Add secp256k1_ec_pubkey_negate and secp256k1_ec_privkey_negate

Author: apoelstra

include/secp256k1.h

@@ -491,6 +491,28 @@ SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_pubkey_create(
     const unsigned char *seckey
 ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
 
+/** Negates a private key in place.
+ *
+ *  Returns: 1 always
+ *  Args:   ctx:        pointer to a context object
+ *  In/Out: pubkey:     pointer to the public key to be negated (cannot be NULL)
+ */
+SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_privkey_negate(
+    const secp256k1_context* ctx,
+    unsigned char *seckey
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2);
+
+/** Negates a public key in place.
+ *
+ *  Returns: 1 always
+ *  Args:   ctx:        pointer to a context object
+ *  In/Out: pubkey:     pointer to the public key to be negated (cannot be NULL)
+ */
+SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_pubkey_negate(
+    const secp256k1_context* ctx,
+    secp256k1_pubkey *pubkey
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2);
+
 /** Tweak a private key by adding tweak to it.
  * Returns: 0 if the tweak was out of range (chance of around 1 in 2^128 for
  *          uniformly random 32-byte arrays, or if the resulting private key

src/secp256k1.c

@@ -424,6 +424,33 @@ int secp256k1_ec_pubkey_create(const secp256k1_context* ctx, secp256k1_pubkey *p
     return ret;
 }
 
+int secp256k1_ec_privkey_negate(const secp256k1_context* ctx, unsigned char *seckey) {
+    secp256k1_scalar sec;
+    VERIFY_CHECK(ctx != NULL);
+    ARG_CHECK(seckey != NULL);
+
+    secp256k1_scalar_set_b32(&sec, seckey, NULL);
+    secp256k1_scalar_negate(&sec, &sec);
+    secp256k1_scalar_get_b32(seckey, &sec);
+
+    return 1;
+}
+
+int secp256k1_ec_pubkey_negate(const secp256k1_context* ctx, secp256k1_pubkey *pubkey) {
+    int ret = 0;
+    secp256k1_ge p;
+    VERIFY_CHECK(ctx != NULL);
+    ARG_CHECK(pubkey != NULL);
+
+    ret = secp256k1_pubkey_load(ctx, &p, pubkey);
+    memset(pubkey, 0, sizeof(*pubkey));
+    if (ret) {
+        secp256k1_ge_neg(&p, &p);
+        secp256k1_pubkey_save(pubkey, &p);
+    }
+    return ret;
+}
+
 int secp256k1_ec_privkey_tweak_add(const secp256k1_context* ctx, unsigned char *seckey, const unsigned char *tweak) {
     secp256k1_scalar term;
     secp256k1_scalar sec;

src/tests.c

@@ -10,6 +10,7 @@
 
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 
 #include <time.h>
 
@@ -135,6 +136,7 @@ void random_scalar_order(secp256k1_scalar *num) {
 
 void run_context_tests(void) {
     secp256k1_pubkey pubkey;
+    secp256k1_pubkey zero_pubkey;
     secp256k1_ecdsa_signature sig;
     unsigned char ctmp[32];
     int32_t ecount;
@@ -149,6 +151,8 @@ void run_context_tests(void) {
     secp256k1_scalar msg, key, nonce;
     secp256k1_scalar sigr, sigs;
 
+    memset(&zero_pubkey, 0, sizeof(zero_pubkey));
+
     ecount = 0;
     ecount2 = 10;
     secp256k1_context_set_illegal_callback(vrfy, counting_illegal_callback_fn, &ecount);
@@ -201,12 +205,20 @@ void run_context_tests(void) {
     CHECK(ecount == 2);
     CHECK(secp256k1_ec_pubkey_tweak_mul(sign, &pubkey, ctmp) == 0);
     CHECK(ecount2 == 13);
-    CHECK(secp256k1_ec_pubkey_tweak_mul(vrfy, &pubkey, ctmp) == 1);
+    CHECK(secp256k1_ec_pubkey_negate(vrfy, &pubkey) == 1);
     CHECK(ecount == 2);
-    CHECK(secp256k1_context_randomize(vrfy, ctmp) == 0);
+    CHECK(secp256k1_ec_pubkey_negate(sign, &pubkey) == 1);
+    CHECK(ecount == 2);
+    CHECK(secp256k1_ec_pubkey_negate(sign, NULL) == 0);
+    CHECK(ecount2 == 14);
+    CHECK(secp256k1_ec_pubkey_negate(vrfy, &zero_pubkey) == 0);
+    CHECK(ecount == 3);
+    CHECK(secp256k1_ec_pubkey_tweak_mul(vrfy, &pubkey, ctmp) == 1);
     CHECK(ecount == 3);
+    CHECK(secp256k1_context_randomize(vrfy, ctmp) == 0);
+    CHECK(ecount == 4);
     CHECK(secp256k1_context_randomize(sign, NULL) == 1);
-    CHECK(ecount2 == 13);
+    CHECK(ecount2 == 14);
     secp256k1_context_set_illegal_callback(vrfy, NULL, NULL);
     secp256k1_context_set_illegal_callback(sign, NULL, NULL);
 
@@ -3436,6 +3448,7 @@ void test_ecdsa_end_to_end(void) {
     unsigned char pubkeyc[65];
     size_t pubkeyclen = 65;
     secp256k1_pubkey pubkey;
+    secp256k1_pubkey pubkey_tmp;
     unsigned char seckey[300];
     size_t seckeylen = 300;
 
@@ -3457,6 +3470,13 @@ void test_ecdsa_end_to_end(void) {
     memset(&pubkey, 0, sizeof(pubkey));
     CHECK(secp256k1_ec_pubkey_parse(ctx, &pubkey, pubkeyc, pubkeyclen) == 1);
 
+    /* Verify negation changes the key and changes it back */
+    memcpy(&pubkey_tmp, &pubkey, sizeof(pubkey));
+    CHECK(secp256k1_ec_pubkey_negate(ctx, &pubkey_tmp) == 1);
+    CHECK(memcmp(&pubkey_tmp, &pubkey, sizeof(pubkey)) != 0);
+    CHECK(secp256k1_ec_pubkey_negate(ctx, &pubkey_tmp) == 1);
+    CHECK(memcmp(&pubkey_tmp, &pubkey, sizeof(pubkey)) == 0);
+
     /* Verify private key import and export. */
     CHECK(ec_privkey_export_der(ctx, seckey, &seckeylen, privkey, secp256k1_rand_bits(1) == 1));
     CHECK(ec_privkey_import_der(ctx, privkey2, seckey, seckeylen) == 1);