Implement adding private and public keys

Secp256k1.xs

@@ -388,7 +388,10 @@ _privkey_negate(self, privkey)
 			new_seckey[i] = seckey_str[i];
 		}
 
-		int result = secp256k1_ec_seckey_negate(ctx->ctx, new_seckey);
+		int result = secp256k1_ec_seckey_negate(
+			ctx->ctx,
+			new_seckey
+		);
 
 		if (!result) {
 			croak("resulting negated privkey is not valid");
@@ -403,14 +406,82 @@ _privkey_negate(self, privkey)
 	OUTPUT:
 		RETVAL
 
-SV*
+void
 _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);
+		int result = secp256k1_ec_pubkey_negate(
+			ctx->ctx,
+			ctx->pubkey
+		);
+
+SV*
+_privkey_add(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("adding 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_add(
+			ctx->ctx,
+			new_seckey,
+			tweak_str
+		);
+
+		if (!result) {
+			croak("resulting added 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
+
+void
+_pubkey_add(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("adding a pubkey requires 32-byte tweak");
+		}
+
+		int result = secp256k1_ec_pubkey_tweak_add(
+			ctx->ctx,
+			ctx->pubkey,
+			tweak_str
+		);
+
+		if (!result) {
+			croak("resulting added pubkey is not valid");
+		}
 
 void
 DESTROY(self)

lib/Bitcoin/Secp256k1.pm

@@ -127,8 +127,24 @@ sub negate_private_key
 {
 	my ($self, $private_key) = @_;
 
-	$private_key = $self->_privkey_negate($private_key);
-	return $private_key;
+	return $self->_privkey_negate($private_key);
+}
+
+sub add_public_key
+{
+	my ($self, $public_key, $tweak) = @_;
+
+	$self->_pubkey($public_key);
+	$self->_pubkey_add($tweak);
+
+	return $self->_pubkey;
+}
+
+sub add_private_key
+{
+	my ($self, $private_key, $tweak) = @_;
+
+	return $self->_privkey_add($private_key, $tweak);
 }
 
 1;
@@ -281,6 +297,25 @@ Negates a private key and returns it.
 
 Negates a public key and returns it.
 
+=head3 add_private_key
+
+	$tweaked = $secp256k1->add_private_key($private_key, $tweak)
+
+Add a C<$tweak> (bytestring of length C<32>) to C<$private_key> (bytestring of
+length C<32>). The result is a bytestring containing tweaked private key.
+
+If the arguments or the resulting key are not valid, an exception will be thrown.
+
+=head3 add_public_key
+
+	$tweaked = $secp256k1->add_public_key($public_key, $tweak)
+
+Add a C<$tweak> (bytestring of length C<32>) to C<$public_key> (bytestring with
+compressed or uncompressed public key). The result is a bytestring containing
+tweaked public key in compressed form.
+
+If the arguments or the resulting key are not valid, an exception will be thrown.
+
 =head1 IMPLEMENTATION
 
 The module consists of two layers:

t/api.t

@@ -62,5 +62,14 @@ subtest 'should negate' => sub {
 	is $secp->negate_private_key($t{privkey}), $negated_privkey, 'negated privkey ok';
 };
 
+subtest 'should add' => sub {
+	my $added_pubkey = pack 'H*', '0260213f6d967636c54d8845c23098e0f63d906b7903d23692efa155a155eda169';
+	my $added_privkey = pack 'H*', '67a239562bcdfa07345b72305eb8567436be572159b3ef64a91d0392388d04bf';
+	my $tweak = "\x06" x 32;
+
+	is $secp->add_public_key($t{pubkey}, $tweak), $added_pubkey, 'added pubkey ok';
+	is $secp->add_private_key($t{privkey}, $tweak), $added_privkey, 'added privkey ok';
+};
+
 done_testing;