Implement multiplying private and public keys

Secp256k1.xs

@@ -483,6 +483,71 @@ _pubkey_add(self, tweak)
 			croak("resulting added pubkey is not valid");
 		}
 
+SV*
+_privkey_mul(self, privkey, tweak)
+		SV *self
+		SV *privkey
+		SV *tweak
+	CODE:
+		secp256k1_perl *ctx = ctx_from_sv(self);
+		size_t seckey_size;
+		unsigned char *seckey_str = (unsigned char*) SvPVbyte(privkey, seckey_size);
+
+		size_t tweak_size;
+		unsigned char *tweak_str = (unsigned char*) SvPVbyte(tweak, tweak_size);
+
+		if (seckey_size != CURVE_SIZE || tweak_size != CURVE_SIZE) {
+			croak("multiplying a privkey requires 32-byte secret key and tweak");
+		}
+
+		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_tweak_mul(
+			ctx->ctx,
+			new_seckey,
+			tweak_str
+		);
+
+		if (!result) {
+			croak("multiplication arguments are 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
+
+void
+_pubkey_mul(self, tweak)
+		SV *self
+		SV *tweak
+	CODE:
+		secp256k1_perl *ctx = ctx_from_sv(self);
+
+		size_t tweak_size;
+		unsigned char *tweak_str = (unsigned char*) SvPVbyte(tweak, tweak_size);
+
+		if (tweak_size != CURVE_SIZE) {
+			croak("multiplying a pubkey requires 32-byte tweak");
+		}
+
+		int result = secp256k1_ec_pubkey_tweak_mul(
+			ctx->ctx,
+			ctx->pubkey,
+			tweak_str
+		);
+
+		if (!result) {
+			croak("multiplication arguments are not valid");
+		}
+
 void
 DESTROY(self)
 		SV *self

lib/Bitcoin/Secp256k1.pm

@@ -147,6 +147,23 @@ sub add_private_key
 	return $self->_privkey_add($private_key, $tweak);
 }
 
+sub multiply_public_key
+{
+	my ($self, $public_key, $tweak) = @_;
+
+	$self->_pubkey($public_key);
+	$self->_pubkey_mul($tweak);
+
+	return $self->_pubkey;
+}
+
+sub multiply_private_key
+{
+	my ($self, $private_key, $tweak) = @_;
+
+	return $self->_privkey_mul($private_key, $tweak);
+}
+
 1;
 
 __END__
@@ -316,6 +333,18 @@ tweaked public key in compressed form.
 
 If the arguments or the resulting key are not valid, an exception will be thrown.
 
+=head3 multiply_private_key
+
+	$tweaked = $secp256k1->multiply_private_key($private_key, $tweak)
+
+Same as L</add_private_key>, but performs multiplication instead of addition.
+
+=head3 multiply_public_key
+
+	$tweaked = $secp256k1->multiply_public_key($public_key, $tweak)
+
+Same as L</add_public_key>, but performs multiplication instead of addition.
+
 =head1 IMPLEMENTATION
 
 The module consists of two layers:

t/api.t

@@ -71,5 +71,14 @@ subtest 'should add' => sub {
 	is $secp->add_private_key($t{privkey}, $tweak), $added_privkey, 'added privkey ok';
 };
 
+subtest 'should multiply' => sub {
+	my $multiplied_pubkey = pack 'H*', '0311ab47c9252066f0ca5946d70c3aaac1486d65969b90cd57207476963c9f9af3';
+	my $multiplied_privkey = pack 'H*', '2d40a33515eb26b0fbce29e0e9645e15d6ff4892a17e59ab31b66b496780a683';
+	my $tweak = "\x06" x 32;
+
+	is $secp->multiply_public_key($t{pubkey}, $tweak), $multiplied_pubkey, 'multiplied pubkey ok';
+	is $secp->multiply_private_key($t{privkey}, $tweak), $multiplied_privkey, 'multiplied privkey ok';
+};
+
 done_testing;