From 6fcb5b845d2832ce019d60507033f74426290768 Mon Sep 17 00:00:00 2001 From: Jonas Nick Date: Wed, 22 Jul 2020 09:09:34 +0000 Subject: [PATCH] extrakeys: Add keypair_xonly_tweak_add --- include/secp256k1_extrakeys.h | 27 +++++++ src/modules/extrakeys/main_impl.h | 31 ++++++++ src/modules/extrakeys/tests_impl.h | 109 +++++++++++++++++++++++++++++ src/valgrind_ctime_test.c | 11 ++- 4 files changed, 177 insertions(+), 1 deletion(-) diff --git a/include/secp256k1_extrakeys.h b/include/secp256k1_extrakeys.h index 96e470d..0c5dff2 100644 --- a/include/secp256k1_extrakeys.h +++ b/include/secp256k1_extrakeys.h @@ -202,6 +202,33 @@ SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_keypair_xonly_pub( const secp256k1_keypair *keypair ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(4); +/** Tweak a keypair by adding tweak32 to the secret key and updating the public + * key accordingly. + * + * Calling this function and then secp256k1_keypair_pub results in the same + * public key as calling secp256k1_keypair_xonly_pub and then + * secp256k1_xonly_pubkey_tweak_add. + * + * Returns: 0 if the arguments are invalid or the resulting keypair would be + * invalid (only when the tweak is the negation of the keypair's + * secret key). 1 otherwise. + * + * Args: ctx: pointer to a context object initialized for verification + * (cannot be NULL) + * In/Out: keypair: pointer to a keypair to apply the tweak to. Will be set to + * an invalid value if this function returns 0 (cannot be + * NULL). + * In: 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_keypair_xonly_tweak_add( + const secp256k1_context* ctx, + secp256k1_keypair *keypair, + const unsigned char *tweak32 +) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3); + #ifdef __cplusplus } #endif diff --git a/src/modules/extrakeys/main_impl.h b/src/modules/extrakeys/main_impl.h index a0c44e9..d319215 100644 --- a/src/modules/extrakeys/main_impl.h +++ b/src/modules/extrakeys/main_impl.h @@ -214,4 +214,35 @@ int secp256k1_keypair_xonly_pub(const secp256k1_context* ctx, secp256k1_xonly_pu return 1; } +int secp256k1_keypair_xonly_tweak_add(const secp256k1_context* ctx, secp256k1_keypair *keypair, const unsigned char *tweak32) { + secp256k1_ge pk; + secp256k1_scalar sk; + int y_parity; + int ret; + + VERIFY_CHECK(ctx != NULL); + ARG_CHECK(secp256k1_ecmult_context_is_built(&ctx->ecmult_ctx)); + ARG_CHECK(keypair != NULL); + ARG_CHECK(tweak32 != NULL); + + ret = secp256k1_keypair_load(ctx, &sk, &pk, keypair); + memset(keypair, 0, sizeof(*keypair)); + + y_parity = secp256k1_extrakeys_ge_even_y(&pk); + if (y_parity == 1) { + secp256k1_scalar_negate(&sk, &sk); + } + + ret &= secp256k1_ec_seckey_tweak_add_helper(&sk, tweak32); + ret &= secp256k1_ec_pubkey_tweak_add_helper(&ctx->ecmult_ctx, &pk, tweak32); + + secp256k1_declassify(ctx, &ret, sizeof(ret)); + if (ret) { + secp256k1_keypair_save(keypair, &sk, &pk); + } + + secp256k1_scalar_clear(&sk); + return ret; +} + #endif diff --git a/src/modules/extrakeys/tests_impl.h b/src/modules/extrakeys/tests_impl.h index af289c8..fc9d40e 100644 --- a/src/modules/extrakeys/tests_impl.h +++ b/src/modules/extrakeys/tests_impl.h @@ -401,6 +401,114 @@ void test_keypair(void) { secp256k1_context_destroy(verify); } +void test_keypair_add(void) { + unsigned char sk[32]; + secp256k1_keypair keypair; + unsigned char overflows[32]; + unsigned char zeros96[96] = { 0 }; + unsigned char tweak[32]; + int i; + int ecount = 0; + secp256k1_context *none = api_test_context(SECP256K1_CONTEXT_NONE, &ecount); + secp256k1_context *sign = api_test_context(SECP256K1_CONTEXT_SIGN, &ecount); + secp256k1_context *verify = api_test_context(SECP256K1_CONTEXT_VERIFY, &ecount); + + CHECK(sizeof(zeros96) == sizeof(keypair)); + secp256k1_rand256(sk); + secp256k1_rand256(tweak); + memset(overflows, 0xFF, 32); + CHECK(secp256k1_keypair_create(ctx, &keypair, sk) == 1); + + CHECK(secp256k1_keypair_xonly_tweak_add(none, &keypair, tweak) == 0); + CHECK(ecount == 1); + CHECK(secp256k1_keypair_xonly_tweak_add(sign, &keypair, tweak) == 0); + CHECK(ecount == 2); + CHECK(secp256k1_keypair_xonly_tweak_add(verify, &keypair, tweak) == 1); + CHECK(secp256k1_keypair_xonly_tweak_add(verify, NULL, tweak) == 0); + CHECK(ecount == 3); + CHECK(secp256k1_keypair_xonly_tweak_add(verify, &keypair, NULL) == 0); + CHECK(ecount == 4); + /* This does not set the keypair to zeroes */ + CHECK(memcmp(&keypair, zeros96, sizeof(keypair)) != 0); + + /* Invalid tweak zeroes the keypair */ + CHECK(secp256k1_keypair_create(ctx, &keypair, sk) == 1); + CHECK(secp256k1_keypair_xonly_tweak_add(ctx, &keypair, overflows) == 0); + CHECK(memcmp(&keypair, zeros96, sizeof(keypair)) == 0); + + /* A zero tweak is fine */ + CHECK(secp256k1_keypair_create(ctx, &keypair, sk) == 1); + CHECK(secp256k1_keypair_xonly_tweak_add(ctx, &keypair, zeros96) == 1); + + /* Fails if the resulting keypair was (sk=0, pk=infinity) */ + for (i = 0; i < count; i++) { + secp256k1_scalar scalar_tweak; + secp256k1_keypair keypair_tmp; + secp256k1_rand256(sk); + CHECK(secp256k1_keypair_create(ctx, &keypair, sk) == 1); + memcpy(&keypair_tmp, &keypair, sizeof(keypair)); + /* Because sk may be negated before adding, we need to try with tweak = + * sk as well as tweak = -sk. */ + secp256k1_scalar_set_b32(&scalar_tweak, sk, NULL); + secp256k1_scalar_negate(&scalar_tweak, &scalar_tweak); + secp256k1_scalar_get_b32(tweak, &scalar_tweak); + CHECK((secp256k1_keypair_xonly_tweak_add(ctx, &keypair, sk) == 0) + || (secp256k1_keypair_xonly_tweak_add(ctx, &keypair_tmp, tweak) == 0)); + CHECK(memcmp(&keypair, zeros96, sizeof(keypair)) == 0 + || memcmp(&keypair_tmp, zeros96, sizeof(keypair_tmp)) == 0); + } + + /* Invalid keypair with a valid tweak */ + memset(&keypair, 0, sizeof(keypair)); + secp256k1_rand256(tweak); + ecount = 0; + CHECK(secp256k1_keypair_xonly_tweak_add(verify, &keypair, tweak) == 0); + CHECK(ecount == 1); + CHECK(memcmp(&keypair, zeros96, sizeof(keypair)) == 0); + /* Only seckey part of keypair invalid */ + CHECK(secp256k1_keypair_create(ctx, &keypair, sk) == 1); + memset(&keypair, 0, 32); + CHECK(secp256k1_keypair_xonly_tweak_add(verify, &keypair, tweak) == 0); + CHECK(ecount == 2); + /* Only pubkey part of keypair invalid */ + CHECK(secp256k1_keypair_create(ctx, &keypair, sk) == 1); + memset(&keypair.data[32], 0, 64); + CHECK(secp256k1_keypair_xonly_tweak_add(verify, &keypair, tweak) == 0); + CHECK(ecount == 3); + + /* Check that the keypair_tweak_add implementation is correct */ + CHECK(secp256k1_keypair_create(ctx, &keypair, sk) == 1); + for (i = 0; i < count; i++) { + secp256k1_xonly_pubkey internal_pk; + secp256k1_xonly_pubkey output_pk; + secp256k1_pubkey output_pk_xy; + secp256k1_pubkey output_pk_expected; + unsigned char pk32[32]; + int pk_parity; + + secp256k1_rand256(tweak); + CHECK(secp256k1_keypair_xonly_pub(ctx, &internal_pk, NULL, &keypair) == 1); + CHECK(secp256k1_keypair_xonly_tweak_add(ctx, &keypair, tweak) == 1); + CHECK(secp256k1_keypair_xonly_pub(ctx, &output_pk, &pk_parity, &keypair) == 1); + + /* Check that it passes xonly_pubkey_tweak_add_check */ + CHECK(secp256k1_xonly_pubkey_serialize(ctx, pk32, &output_pk) == 1); + CHECK(secp256k1_xonly_pubkey_tweak_add_check(ctx, pk32, pk_parity, &internal_pk, tweak) == 1); + + /* Check that the resulting pubkey matches xonly_pubkey_tweak_add */ + CHECK(secp256k1_keypair_pub(ctx, &output_pk_xy, &keypair) == 1); + CHECK(secp256k1_xonly_pubkey_tweak_add(ctx, &output_pk_expected, &internal_pk, tweak) == 1); + CHECK(memcmp(&output_pk_xy, &output_pk_expected, sizeof(output_pk_xy)) == 0); + + /* Check that the secret key in the keypair is tweaked correctly */ + CHECK(secp256k1_ec_pubkey_create(ctx, &output_pk_expected, &keypair.data[0]) == 1); + CHECK(memcmp(&output_pk_xy, &output_pk_expected, sizeof(output_pk_xy)) == 0); + } + secp256k1_context_destroy(none); + secp256k1_context_destroy(sign); + secp256k1_context_destroy(verify); +} + void run_extrakeys_tests(void) { /* xonly key test cases */ test_xonly_pubkey(); @@ -410,6 +518,7 @@ void run_extrakeys_tests(void) { /* keypair tests */ test_keypair(); + test_keypair_add(); } #endif diff --git a/src/valgrind_ctime_test.c b/src/valgrind_ctime_test.c index 09b578a..3d70563 100644 --- a/src/valgrind_ctime_test.c +++ b/src/valgrind_ctime_test.c @@ -57,7 +57,9 @@ int main(void) { msg[i] = i + 1; } - ctx = secp256k1_context_create(SECP256K1_CONTEXT_SIGN | SECP256K1_CONTEXT_DECLASSIFY); + ctx = secp256k1_context_create(SECP256K1_CONTEXT_SIGN + | SECP256K1_CONTEXT_VERIFY + | SECP256K1_CONTEXT_DECLASSIFY); /* Test keygen. */ VALGRIND_MAKE_MEM_UNDEFINED(key, 32); @@ -122,11 +124,18 @@ int main(void) { VALGRIND_MAKE_MEM_DEFINED(&ret, sizeof(ret)); CHECK(ret); + /* Test keypair_create and keypair_xonly_tweak_add. */ #if ENABLE_MODULE_EXTRAKEYS VALGRIND_MAKE_MEM_UNDEFINED(key, 32); ret = secp256k1_keypair_create(ctx, &keypair, key); VALGRIND_MAKE_MEM_DEFINED(&ret, sizeof(ret)); CHECK(ret == 1); + + /* The tweak is not treated as a secret in keypair_tweak_add */ + VALGRIND_MAKE_MEM_DEFINED(msg, 32); + ret = secp256k1_keypair_xonly_tweak_add(ctx, &keypair, msg); + VALGRIND_MAKE_MEM_DEFINED(&ret, sizeof(ret)); + CHECK(ret == 1); #endif secp256k1_context_destroy(ctx);