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Add module "musig" that implements MuSig2 multi-signatures (BIP 327)
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| Notes on the musig module API | ||
| =========================== | ||
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| The following sections contain additional notes on the API of the musig module (`include/secp256k1_musig.h`). | ||
| A usage example can be found in `examples/musig.c`. | ||
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| ## API misuse | ||
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| The musig API is designed with a focus on misuse resistance. | ||
| However, due to the interactive nature of the MuSig protocol, there are additional failure modes that are not present in regular (single-party) Schnorr signature creation. | ||
| While the results can be catastrophic (e.g. leaking of the secret key), it is unfortunately not possible for the musig implementation to prevent all such failure modes. | ||
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| Therefore, users of the musig module must take great care to make sure of the following: | ||
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| 1. A unique nonce per signing session is generated in `secp256k1_musig_nonce_gen`. | ||
| See the corresponding comment in `include/secp256k1_musig.h` for how to ensure that. | ||
| 2. The `secp256k1_musig_secnonce` structure is never copied or serialized. | ||
| See also the comment on `secp256k1_musig_secnonce` in `include/secp256k1_musig.h`. | ||
| 3. Opaque data structures are never written to or read from directly. | ||
| Instead, only the provided accessor functions are used. | ||
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| ## Key Aggregation and (Taproot) Tweaking | ||
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| Given a set of public keys, the aggregate public key is computed with `secp256k1_musig_pubkey_agg`. | ||
| A plain tweak can be added to the resulting public key with `secp256k1_ec_pubkey_tweak_add` by setting the `tweak32` argument to the hash defined in BIP 32. Similarly, a Taproot tweak can be added with `secp256k1_xonly_pubkey_tweak_add` by setting the `tweak32` argument to the TapTweak hash defined in BIP 341. | ||
| Both types of tweaking can be combined and invoked multiple times if the specific application requires it. | ||
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| ## Signing | ||
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| This is covered by `examples/musig.c`. | ||
| Essentially, the protocol proceeds in the following steps: | ||
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| 1. Generate a keypair with `secp256k1_keypair_create` and obtain the public key with `secp256k1_keypair_pub`. | ||
| 2. Call `secp256k1_musig_pubkey_agg` with the pubkeys of all participants. | ||
| 3. Optionally add a (Taproot) tweak with `secp256k1_musig_pubkey_xonly_tweak_add` and a plain tweak with `secp256k1_musig_pubkey_ec_tweak_add`. | ||
| 4. Generate a pair of secret and public nonce with `secp256k1_musig_nonce_gen` and send the public nonce to the other signers. | ||
| 5. Someone (not necessarily the signer) aggregates the public nonces with `secp256k1_musig_nonce_agg` and sends it to the signers. | ||
| 6. Process the aggregate nonce with `secp256k1_musig_nonce_process`. | ||
| 7. Create a partial signature with `secp256k1_musig_partial_sign`. | ||
| 8. Verify the partial signatures (optional in some scenarios) with `secp256k1_musig_partial_sig_verify`. | ||
| 9. Someone (not necessarily the signer) obtains all partial signatures and aggregates them into the final Schnorr signature using `secp256k1_musig_partial_sig_agg`. | ||
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| The aggregate signature can be verified with `secp256k1_schnorrsig_verify`. | ||
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| Steps 1 through 5 above can occur before or after the signers are aware of the message to be signed. | ||
| Whenever possible, it is recommended to generate the nonces only after the message is known. | ||
| This provides enhanced defense-in-depth measures, protecting against potential API misuse in certain scenarios. | ||
| However, it does require two rounds of communication during the signing process. | ||
| The alternative, generating the nonces in a pre-processing step before the message is known, eliminates these additional protective measures but allows for non-interactive signing. | ||
| Similarly, the API supports an alternative protocol flow where generating the aggregate key (steps 1 to 3) is allowed to happen after exchanging nonces (steps 4 to 5). | ||
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| ## Verification | ||
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| A participant who wants to verify the partial signatures, but does not sign itself may do so using the above instructions except that the verifier skips steps 1, 4 and 7. |
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