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musig: add pubkey_tweak_add function to allow taproot tweaking

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This commit is contained in:
Jonas Nick
2020-11-10 22:33:47 +00:00
parent 38a8b20991
commit b9d91b3ecb
3 changed files with 250 additions and 15 deletions
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50
include/secp256k1_musig.h
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@@ -27,11 +27,18 @@ extern "C" {
* pk_hash: The 32-byte hash of the original public keys
* pk_parity: Whether the MuSig-aggregated point was negated when
* converting it to the combined xonly pubkey.
* is_tweaked: Whether the combined pubkey was tweaked
* tweak: If is_tweaked, array with the 32-byte tweak
* internal_key_parity: If is_tweaked, the parity of the combined pubkey
* before tweaking
*/
typedef struct {
uint64_t magic;
unsigned char pk_hash[32];
int pk_parity;
int is_tweaked;
unsigned char tweak[32];
int internal_key_parity;
} secp256k1_musig_pre_session;
/** Data structure containing data related to a signing session resulting in a single
@@ -139,7 +146,7 @@ typedef struct {
* multiexponentiation. If NULL, an inefficient algorithm is used.
* Out: combined_pk: the MuSig-combined xonly public key (cannot be NULL)
* pre_session: if non-NULL, pointer to a musig_pre_session struct to be used in
* `musig_session_init`.
* `musig_session_init` or `musig_pubkey_tweak_add`.
* In: pubkeys: input array of public keys to combine. The order is important;
* a different order will result in a different combined public
* key (cannot be NULL)
@@ -154,6 +161,42 @@ SECP256K1_API int secp256k1_musig_pubkey_combine(
size_t n_pubkeys
) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(5);
/** Tweak an x-only public key by adding the generator multiplied with tweak32
* to it. The resulting output_pubkey with the given internal_pubkey and tweak
* passes `secp256k1_xonly_pubkey_tweak_test`.
*
* This function is only useful before initializing a signing session. If you
* are only computing a public key, but not intending to create a signature for
* it, you can just use `secp256k1_xonly_pubkey_tweak_add`. Can only be called
* once with a given pre_session.
*
* Returns: 0 if the arguments are invalid or the resulting public key would be
* invalid (only when the tweak is the negation of the corresponding
* secret key). 1 otherwise.
* Args: ctx: pointer to a context object initialized for verification
* (cannot be NULL)
* pre_session: pointer to a `musig_pre_session` struct initialized in
* `musig_pubkey_combine` (cannot be NULL)
* Out: output_pubkey: pointer to a public key to store the result. Will be set
* to an invalid value if this function returns 0 (cannot
* be NULL)
* In: internal_pubkey: pointer to the `combined_pk` from
* `musig_pubkey_combine` to which the tweak is applied.
* (cannot be NULL).
* tweak32: pointer to a 32-byte tweak. If the tweak is invalid
* according to secp256k1_ec_seckey_verify, this function
* returns 0. For uniformly random 32-byte arrays the
* chance of being invalid is negligible (around 1 in
* 2^128) (cannot be NULL).
*/
SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_musig_pubkey_tweak_add(
const secp256k1_context* ctx,
secp256k1_musig_pre_session *pre_session,
secp256k1_pubkey *output_pubkey,
const secp256k1_xonly_pubkey *internal_pubkey,
const unsigned char *tweak32
) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4) SECP256K1_ARG_NONNULL(5);
/** Initializes a signing session for a signer
*
* Returns: 1: session is successfully initialized
@@ -173,8 +216,9 @@ SECP256K1_API int secp256k1_musig_pubkey_combine(
* because it reduces nonce misuse resistance. If NULL, must be
* set with `musig_session_get_public_nonce`.
* combined_pk: the combined xonly public key of all signers (cannot be NULL)
* pre_session: pointer to a musig_pre_session struct from
* `musig_pubkey_combine` (cannot be NULL)
* pre_session: pointer to a musig_pre_session struct after initializing
* it with `musig_pubkey_combine` and optionally provided to
* `musig_pubkey_tweak_add` (cannot be NULL).
* n_signers: length of signers array. Number of signers participating in
* the MuSig. Must be greater than 0 and at most 2^32 - 1.
* my_index: index of this signer in the signers array. Must be less