Implement negation of keys

Secp256k1.xs

@@ -370,6 +370,48 @@ _verify_privkey(self, privkey)
 	OUTPUT:
 		RETVAL
 
+SV*
+_privkey_negate(self, privkey)
+		SV *self
+		SV *privkey
+	CODE:
+		secp256k1_perl *ctx = ctx_from_sv(self);
+		size_t seckey_size;
+		unsigned char *seckey_str = (unsigned char*) SvPVbyte(privkey, seckey_size);
+
+		if (seckey_size != CURVE_SIZE) {
+			croak("negating a privkey requires a 32-byte secret key");
+		}
+
+		unsigned char new_seckey[CURVE_SIZE];
+		for (int i = 0; i < CURVE_SIZE; ++i) {
+			new_seckey[i] = seckey_str[i];
+		}
+
+		int result = secp256k1_ec_seckey_negate(ctx->ctx, new_seckey);
+
+		if (!result) {
+			croak("resulting negated privkey is not valid");
+		}
+
+		RETVAL = newSVpv((char*) new_seckey, CURVE_SIZE);
+
+		/* Clean up the secret, since we copied it to the stack */
+		for (int i = 0; i < CURVE_SIZE; ++i) {
+			new_seckey[i] = '\0';
+		}
+	OUTPUT:
+		RETVAL
+
+SV*
+_pubkey_negate(self)
+		SV *self
+	CODE:
+		secp256k1_perl *ctx = ctx_from_sv(self);
+
+		/* NOTE: result is always 1 */
+		int result = secp256k1_ec_pubkey_negate(ctx->ctx, ctx->pubkey);
+
 void
 DESTROY(self)
 		SV *self

lib/Bitcoin/Secp256k1.pm

@@ -113,6 +113,24 @@ sub verify_digest
 	return $self->_verify($digest);
 }
 
+sub negate_public_key
+{
+	my ($self, $public_key) = @_;
+
+	$self->_pubkey($public_key);
+	$self->_pubkey_negate;
+
+	return $self->_pubkey;
+}
+
+sub negate_private_key
+{
+	my ($self, $private_key) = @_;
+
+	$private_key = $self->_privkey_negate($private_key);
+	return $private_key;
+}
+
 1;
 
 __END__
@@ -251,6 +269,18 @@ accept or reject malleable signatures explicitly.
 Same as L</verify_message>, but it does not perform double SHA256 on its input.
 Because of that, C<$message_digest> must be a bytestring of length C<32>.
 
+=head3 negate_private_key
+
+	$negated_private_key = $secp256k1->negated_private_key($private_key)
+
+Negates a private key and returns it.
+
+=head3 negate_public_key
+
+	$negated_public_key = $secp256k1->negate_public_key($public_key)
+
+Negates a public key and returns it.
+
 =head1 IMPLEMENTATION
 
 The module consists of two layers:

t/api.t

@@ -54,5 +54,13 @@ subtest 'should sign and verify a digest' => sub {
 	}, 1, 'unnormalized signature warning ok';
 };
 
+subtest 'should negate' => sub {
+	my $negated_pubkey = pack 'H*', '035476c2e83188368da1ff3e292e7acafcdb3566bb0ad253f62fc70f07aeee6357';
+	my $negated_privkey = pack 'H*', '9e63ccafda380bfed1aa93d5a74daf9089f68bcb5b9ab6dd1cbb61009daf4288';
+
+	is $secp->negate_public_key($t{pubkey}), $negated_pubkey, 'negated pubkey ok';
+	is $secp->negate_private_key($t{privkey}), $negated_privkey, 'negated privkey ok';
+};
+
 done_testing;