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Add secp256k1_split_lambda_verify

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Russell O'Connor 2020-09-23 12:52:29 -04:00 committed by Pieter Wuille
parent acab934d24
commit 9aca2f7f07
1 changed files with 47 additions and 0 deletions
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47
src/scalar_impl.h
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@ -7,6 +7,10 @@
#ifndef SECP256K1_SCALAR_IMPL_H
#define SECP256K1_SCALAR_IMPL_H
#ifdef VERIFY
#include <string.h>
#endif
#include "scalar.h"
#include "util.h"
@ -430,6 +434,45 @@ static void secp256k1_scalar_split_lambda(secp256k1_scalar *r1, secp256k1_scalar
* Q.E.D.
*/
#ifdef VERIFY
static void secp256k1_scalar_split_lambda_verify(const secp256k1_scalar *r1, const secp256k1_scalar *r2, const secp256k1_scalar *k) {
secp256k1_scalar s;
unsigned char buf1[32];
unsigned char buf2[32];
static const secp256k1_scalar lambda = SECP256K1_SCALAR_CONST(
0x5363AD4CUL, 0xC05C30E0UL, 0xA5261C02UL, 0x8812645AUL,
0x122E22EAUL, 0x20816678UL, 0xDF02967CUL, 0x1B23BD72UL
);
/* (a1 + a2 + 1)/2 is 0xa2a8918ca85bafe22016d0b917e4dd77 */
static const unsigned char k1_bound[32] = {
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0xa2, 0xa8, 0x91, 0x8c, 0xa8, 0x5b, 0xaf, 0xe2, 0x20, 0x16, 0xd0, 0xb9, 0x17, 0xe4, 0xdd, 0x77
};
/* (-b1 + b2)/2 + 1 is 0x8a65287bd47179fb2be08846cea267ed */
static const unsigned char k2_bound[32] = {
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x8a, 0x65, 0x28, 0x7b, 0xd4, 0x71, 0x79, 0xfb, 0x2b, 0xe0, 0x88, 0x46, 0xce, 0xa2, 0x67, 0xed
};
secp256k1_scalar_mul(&s, &lambda, r2);
secp256k1_scalar_add(&s, &s, r1);
VERIFY_CHECK(secp256k1_scalar_eq(&s, k));
secp256k1_scalar_negate(&s, r1);
secp256k1_scalar_get_b32(buf1, r1);
secp256k1_scalar_get_b32(buf2, &s);
VERIFY_CHECK(secp256k1_memcmp_var(buf1, k1_bound, 32) < 0 || secp256k1_memcmp_var(buf2, k1_bound, 32) < 0);
secp256k1_scalar_negate(&s, r2);
secp256k1_scalar_get_b32(buf1, r2);
secp256k1_scalar_get_b32(buf2, &s);
VERIFY_CHECK(secp256k1_memcmp_var(buf1, k2_bound, 32) < 0 || secp256k1_memcmp_var(buf2, k2_bound, 32) < 0);
}
#endif
static void secp256k1_scalar_split_lambda(secp256k1_scalar *r1, secp256k1_scalar *r2, const secp256k1_scalar *k) {
secp256k1_scalar c1, c2;
static const secp256k1_scalar minus_lambda = SECP256K1_SCALAR_CONST(
@ -462,6 +505,10 @@ static void secp256k1_scalar_split_lambda(secp256k1_scalar *r1, secp256k1_scalar
secp256k1_scalar_add(r2, &c1, &c2);
secp256k1_scalar_mul(r1, r2, &minus_lambda);
secp256k1_scalar_add(r1, r1, k);
#ifdef VERIFY
secp256k1_scalar_split_lambda_verify(r1, r2, k);
#endif
}
#endif
#endif