1 #ifndef SECP256K1_SCHNORRSIG_H
2 #define SECP256K1_SCHNORRSIG_H
3 4 #include "secp256k1.h"
5 #include "secp256k1_extrakeys.h"
6 7 #ifdef __cplusplus
8 extern "C" {
9 #endif
10 11 /** This module implements a variant of Schnorr signatures compliant with
12 * Bitcoin Improvement Proposal 340 "Schnorr Signatures for secp256k1"
13 * (https://github.com/bitcoin/bips/blob/master/bip-0340.mediawiki).
14 */
15 16 /** A pointer to a function to deterministically generate a nonce.
17 *
18 * Same as secp256k1_nonce function with the exception of accepting an
19 * additional pubkey argument and not requiring an attempt argument. The pubkey
20 * argument can protect signature schemes with key-prefixed challenge hash
21 * inputs against reusing the nonce when signing with the wrong precomputed
22 * pubkey.
23 *
24 * Returns: 1 if a nonce was successfully generated. 0 will cause signing to
25 * return an error.
26 * Out: nonce32: pointer to a 32-byte array to be filled by the function
27 * In: msg: the message being verified. Is NULL if and only if msglen
28 * is 0.
29 * msglen: the length of the message
30 * key32: pointer to a 32-byte secret key (will not be NULL)
31 * xonly_pk32: the 32-byte serialized xonly pubkey corresponding to key32
32 * (will not be NULL)
33 * algo: pointer to an array describing the signature
34 * algorithm (will not be NULL)
35 * algolen: the length of the algo array
36 * data: arbitrary data pointer that is passed through
37 *
38 * Except for test cases, this function should compute some cryptographic hash of
39 * the message, the key, the pubkey, the algorithm description, and data.
40 */
41 typedef int (*secp256k1_nonce_function_hardened)(
42 unsigned char *nonce32,
43 const unsigned char *msg,
44 size_t msglen,
45 const unsigned char *key32,
46 const unsigned char *xonly_pk32,
47 const unsigned char *algo,
48 size_t algolen,
49 void *data
50 );
51 52 /** An implementation of the nonce generation function as defined in Bitcoin
53 * Improvement Proposal 340 "Schnorr Signatures for secp256k1"
54 * (https://github.com/bitcoin/bips/blob/master/bip-0340.mediawiki).
55 *
56 * If a data pointer is passed, it is assumed to be a pointer to 32 bytes of
57 * auxiliary random data as defined in BIP-340. If the data pointer is NULL,
58 * the nonce derivation procedure follows BIP-340 by setting the auxiliary
59 * random data to zero. The algo argument must be non-NULL, otherwise the
60 * function will fail and return 0. The hash will be tagged with algo.
61 * Therefore, to create BIP-340 compliant signatures, algo must be set to
62 * "BIP0340/nonce" and algolen to 13.
63 */
64 SECP256K1_API const secp256k1_nonce_function_hardened secp256k1_nonce_function_bip340;
65 66 /** Data structure that contains additional arguments for schnorrsig_sign_custom.
67 *
68 * A schnorrsig_extraparams structure object can be initialized correctly by
69 * setting it to SECP256K1_SCHNORRSIG_EXTRAPARAMS_INIT.
70 *
71 * Members:
72 * magic: set to SECP256K1_SCHNORRSIG_EXTRAPARAMS_MAGIC at initialization
73 * and has no other function than making sure the object is
74 * initialized.
75 * noncefp: pointer to a nonce generation function. If NULL,
76 * secp256k1_nonce_function_bip340 is used
77 * ndata: pointer to arbitrary data used by the nonce generation function
78 * (can be NULL). If it is non-NULL and
79 * secp256k1_nonce_function_bip340 is used, then ndata must be a
80 * pointer to 32-byte auxiliary randomness as per BIP-340.
81 */
82 typedef struct secp256k1_schnorrsig_extraparams {
83 unsigned char magic[4];
84 secp256k1_nonce_function_hardened noncefp;
85 void *ndata;
86 } secp256k1_schnorrsig_extraparams;
87 88 #define SECP256K1_SCHNORRSIG_EXTRAPARAMS_MAGIC { 0xda, 0x6f, 0xb3, 0x8c }
89 #define SECP256K1_SCHNORRSIG_EXTRAPARAMS_INIT {\
90 SECP256K1_SCHNORRSIG_EXTRAPARAMS_MAGIC,\
91 NULL,\
92 NULL\
93 }
94 95 /** Create a Schnorr signature.
96 *
97 * Does _not_ strictly follow BIP-340 because it does not verify the resulting
98 * signature. Instead, you can manually use secp256k1_schnorrsig_verify and
99 * abort if it fails.
100 *
101 * This function only signs 32-byte messages. If you have messages of a
102 * different size (or the same size but without a context-specific tag
103 * prefix), it is recommended to create a 32-byte message hash with
104 * secp256k1_tagged_sha256 and then sign the hash. Tagged hashing allows
105 * providing an context-specific tag for domain separation. This prevents
106 * signatures from being valid in multiple contexts by accident.
107 *
108 * Returns 1 on success, 0 on failure.
109 * Args: ctx: pointer to a context object (not secp256k1_context_static).
110 * Out: sig64: pointer to a 64-byte array to store the serialized signature.
111 * In: msg32: the 32-byte message being signed.
112 * keypair: pointer to an initialized keypair.
113 * aux_rand32: 32 bytes of fresh randomness. While recommended to provide
114 * this, it is only supplemental to security and can be NULL. A
115 * NULL argument is treated the same as an all-zero one. See
116 * BIP-340 "Default Signing" for a full explanation of this
117 * argument and for guidance if randomness is expensive.
118 */
119 SECP256K1_API int secp256k1_schnorrsig_sign32(
120 const secp256k1_context *ctx,
121 unsigned char *sig64,
122 const unsigned char *msg32,
123 const secp256k1_keypair *keypair,
124 const unsigned char *aux_rand32
125 ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4);
126 127 /** Same as secp256k1_schnorrsig_sign32, but DEPRECATED. Will be removed in
128 * future versions. */
129 SECP256K1_API int secp256k1_schnorrsig_sign(
130 const secp256k1_context *ctx,
131 unsigned char *sig64,
132 const unsigned char *msg32,
133 const secp256k1_keypair *keypair,
134 const unsigned char *aux_rand32
135 ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4)
136 SECP256K1_DEPRECATED("Use secp256k1_schnorrsig_sign32 instead");
137 138 /** Create a Schnorr signature with a more flexible API.
139 *
140 * Same arguments as secp256k1_schnorrsig_sign except that it allows signing
141 * variable length messages and accepts a pointer to an extraparams object that
142 * allows customizing signing by passing additional arguments.
143 *
144 * Equivalent to secp256k1_schnorrsig_sign32(..., aux_rand32) if msglen is 32
145 * and extraparams is initialized as follows:
146 * ```
147 * secp256k1_schnorrsig_extraparams extraparams = SECP256K1_SCHNORRSIG_EXTRAPARAMS_INIT;
148 * extraparams.ndata = (unsigned char*)aux_rand32;
149 * ```
150 *
151 * Returns 1 on success, 0 on failure.
152 * Args: ctx: pointer to a context object (not secp256k1_context_static).
153 * Out: sig64: pointer to a 64-byte array to store the serialized signature.
154 * In: msg: the message being signed. Can only be NULL if msglen is 0.
155 * msglen: length of the message.
156 * keypair: pointer to an initialized keypair.
157 * extraparams: pointer to an extraparams object (can be NULL).
158 */
159 SECP256K1_API int secp256k1_schnorrsig_sign_custom(
160 const secp256k1_context *ctx,
161 unsigned char *sig64,
162 const unsigned char *msg,
163 size_t msglen,
164 const secp256k1_keypair *keypair,
165 secp256k1_schnorrsig_extraparams *extraparams
166 ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(5);
167 168 /** Verify a Schnorr signature.
169 *
170 * Returns: 1: correct signature
171 * 0: incorrect signature
172 * Args: ctx: pointer to a context object.
173 * In: sig64: pointer to the 64-byte signature to verify.
174 * msg: the message being verified. Can only be NULL if msglen is 0.
175 * msglen: length of the message
176 * pubkey: pointer to an x-only public key to verify with
177 */
178 SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_schnorrsig_verify(
179 const secp256k1_context *ctx,
180 const unsigned char *sig64,
181 const unsigned char *msg,
182 size_t msglen,
183 const secp256k1_xonly_pubkey *pubkey
184 ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(5);
185 186 #ifdef __cplusplus
187 }
188 #endif
189 190 #endif /* SECP256K1_SCHNORRSIG_H */
191