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[ECDH API change] Support custom hash function
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@fanatid
fanatid committed May 16, 2018
1 parent 452d8e4 commit b00be65
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Showing 5 changed files with 93 additions and 35 deletions.
28 changes: 24 additions & 4 deletions include/secp256k1_ecdh.h
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Expand Up @@ -7,21 +7,41 @@
extern "C" {
#endif

/** A pointer to a function that applies hash function to a point
*
* Returns: 1 if a point was successfully hashed. 0 will cause ecdh to fail
* Out: output: pointer to an array to be filled by the function
* In: x: pointer to a 32-byte x coordinate
* y: pointer to a 32-byte y coordinate
*/
typedef int (*secp256k1_ecdh_hash_function)(
unsigned char *output,
const unsigned char *x,
const unsigned char *y
);

/** An implementation of SHA256 hash function that applies to compressed public key. */
SECP256K1_API extern const secp256k1_ecdh_hash_function secp256k1_ecdh_hash_function_sha256;

/** A default ecdh hash function (currently equal to secp256k1_ecdh_hash_function_sha256). */
SECP256K1_API extern const secp256k1_ecdh_hash_function secp256k1_ecdh_hash_function_default;

/** Compute an EC Diffie-Hellman secret in constant time
* Returns: 1: exponentiation was successful
* 0: scalar was invalid (zero or overflow)
* Args: ctx: pointer to a context object (cannot be NULL)
* Out: result: a 32-byte array which will be populated by an ECDH
* secret computed from the point and scalar
* Out: output: pointer to an array to be filled by the function
* In: pubkey: a pointer to a secp256k1_pubkey containing an
* initialized public key
* privkey: a 32-byte scalar with which to multiply the point
* hashfp: pointer to a hash function. If NULL, secp256k1_ecdh_hash_function_sha256 is used
*/
SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_ecdh(
const secp256k1_context* ctx,
unsigned char *result,
unsigned char *output,
const secp256k1_pubkey *pubkey,
const unsigned char *privkey
const unsigned char *privkey,
secp256k1_ecdh_hash_function hashfp
) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4);

#ifdef __cplusplus
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2 changes: 1 addition & 1 deletion src/bench_ecdh.c
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Original file line number Diff line number Diff line change
Expand Up @@ -42,7 +42,7 @@ static void bench_ecdh(void* arg) {
bench_ecdh_data *data = (bench_ecdh_data*)arg;

for (i = 0; i < 20000; i++) {
CHECK(secp256k1_ecdh(data->ctx, res, &data->point, data->scalar) == 1);
CHECK(secp256k1_ecdh(data->ctx, res, &data->point, data->scalar, NULL) == 1);
}
}

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5 changes: 3 additions & 2 deletions src/java/org_bitcoin_NativeSecp256k1.c
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Expand Up @@ -83,7 +83,7 @@ SECP256K1_API jobjectArray JNICALL Java_org_bitcoin_NativeSecp256k1_secp256k1_1e

secp256k1_ecdsa_signature sig[72];

int ret = secp256k1_ecdsa_sign(ctx, sig, data, secKey, NULL, NULL );
int ret = secp256k1_ecdsa_sign(ctx, sig, data, secKey, NULL, NULL);

unsigned char outputSer[72];
size_t outputLen = 72;
Expand Down Expand Up @@ -353,7 +353,8 @@ SECP256K1_API jobjectArray JNICALL Java_org_bitcoin_NativeSecp256k1_secp256k1_1e
ctx,
nonce_res,
&pubkey,
secdata
secdata,
NULL
);
}

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38 changes: 25 additions & 13 deletions src/modules/ecdh/main_impl.h
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Expand Up @@ -10,41 +10,53 @@
#include "include/secp256k1_ecdh.h"
#include "ecmult_const_impl.h"

int secp256k1_ecdh(const secp256k1_context* ctx, unsigned char *result, const secp256k1_pubkey *point, const unsigned char *scalar) {
static int ecdh_hash_function_sha256(unsigned char *output, const unsigned char *x, const unsigned char *y) {
unsigned char version = (y[31] & 0x01) | 0x02;
secp256k1_sha256 sha;

secp256k1_sha256_initialize(&sha);
secp256k1_sha256_write(&sha, &version, 1);
secp256k1_sha256_write(&sha, x, 32);
secp256k1_sha256_finalize(&sha, output);

return 1;
}

const secp256k1_ecdh_hash_function secp256k1_ecdh_hash_function_sha256 = ecdh_hash_function_sha256;
const secp256k1_ecdh_hash_function secp256k1_ecdh_hash_function_default = ecdh_hash_function_sha256;

int secp256k1_ecdh(const secp256k1_context* ctx, unsigned char *output, const secp256k1_pubkey *point, const unsigned char *scalar, secp256k1_ecdh_hash_function hashfp) {
int ret = 0;
int overflow = 0;
secp256k1_gej res;
secp256k1_ge pt;
secp256k1_scalar s;
VERIFY_CHECK(ctx != NULL);
ARG_CHECK(result != NULL);
ARG_CHECK(output != NULL);
ARG_CHECK(point != NULL);
ARG_CHECK(scalar != NULL);
if (hashfp == NULL) {
hashfp = secp256k1_ecdh_hash_function_default;
}

secp256k1_pubkey_load(ctx, &pt, point);
secp256k1_scalar_set_b32(&s, scalar, &overflow);
if (overflow || secp256k1_scalar_is_zero(&s)) {
ret = 0;
} else {
unsigned char x[32];
unsigned char y[1];
secp256k1_sha256 sha;
unsigned char y[32];

secp256k1_ecmult_const(&res, &pt, &s, 256);
secp256k1_ge_set_gej(&pt, &res);
/* Compute a hash of the point in compressed form
* Note we cannot use secp256k1_eckey_pubkey_serialize here since it does not
* expect its output to be secret and has a timing sidechannel. */

/* Compute a hash of the point */
secp256k1_fe_normalize(&pt.x);
secp256k1_fe_normalize(&pt.y);
secp256k1_fe_get_b32(x, &pt.x);
y[0] = 0x02 | secp256k1_fe_is_odd(&pt.y);
secp256k1_fe_get_b32(y, &pt.y);

secp256k1_sha256_initialize(&sha);
secp256k1_sha256_write(&sha, y, sizeof(y));
secp256k1_sha256_write(&sha, x, sizeof(x));
secp256k1_sha256_finalize(&sha, result);
ret = 1;
ret = hashfp(output, x, y);
}

secp256k1_scalar_clear(&s);
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55 changes: 40 additions & 15 deletions src/modules/ecdh/tests_impl.h
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Expand Up @@ -7,6 +7,21 @@
#ifndef SECP256K1_MODULE_ECDH_TESTS_H
#define SECP256K1_MODULE_ECDH_TESTS_H

int ecdh_hash_function_test_fail(unsigned char *output, const unsigned char *x, const unsigned char *y) {
(void)output;
(void)x;
(void)y;
return 0;
}

int ecdh_hash_function_custom(unsigned char *output, const unsigned char *x, const unsigned char *y) {
/* Save x and y as uncompressed public key */
output[0] = 0x04;
memcpy(output + 1, x, 32);
memcpy(output + 33, y, 32);
return 1;
}

void test_ecdh_api(void) {
/* Setup context that just counts errors */
secp256k1_context *tctx = secp256k1_context_create(SECP256K1_CONTEXT_SIGN);
Expand All @@ -21,15 +36,15 @@ void test_ecdh_api(void) {
CHECK(secp256k1_ec_pubkey_create(tctx, &point, s_one) == 1);

/* Check all NULLs are detected */
CHECK(secp256k1_ecdh(tctx, res, &point, s_one) == 1);
CHECK(secp256k1_ecdh(tctx, res, &point, s_one, NULL) == 1);
CHECK(ecount == 0);
CHECK(secp256k1_ecdh(tctx, NULL, &point, s_one) == 0);
CHECK(secp256k1_ecdh(tctx, NULL, &point, s_one, NULL) == 0);
CHECK(ecount == 1);
CHECK(secp256k1_ecdh(tctx, res, NULL, s_one) == 0);
CHECK(secp256k1_ecdh(tctx, res, NULL, s_one, NULL) == 0);
CHECK(ecount == 2);
CHECK(secp256k1_ecdh(tctx, res, &point, NULL) == 0);
CHECK(secp256k1_ecdh(tctx, res, &point, NULL, NULL) == 0);
CHECK(ecount == 3);
CHECK(secp256k1_ecdh(tctx, res, &point, s_one) == 1);
CHECK(secp256k1_ecdh(tctx, res, &point, s_one, NULL) == 1);
CHECK(ecount == 3);

/* Cleanup */
Expand All @@ -46,27 +61,34 @@ void test_ecdh_generator_basepoint(void) {
for (i = 0; i < 100; ++i) {
secp256k1_sha256 sha;
unsigned char s_b32[32];
unsigned char output_ecdh[32];
unsigned char output_ecdh[65];
unsigned char output_ser[32];
unsigned char point_ser[33];
unsigned char point_ser[65];
size_t point_ser_len = sizeof(point_ser);
secp256k1_scalar s;

random_scalar_order(&s);
secp256k1_scalar_get_b32(s_b32, &s);

/* compute using ECDH function */
CHECK(secp256k1_ec_pubkey_create(ctx, &point[0], s_one) == 1);
CHECK(secp256k1_ecdh(ctx, output_ecdh, &point[0], s_b32) == 1);
/* compute "explicitly" */
CHECK(secp256k1_ec_pubkey_create(ctx, &point[1], s_b32) == 1);

/* compute using ECDH function with custom hash function */
CHECK(secp256k1_ecdh(ctx, output_ecdh, &point[0], s_b32, ecdh_hash_function_custom) == 1);
/* compute "explicitly" */
CHECK(secp256k1_ec_pubkey_serialize(ctx, point_ser, &point_ser_len, &point[1], SECP256K1_EC_UNCOMPRESSED) == 1);
/* compare */
CHECK(memcmp(output_ecdh, point_ser, 65) == 0);

/* compute using ECDH function with default hash function */
CHECK(secp256k1_ecdh(ctx, output_ecdh, &point[0], s_b32, NULL) == 1);
/* compute "explicitly" */
CHECK(secp256k1_ec_pubkey_serialize(ctx, point_ser, &point_ser_len, &point[1], SECP256K1_EC_COMPRESSED) == 1);
CHECK(point_ser_len == sizeof(point_ser));
secp256k1_sha256_initialize(&sha);
secp256k1_sha256_write(&sha, point_ser, point_ser_len);
secp256k1_sha256_finalize(&sha, output_ser);
/* compare */
CHECK(memcmp(output_ecdh, output_ser, sizeof(output_ser)) == 0);
CHECK(memcmp(output_ecdh, output_ser, 32) == 0);
}
}

Expand All @@ -89,11 +111,14 @@ void test_bad_scalar(void) {
CHECK(secp256k1_ec_pubkey_create(ctx, &point, s_rand) == 1);

/* Try to multiply it by bad values */
CHECK(secp256k1_ecdh(ctx, output, &point, s_zero) == 0);
CHECK(secp256k1_ecdh(ctx, output, &point, s_overflow) == 0);
CHECK(secp256k1_ecdh(ctx, output, &point, s_zero, NULL) == 0);
CHECK(secp256k1_ecdh(ctx, output, &point, s_overflow, NULL) == 0);
/* ...and a good one */
s_overflow[31] -= 1;
CHECK(secp256k1_ecdh(ctx, output, &point, s_overflow) == 1);
CHECK(secp256k1_ecdh(ctx, output, &point, s_overflow, NULL) == 1);

/* Hash function failure results in ecdh failure */
CHECK(secp256k1_ecdh(ctx, output, &point, s_overflow, ecdh_hash_function_test_fail) == 0);
}

void run_ecdh_tests(void) {
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