Bitcoin Core  0.20.99
P2P Digital Currency
main_impl.h
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1 /**********************************************************************
2  * Copyright (c) 2018-2020 Andrew Poelstra, Jonas Nick *
3  * Distributed under the MIT software license, see the accompanying *
4  * file COPYING or http://www.opensource.org/licenses/mit-license.php.*
5  **********************************************************************/
6 
7 #ifndef _SECP256K1_MODULE_SCHNORRSIG_MAIN_
8 #define _SECP256K1_MODULE_SCHNORRSIG_MAIN_
9 
10 #include "include/secp256k1.h"
12 #include "hash.h"
13 
14 /* Initializes SHA256 with fixed midstate. This midstate was computed by applying
15  * SHA256 to SHA256("BIP0340/nonce")||SHA256("BIP0340/nonce"). */
18  sha->s[0] = 0x46615b35ul;
19  sha->s[1] = 0xf4bfbff7ul;
20  sha->s[2] = 0x9f8dc671ul;
21  sha->s[3] = 0x83627ab3ul;
22  sha->s[4] = 0x60217180ul;
23  sha->s[5] = 0x57358661ul;
24  sha->s[6] = 0x21a29e54ul;
25  sha->s[7] = 0x68b07b4cul;
26 
27  sha->bytes = 64;
28 }
29 
30 /* Initializes SHA256 with fixed midstate. This midstate was computed by applying
31  * SHA256 to SHA256("BIP0340/aux")||SHA256("BIP0340/aux"). */
34  sha->s[0] = 0x24dd3219ul;
35  sha->s[1] = 0x4eba7e70ul;
36  sha->s[2] = 0xca0fabb9ul;
37  sha->s[3] = 0x0fa3166dul;
38  sha->s[4] = 0x3afbe4b1ul;
39  sha->s[5] = 0x4c44df97ul;
40  sha->s[6] = 0x4aac2739ul;
41  sha->s[7] = 0x249e850aul;
42 
43  sha->bytes = 64;
44 }
45 
46 /* algo16 argument for nonce_function_bip340 to derive the nonce exactly as stated in BIP-340
47  * by using the correct tagged hash function. */
48 static const unsigned char bip340_algo16[16] = "BIP0340/nonce\0\0\0";
49 
50 static int nonce_function_bip340(unsigned char *nonce32, const unsigned char *msg32, const unsigned char *key32, const unsigned char *xonly_pk32, const unsigned char *algo16, void *data) {
51  secp256k1_sha256 sha;
52  unsigned char masked_key[32];
53  int i;
54 
55  if (algo16 == NULL) {
56  return 0;
57  }
58 
59  if (data != NULL) {
61  secp256k1_sha256_write(&sha, data, 32);
62  secp256k1_sha256_finalize(&sha, masked_key);
63  for (i = 0; i < 32; i++) {
64  masked_key[i] ^= key32[i];
65  }
66  }
67 
68  /* Tag the hash with algo16 which is important to avoid nonce reuse across
69  * algorithms. If this nonce function is used in BIP-340 signing as defined
70  * in the spec, an optimized tagging implementation is used. */
71  if (secp256k1_memcmp_var(algo16, bip340_algo16, 16) == 0) {
73  } else {
74  int algo16_len = 16;
75  /* Remove terminating null bytes */
76  while (algo16_len > 0 && !algo16[algo16_len - 1]) {
77  algo16_len--;
78  }
79  secp256k1_sha256_initialize_tagged(&sha, algo16, algo16_len);
80  }
81 
82  /* Hash (masked-)key||pk||msg using the tagged hash as per the spec */
83  if (data != NULL) {
84  secp256k1_sha256_write(&sha, masked_key, 32);
85  } else {
86  secp256k1_sha256_write(&sha, key32, 32);
87  }
88  secp256k1_sha256_write(&sha, xonly_pk32, 32);
89  secp256k1_sha256_write(&sha, msg32, 32);
90  secp256k1_sha256_finalize(&sha, nonce32);
91  return 1;
92 }
93 
95 
96 /* Initializes SHA256 with fixed midstate. This midstate was computed by applying
97  * SHA256 to SHA256("BIP0340/challenge")||SHA256("BIP0340/challenge"). */
100  sha->s[0] = 0x9cecba11ul;
101  sha->s[1] = 0x23925381ul;
102  sha->s[2] = 0x11679112ul;
103  sha->s[3] = 0xd1627e0ful;
104  sha->s[4] = 0x97c87550ul;
105  sha->s[5] = 0x003cc765ul;
106  sha->s[6] = 0x90f61164ul;
107  sha->s[7] = 0x33e9b66aul;
108  sha->bytes = 64;
109 }
110 
111 static void secp256k1_schnorrsig_challenge(secp256k1_scalar* e, const unsigned char *r32, const unsigned char *msg32, const unsigned char *pubkey32)
112 {
113  unsigned char buf[32];
114  secp256k1_sha256 sha;
115 
116  /* tagged hash(r.x, pk.x, msg32) */
118  secp256k1_sha256_write(&sha, r32, 32);
119  secp256k1_sha256_write(&sha, pubkey32, 32);
120  secp256k1_sha256_write(&sha, msg32, 32);
121  secp256k1_sha256_finalize(&sha, buf);
122  /* Set scalar e to the challenge hash modulo the curve order as per
123  * BIP340. */
124  secp256k1_scalar_set_b32(e, buf, NULL);
125 }
126 
127 int secp256k1_schnorrsig_sign(const secp256k1_context* ctx, unsigned char *sig64, const unsigned char *msg32, const secp256k1_keypair *keypair, secp256k1_nonce_function_hardened noncefp, void *ndata) {
128  secp256k1_scalar sk;
131  secp256k1_gej rj;
132  secp256k1_ge pk;
133  secp256k1_ge r;
134  unsigned char buf[32] = { 0 };
135  unsigned char pk_buf[32];
136  unsigned char seckey[32];
137  int ret = 1;
138 
139  VERIFY_CHECK(ctx != NULL);
141  ARG_CHECK(sig64 != NULL);
142  ARG_CHECK(msg32 != NULL);
143  ARG_CHECK(keypair != NULL);
144 
145  if (noncefp == NULL) {
147  }
148 
149  ret &= secp256k1_keypair_load(ctx, &sk, &pk, keypair);
150  /* Because we are signing for a x-only pubkey, the secret key is negated
151  * before signing if the point corresponding to the secret key does not
152  * have an even Y. */
153  if (secp256k1_fe_is_odd(&pk.y)) {
154  secp256k1_scalar_negate(&sk, &sk);
155  }
156 
157  secp256k1_scalar_get_b32(seckey, &sk);
158  secp256k1_fe_get_b32(pk_buf, &pk.x);
159  ret &= !!noncefp(buf, msg32, seckey, pk_buf, bip340_algo16, ndata);
160  secp256k1_scalar_set_b32(&k, buf, NULL);
161  ret &= !secp256k1_scalar_is_zero(&k);
163 
164  secp256k1_ecmult_gen(&ctx->ecmult_gen_ctx, &rj, &k);
165  secp256k1_ge_set_gej(&r, &rj);
166 
167  /* We declassify r to allow using it as a branch point. This is fine
168  * because r is not a secret. */
169  secp256k1_declassify(ctx, &r, sizeof(r));
171  if (secp256k1_fe_is_odd(&r.y)) {
172  secp256k1_scalar_negate(&k, &k);
173  }
175  secp256k1_fe_get_b32(&sig64[0], &r.x);
176 
177  secp256k1_schnorrsig_challenge(&e, &sig64[0], msg32, pk_buf);
178  secp256k1_scalar_mul(&e, &e, &sk);
179  secp256k1_scalar_add(&e, &e, &k);
180  secp256k1_scalar_get_b32(&sig64[32], &e);
181 
182  memczero(sig64, 64, !ret);
185  memset(seckey, 0, sizeof(seckey));
186 
187  return ret;
188 }
189 
190 int secp256k1_schnorrsig_verify(const secp256k1_context* ctx, const unsigned char *sig64, const unsigned char *msg32, const secp256k1_xonly_pubkey *pubkey) {
193  secp256k1_gej rj;
194  secp256k1_ge pk;
195  secp256k1_gej pkj;
196  secp256k1_fe rx;
197  secp256k1_ge r;
198  unsigned char buf[32];
199  int overflow;
200 
201  VERIFY_CHECK(ctx != NULL);
203  ARG_CHECK(sig64 != NULL);
204  ARG_CHECK(msg32 != NULL);
205  ARG_CHECK(pubkey != NULL);
206 
207  if (!secp256k1_fe_set_b32(&rx, &sig64[0])) {
208  return 0;
209  }
210 
211  secp256k1_scalar_set_b32(&s, &sig64[32], &overflow);
212  if (overflow) {
213  return 0;
214  }
215 
216  if (!secp256k1_xonly_pubkey_load(ctx, &pk, pubkey)) {
217  return 0;
218  }
219 
220  /* Compute e. */
221  secp256k1_fe_get_b32(buf, &pk.x);
222  secp256k1_schnorrsig_challenge(&e, &sig64[0], msg32, buf);
223 
224  /* Compute rj = s*G + (-e)*pkj */
225  secp256k1_scalar_negate(&e, &e);
226  secp256k1_gej_set_ge(&pkj, &pk);
227  secp256k1_ecmult(&ctx->ecmult_ctx, &rj, &pkj, &e, &s);
228 
229  secp256k1_ge_set_gej_var(&r, &rj);
230  if (secp256k1_ge_is_infinity(&r)) {
231  return 0;
232  }
233 
235  return !secp256k1_fe_is_odd(&r.y) &&
236  secp256k1_fe_equal_var(&rx, &r.x);
237 }
238 
239 #endif
static void secp256k1_scalar_mul(secp256k1_scalar *r, const secp256k1_scalar *a, const secp256k1_scalar *b)
Multiply two scalars (modulo the group order).
static int secp256k1_ecmult_context_is_built(const secp256k1_ecmult_context *ctx)
static int secp256k1_ge_is_infinity(const secp256k1_ge *a)
Check whether a group element is the point at infinity.
#define VERIFY_CHECK(cond)
Definition: util.h:68
static void secp256k1_nonce_function_bip340_sha256_tagged(secp256k1_sha256 *sha)
Definition: main_impl.h:16
static void secp256k1_ecmult_gen(const secp256k1_ecmult_gen_context *ctx, secp256k1_gej *r, const secp256k1_scalar *a)
Multiply with the generator: R = a*G.
static void secp256k1_fe_normalize_var(secp256k1_fe *r)
Normalize a field element, without constant-time guarantee.
static void secp256k1_ge_set_gej_var(secp256k1_ge *r, secp256k1_gej *a)
Definition: group_impl.h:102
static void secp256k1_scalar_negate(secp256k1_scalar *r, const secp256k1_scalar *a)
Compute the complement of a scalar (modulo the group order).
static int secp256k1_scalar_is_zero(const secp256k1_scalar *a)
Check whether a scalar equals zero.
const secp256k1_nonce_function_hardened secp256k1_nonce_function_bip340
An implementation of the nonce generation function as defined in Bitcoin Improvement Proposal 340 "Sc...
Definition: main_impl.h:94
static void secp256k1_ecmult(const secp256k1_ecmult_context *ctx, secp256k1_gej *r, const secp256k1_gej *a, const secp256k1_scalar *na, const secp256k1_scalar *ng)
Double multiply: R = na*A + ng*G.
static void secp256k1_scalar_set_b32(secp256k1_scalar *r, const unsigned char *bin, int *overflow)
Set a scalar from a big endian byte array.
A group element of the secp256k1 curve, in jacobian coordinates.
Definition: group.h:24
int(* secp256k1_nonce_function_hardened)(unsigned char *nonce32, const unsigned char *msg32, const unsigned char *key32, const unsigned char *xonly_pk32, const unsigned char *algo16, void *data)
This module implements a variant of Schnorr signatures compliant with Bitcoin Improvement Proposal 34...
static const unsigned char bip340_algo16[16]
Definition: main_impl.h:48
static int secp256k1_keypair_load(const secp256k1_context *ctx, secp256k1_scalar *sk, secp256k1_ge *pk, const secp256k1_keypair *keypair)
Definition: main_impl.h:151
static int secp256k1_fe_is_odd(const secp256k1_fe *a)
Check the "oddness" of a field element.
int secp256k1_schnorrsig_sign(const secp256k1_context *ctx, unsigned char *sig64, const unsigned char *msg32, const secp256k1_keypair *keypair, secp256k1_nonce_function_hardened noncefp, void *ndata)
Create a Schnorr signature.
Definition: main_impl.h:127
static SECP256K1_INLINE void secp256k1_declassify(const secp256k1_context *ctx, const void *p, size_t len)
Definition: secp256k1.c:234
Opaque data structure that holds a parsed and valid "x-only" public key.
uint32_t s[8]
Definition: hash.h:14
secp256k1_ecmult_gen_context ecmult_gen_ctx
Definition: secp256k1.c:72
#define ARG_CHECK(cond)
Definition: secp256k1.c:29
static secp256k1_context * ctx
Definition: tests.c:36
static void secp256k1_ge_set_gej(secp256k1_ge *r, secp256k1_gej *a)
Set a group element equal to another which is given in jacobian coordinates.
secp256k1_ecmult_context ecmult_ctx
Definition: secp256k1.c:71
static void secp256k1_scalar_clear(secp256k1_scalar *r)
Clear a scalar to prevent the leak of sensitive data.
A group element of the secp256k1 curve, in affine coordinates.
Definition: group.h:14
secp256k1_fe x
Definition: group.h:15
A scalar modulo the group order of the secp256k1 curve.
Definition: scalar_4x64.h:13
static void secp256k1_nonce_function_bip340_sha256_tagged_aux(secp256k1_sha256 *sha)
Definition: main_impl.h:32
Opaque data structure that holds a keypair consisting of a secret and a public key.
static void secp256k1_scalar_get_b32(unsigned char *bin, const secp256k1_scalar *a)
Convert a scalar to a byte array.
static void secp256k1_sha256_write(secp256k1_sha256 *hash, const unsigned char *data, size_t size)
static void secp256k1_sha256_initialize_tagged(secp256k1_sha256 *hash, const unsigned char *tag, size_t taglen)
Definition: hash_impl.h:169
static int secp256k1_fe_set_b32(secp256k1_fe *r, const unsigned char *a)
Set a field element equal to 32-byte big endian value.
static int secp256k1_fe_equal_var(const secp256k1_fe *a, const secp256k1_fe *b)
Same as secp256k1_fe_equal, but may be variable time.
static int secp256k1_scalar_add(secp256k1_scalar *r, const secp256k1_scalar *a, const secp256k1_scalar *b)
Add two scalars together (modulo the group order).
static int nonce_function_bip340(unsigned char *nonce32, const unsigned char *msg32, const unsigned char *key32, const unsigned char *xonly_pk32, const unsigned char *algo16, void *data)
Definition: main_impl.h:50
static SECP256K1_INLINE int secp256k1_memcmp_var(const void *s1, const void *s2, size_t n)
Semantics like memcmp.
Definition: util.h:224
static void secp256k1_fe_get_b32(unsigned char *r, const secp256k1_fe *a)
Convert a field element to a 32-byte big endian value.
static void secp256k1_schnorrsig_challenge(secp256k1_scalar *e, const unsigned char *r32, const unsigned char *msg32, const unsigned char *pubkey32)
Definition: main_impl.h:111
static void secp256k1_sha256_initialize(secp256k1_sha256 *hash)
static void secp256k1_gej_set_ge(secp256k1_gej *r, const secp256k1_ge *a)
Set a group element (jacobian) equal to another which is given in affine coordinates.
static SECP256K1_INLINE int secp256k1_xonly_pubkey_load(const secp256k1_context *ctx, secp256k1_ge *ge, const secp256k1_xonly_pubkey *pubkey)
Definition: main_impl.h:13
secp256k1_fe y
Definition: group.h:16
static void secp256k1_scalar_cmov(secp256k1_scalar *r, const secp256k1_scalar *a, int flag)
If flag is true, set *r equal to *a; otherwise leave it.
int secp256k1_schnorrsig_verify(const secp256k1_context *ctx, const unsigned char *sig64, const unsigned char *msg32, const secp256k1_xonly_pubkey *pubkey)
Verify a Schnorr signature.
Definition: main_impl.h:190
static const secp256k1_scalar secp256k1_scalar_one
Definition: scalar_impl.h:31
static SECP256K1_INLINE void memczero(void *s, size_t len, int flag)
Definition: util.h:205
static void secp256k1_sha256_finalize(secp256k1_sha256 *hash, unsigned char *out32)
static int secp256k1_ecmult_gen_context_is_built(const secp256k1_ecmult_gen_context *ctx)
size_t bytes
Definition: hash.h:16
static void secp256k1_schnorrsig_sha256_tagged(secp256k1_sha256 *sha)
Definition: main_impl.h:98