Expose API for constant time point multiplication

Author: apoelstra

include/secp256k1.h

@@ -217,6 +217,24 @@ SECP256K1_WARN_UNUSED_RESULT int secp256k1_ecdsa_recover_compact(
   int recid
 ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4) SECP256K1_ARG_NONNULL(5);
 
+
+/** Do an elliptic curve scalar multiplication in constant time.
+ *  Returns: 1: exponentiation was successful
+ *          -1: scalar was zero (cannot serialize output point)
+ *          -2: scalar overflow
+ *          -3: invalid input point
+ *  In:      scalar:   a 32-byte scalar with which to multiple the point
+ *  In/Out:  point:    pointer to 33 or 65 byte array containing an EC point
+ *                     which will be updated in place
+ *           pointlen: length of the point array, which will be updated by
+ *                     the multiplication
+ */
+SECP256K1_WARN_UNUSED_RESULT int secp256k1_point_multiply(
+  unsigned char *point,
+  int *pointlen,
+  const unsigned char *scalar
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
+
 /** Verify an ECDSA secret key.
  *  Returns: 1: secret key is valid
  *           0: secret key is invalid

src/secp256k1.c

@@ -225,6 +225,32 @@ int secp256k1_ecdsa_recover_compact(const secp256k1_context_t* ctx, const unsign
     return ret;
 }
 
+int secp256k1_point_multiply(unsigned char *point, int *pointlen, const unsigned char *scalar) {
+    int ret = 0;
+    int overflow = 0;
+    secp256k1_gej_t res;
+    secp256k1_ge_t pt;
+    secp256k1_scalar_t s;
+    DEBUG_CHECK(point != NULL);
+    DEBUG_CHECK(pointlen != NULL);
+    DEBUG_CHECK(scalar != NULL);
+
+    if (secp256k1_eckey_pubkey_parse(&pt, point, *pointlen)) {
+        secp256k1_scalar_set_b32(&s, scalar, &overflow);
+        if (overflow) {
+            ret = -2;
+        } else {
+            secp256k1_ecdh_point_multiply(&res, &pt, &s);
+            secp256k1_ge_set_gej(&pt, &res);
+            ret = secp256k1_eckey_pubkey_serialize(&pt, point, pointlen, *pointlen <= 33);
+        }
+    } else {
+        ret = -3;
+    }
+    secp256k1_scalar_clear(&s);
+    return ret;
+}
+
 int secp256k1_ec_seckey_verify(const secp256k1_context_t* ctx, const unsigned char *seckey) {
     secp256k1_scalar_t sec;
     int ret;

src/tests.c

@@ -1314,6 +1314,51 @@ void run_ecdh_tests(void) {
     ecdh_commutativity();
 }
 
+void run_ecdh_api_tests(void) {
+    const unsigned char scalar[] = {0x49, 0x68, 0xd5, 0x24, 0x2a, 0xbf, 0x9b, 0x7a,
+                                    0x46, 0x6a, 0xbb, 0xcf, 0x34, 0xb1, 0x1b, 0x6d,
+                                    0xcd, 0x83, 0xd3, 0x07, 0x82, 0x7b, 0xed, 0x62,
+                                    0x05, 0xfa, 0xd0, 0xce, 0x18, 0xfa, 0xe6, 0x3b};
+    const unsigned char expected_uncomp[] = {
+        0x04,
+        /* x */
+        0x54, 0x94, 0xc1, 0x5d, 0x32, 0x09, 0x97, 0x06,
+        0xc2, 0x39, 0x5f, 0x94, 0x34, 0x87, 0x45, 0xfd,
+        0x75, 0x7c, 0xe3, 0x0e, 0x4e, 0x8c, 0x90, 0xfb,
+        0xa2, 0xba, 0xd1, 0x84, 0xf8, 0x83, 0xc6, 0x9f,
+        /* y */
+        0x5d, 0x19, 0x5d, 0x20, 0xe1, 0x91, 0xbf, 0x7f,
+        0x1b, 0xe3, 0xe5, 0x5f, 0x56, 0xa8, 0x01, 0x96,
+        0x60, 0x71, 0xad, 0x01, 0xf1, 0x46, 0x2f, 0x66,
+        0xc9, 0x97, 0xfa, 0x94, 0xdb, 0x85, 0x84, 0x35
+    };
+    const unsigned char expected_comp[] = {
+        /* y */
+        0x03,
+        /* x */
+        0x54, 0x94, 0xc1, 0x5d, 0x32, 0x09, 0x97, 0x06,
+        0xc2, 0x39, 0x5f, 0x94, 0x34, 0x87, 0x45, 0xfd,
+        0x75, 0x7c, 0xe3, 0x0e, 0x4e, 0x8c, 0x90, 0xfb,
+        0xa2, 0xba, 0xd1, 0x84, 0xf8, 0x83, 0xc6, 0x9f
+    };
+    unsigned char point[65];
+    int pointlen = sizeof(point);
+    secp256k1_ge_t gen = secp256k1_ge_const_g;
+    int i;
+    /* uncompressed */
+    secp256k1_eckey_pubkey_serialize(&gen, point, &pointlen, 0);
+    for (i = 0; i < 100; ++i) {
+        CHECK(secp256k1_point_multiply(point, &pointlen, scalar) == 1);
+    }
+    CHECK(memcmp(point, expected_uncomp, 65) == 0);
+    /* compressed */
+    secp256k1_eckey_pubkey_serialize(&gen, point, &pointlen, 1);
+    for (i = 0; i < 100; ++i) {
+        CHECK(secp256k1_point_multiply(point, &pointlen, scalar) == 1);
+    }
+    CHECK(memcmp(point, expected_comp, 33) == 0);
+}
+
 /***** ECMULT TESTS *****/
 
 void run_ecmult_chain(void) {
@@ -2409,6 +2454,7 @@ int main(int argc, char **argv) {
 
     /* ecdh tests */
     run_ecdh_tests();
+    run_ecdh_api_tests();
 
     /* ecdsa tests */
     run_random_pubkeys();