\n",
	       type_to_string(tmpctx, struct pubkey, key));
#endif
	return true;
}

/* BOLT #3:
 *
 * The corresponding private keys can be derived similarly if the basepoint
 * secrets are known (i.e., `localkey` and `local-delayedkey` only):
 *
 *     secretkey = basepoint-secret + SHA256(per-commitment-point || basepoint)
 */
bool derive_simple_privkey(const struct privkey *base_secret,
			const struct pubkey *basepoint,
			const struct pubkey *per_commitment_point,
			struct privkey *key)
{
	struct sha256 sha;
	unsigned char der_keys[PUBKEY_DER_LEN * 2];

	pubkey_to_der(der_keys, per_commitment_point);
	pubkey_to_der(der_keys + PUBKEY_DER_LEN, basepoint);
	sha256(&sha, der_keys, sizeof(der_keys));
#ifdef SUPERVERBOSE
	printf("# SHA256(per-commitment-point || basepoint)\n");
	printf("# => SHA256(0x%s || 0x%s)\n",
	       tal_hexstr(tmpctx, der_keys, PUBKEY_DER_LEN),
	       tal_hexstr(tmpctx, der_keys + PUBKEY_DER_LEN, PUBKEY_DER_LEN));
	printf("# = 0x%s\n", tal_hexstr(tmpctx, &sha, sizeof(sha)));
#endif

	*key = *base_secret;
	if (secp256k1_ec_privkey_tweak_add(secp256k1_ctx, key->secret,
					   sha.u.u8) != 1)
		return false;
#ifdef SUPERVERBOSE
	printf("# + basepoint_secret (0x%s)\n",
	       tal_hexstr(tmpctx, base_secret, sizeof(*base_secret)));
	printf("# = 0x%s\n",
	       tal_hexstr(tmpctx, key, sizeof(*key)));
#endif
	return true;
}

/* BOLT #3:
 *
 * The revocationkey is a blinded key: the remote node provides the base,
 * and the local node provides the blinding factor which it later
 * reveals, so the remote node can use the secret revocationkey for a
 * penalty transaction.
 *
 * The `per-commitment-point` is generated using EC multiplication:
 *
 * 	per-commitment-point = per-commitment-secret * G
 *
 * And this is used to derive the revocation key from the remote node's
 * `revocation-basepoint`:
 *
 * 	revocationkey = revocation-basepoint * SHA256(revocation-basepoint || per-commitment-point) + per-commitment-point*SHA256(per-commitment-point || revocation-basepoint)
 */
bool derive_revocation_key(const struct pubkey *basepoint,
			const struct pubkey *per_commitment_point,
			struct pubkey *key)
{
	struct sha256 sha;
	unsigned char der_keys[PUBKEY_DER_LEN * 2];
	secp256k1_pubkey add[2];
	const secp256k1_pubkey *args[2];

	pubkey_to_der(der_keys, basepoint);
	pubkey_to_der(der_keys + PUBKEY_DER_LEN, per_commitment_point);
	sha256(&sha, der_keys, sizeof(der_keys));
#ifdef SUPERVERBOSE
	printf("# SHA256(revocation-basepoint || per-commitment-point)\n");
	printf("# => SHA256(0x%s || 0x%s)\n",
	       tal_hexstr(tmpctx, der_keys, PUBKEY_DER_LEN),
	       tal_hexstr(tmpctx, der_keys + PUBKEY_DER_LEN, PUBKEY_DER_LEN));
	printf("# = 0x%s\n", tal_hexstr(tmpctx, sha.u.u8, sizeof(sha.u.u8))),
#endif

	add[0] = basepoint->pubkey;
	if (secp256k1_ec_pubkey_tweak_mul(secp256k1_ctx, &add[0], sha.u.u8) != 1)
		return false;
#ifdef SUPERVERBOSE
	printf("# x revocation-basepoint = 0x%s\n",
	       type_to_string(tmpctx, secp256k1_pubkey, &add[0]));
#endif

	pubkey_to_der(der_keys, per_commitment_point);
	pubkey_to_der(der_keys + PUBKEY_DER_LEN, basepoint);
	sha256(&sha, der_keys, sizeof(der_keys));
#ifdef SUPERVERBOSE
	printf("# SHA256(per-commitment-point || revocation-basepoint)\n");
	printf("# => SHA256(0x%s || 0x%s)\n",
	       tal_hexstr(tmpctx, der_keys, PUBKEY_DER_LEN),
	       tal_hexstr(tmpctx, der_keys + PUBKEY_DER_LEN, PUBKEY_DER_LEN));
	printf("# = 0x%s\n", tal_hexstr(tmpctx, sha.u.u8, sizeof(sha.u.u8))),
#endif

	add[1] = per_commitment_point->pubkey;
	if (secp256k1_ec_pubkey_tweak_mul(secp256k1_ctx, &add[1], sha.u.u8) != 1)
		return false;
#ifdef SUPERVERBOSE
	printf("# x per-commitment-point = 0x%s\n",
	       type_to_string(tmpctx, secp256k1_pubkey, &add[1]));
#endif

	args[0] = &add[0];
	args[1] = &add[1];
	if (secp256k1_ec_pubkey_combine(secp256k1_ctx, &key->pubkey, args, 2)
	    != 1)
		return false;

#ifdef SUPERVERBOSE
	printf("# 0x%s + 0x%s => 0x%s\n",
	       type_to_string(tmpctx, secp256k1_pubkey, args[0]),
	       type_to_string(tmpctx, secp256k1_pubkey, args[1]),
	       type_to_string(tmpctx, struct pubkey, key));
#endif
	return true;
}

/* BOLT #3:
 *
 * The corresponding private key can be derived once the `per-commitment-secret`
 * is known:
 *
 *     revocationsecretkey = revocation-basepoint-secret * SHA256(revocation-basepoint || per-commitment-point) + per-commitment-secret*SHA256(per-commitment-point || revocation-basepoint)
 */
bool derive_revocation_privkey(const struct privkey *base_secret,
			       const struct privkey *per_commitment_secret,
			       const struct pubkey *basepoint,
			       const struct pubkey *per_commitment_point,
			       struct privkey *key)
{
	struct sha256 sha;
	unsigned char der_keys[PUBKEY_DER_LEN * 2];
	struct privkey part2;

	pubkey_to_der(der_keys, basepoint);
	pubkey_to_der(der_keys + PUBKEY_DER_LEN, per_commitment_point);
	sha256(&sha, der_keys, sizeof(der_keys));
#ifdef SUPERVERBOSE
	printf("# SHA256(revocation-basepoint || per-commitment-point)\n");
	printf("# => SHA256(0x%s || 0x%s)\n",
	       tal_hexstr(tmpctx, der_keys, PUBKEY_DER_LEN),
	       tal_hexstr(tmpctx, der_keys + PUBKEY_DER_LEN, PUBKEY_DER_LEN));
	printf("# = 0x%s\n", tal_hexstr(tmpctx, sha.u.u8, sizeof(sha.u.u8))),
#endif

	*key = *base_secret;
	if (secp256k1_ec_privkey_tweak_mul(secp256k1_ctx, key->secret, sha.u.u8)
	    != 1)
		return false;
#ifdef SUPERVERBOSE
	printf("# * revocation-basepoint-secret (0x%s)",
	       tal_hexstr(tmpctx, base_secret, sizeof(*base_secret))),
	printf("# = 0x%s\n", tal_hexstr(tmpctx, key, sizeof(*key))),
#endif

	pubkey_to_der(der_keys, per_commitment_point);
	pubkey_to_der(der_keys + PUBKEY_DER_LEN, basepoint);
	sha256(&sha, der_keys, sizeof(der_keys));
#ifdef SUPERVERBOSE
	printf("# SHA256(per-commitment-point || revocation-basepoint)\n");
	printf("# => SHA256(0x%s || 0x%s)\n",
	       tal_hexstr(tmpctx, der_keys, PUBKEY_DER_LEN),
	       tal_hexstr(tmpctx, der_keys + PUBKEY_DER_LEN, PUBKEY_DER_LEN));
	printf("# = 0x%s\n", tal_hexstr(tmpctx, sha.u.u8, sizeof(sha.u.u8))),
#endif

	part2 = *per_commitment_secret;
	if (secp256k1_ec_privkey_tweak_mul(secp256k1_ctx, part2.secret,
					   sha.u.u8) != 1)
		return false;
#ifdef SUPERVERBOSE
	printf("# * per-commitment-secret (0x%s)",
	       tal_hexstr(tmpctx, per_commitment_secret,
			  sizeof(*per_commitment_secret))),
		printf("# = 0x%s\n", tal_hexstr(tmpctx, &part2, sizeof(part2)));
#endif

	if (secp256k1_ec_privkey_tweak_add(secp256k1_ctx, key->secret,
					   part2.secret) != 1)
		return false;

#ifdef SUPERVERBOSE
	printf("# => 0x%s\n", tal_hexstr(tmpctx, key, sizeof(*key)));
#endif
	return true;
}