Implement message signing

No erasing of the seckey, since it is allocated by Perl somewhere on the
heap. If this level of security is required, plain C should be used.

Secp256k1.xs

@@ -242,6 +242,43 @@ _verify(self, message)
 	OUTPUT:
 		RETVAL
 
+SV*
+_sign(self, privkey, message)
+		SV* self
+		SV* privkey
+		SV* message
+	CODE:
+		secp256k1_perl *ctx = ctx_from_sv(self);
+
+		size_t message_size;
+		unsigned char *message_str = (unsigned char*) SvPVbyte(message, message_size);
+		if (message_size != CURVE_SIZE) {
+			croak("signing requires a 32-byte message hash");
+		}
+
+		size_t seckey_size;
+		unsigned char *seckey_str = (unsigned char*) SvPVbyte(privkey, seckey_size);
+		if (seckey_size != CURVE_SIZE) {
+			croak("signing requires a 32-byte secret key");
+		}
+
+		secp256k1_ecdsa_signature *result_signature = malloc(sizeof *result_signature);
+		int result = secp256k1_ecdsa_sign(
+			ctx->ctx,
+			result_signature,
+			message_str,
+			seckey_str,
+			NULL,
+			NULL
+		);
+
+		if (!result) {
+			free(result_signature);
+			croak("signing failed (nonce generation problem?)");
+		}
+
+		secp256k1_perl_replace_signature(ctx, result_signature);
+
 void
 DESTROY(self)
 		SV *self

t/base.t

@@ -10,45 +10,54 @@ use Digest::SHA qw(sha256);
 
 my $secp;
 
-subtest 'constructor works' => sub {
+my $sample_privkey = pack 'H*',
+	'619c335025c7f4012e556c2a58b2506e30b8511b53ade95ea316fd8c3286feb9';
+my $sample_pubkey = pack 'H*', '025476c2e83188368da1ff3e292e7acafcdb3566bb0ad253f62fc70f07aeee6357';
+my $sample_pubkey_unc = pack 'H*',
+	'045476c2e83188368da1ff3e292e7acafcdb3566bb0ad253f62fc70f07aeee6357fd57dee6b46a6b010a3e4a70961ecf44a40e18b279ec9e9fba9c1dbc64896198';
+my $sample_preimage = pack 'H*',
+	'0100000096b827c8483d4e9b96712b6713a7b68d6e8003a781feba36c31143470b4efd3752b0a642eea2fb7ae638c36f6252b6750293dbe574a806984b8e4d8548339a3bef51e1b804cc89d182d279655c3aa89e815b1b309fe287d9b2b55d57b90ec68a010000001976a9141d0f172a0ecb48aee1be1f2687d2963ae33f71a188ac0046c32300000000ffffffff863ef3e1a92afbfdb97f31ad0fc7683ee943e9abcf2501590ff8f6551f47e5e51100000001000000';
+my $sample_sig = pack 'H*',
+	'304402203609e17b84f6a7d30c80bfa610b5b4542f32a8a0d5447a12fb1366d7f01cc44a0220573a954c4518331561406f90300e8f3358f51928d43c212a8caed02de67eebee';
+
+subtest 'should create and destroy' => sub {
 	$secp = Bitcoin::Secp256k1->new();
 	isa_ok $secp, 'Bitcoin::Secp256k1';
 };
 
-subtest 'can import and export pubkey' => sub {
-	my $sample_pubkey = '025476c2e83188368da1ff3e292e7acafcdb3566bb0ad253f62fc70f07aeee6357';
-	my $sample_pubkey_unc =
-		'045476c2e83188368da1ff3e292e7acafcdb3566bb0ad253f62fc70f07aeee6357fd57dee6b46a6b010a3e4a70961ecf44a40e18b279ec9e9fba9c1dbc64896198';
-	my $pubkey_bytes = pack 'H*', $sample_pubkey;
-	my $pubkey_unc_bytes = pack 'H*', $sample_pubkey_unc;
-
+subtest 'should import and export pubkey' => sub {
 	is $secp->_pubkey, undef, 'starting pubkey ok';
-	is $secp->_pubkey($pubkey_bytes), $pubkey_bytes, 'setter ok';
-	is $secp->_pubkey, $pubkey_bytes, 'getter ok';
-	is $secp->_pubkey(undef, 1), $pubkey_bytes, 'getter with explicit compression ok';
-	is $secp->_pubkey(undef, 0), $pubkey_unc_bytes, 'getter with explicit (un)compression ok';
-	is $secp->_pubkey($pubkey_unc_bytes), $pubkey_bytes, 'setter with uncompressed input, compressed output ok';
+	is $secp->_pubkey($sample_pubkey), $sample_pubkey, 'setter ok';
+	is $secp->_pubkey, $sample_pubkey, 'getter ok';
+	is $secp->_pubkey(undef, 1), $sample_pubkey, 'getter with explicit compression ok';
+	is $secp->_pubkey(undef, 0), $sample_pubkey_unc, 'getter with explicit (un)compression ok';
+	is $secp->_pubkey($sample_pubkey_unc), $sample_pubkey, 'setter with uncompressed input, compressed output ok';
 };
 
-subtest 'can import and export signature' => sub {
-	my $sample_sig =
-		'304402203609e17b84f6a7d30c80bfa610b5b4542f32a8a0d5447a12fb1366d7f01cc44a0220573a954c4518331561406f90300e8f3358f51928d43c212a8caed02de67eebee';
-	my $sig_bytes = pack 'H*', $sample_sig;
-
+subtest 'should import and export signature' => sub {
 	is $secp->_signature, undef, 'starting sig ok';
-	is $secp->_signature($sig_bytes), $sig_bytes, 'setter ok';
-	is $secp->_signature, $sig_bytes, 'getter ok';
+	is $secp->_signature($sample_sig), $sample_sig, 'setter ok';
+	is $secp->_signature, $sample_sig, 'getter ok';
 };
 
-subtest 'can verify a signature' => sub {
-	my $sample_preimage =
-		'0100000096b827c8483d4e9b96712b6713a7b68d6e8003a781feba36c31143470b4efd3752b0a642eea2fb7ae638c36f6252b6750293dbe574a806984b8e4d8548339a3bef51e1b804cc89d182d279655c3aa89e815b1b309fe287d9b2b55d57b90ec68a010000001976a9141d0f172a0ecb48aee1be1f2687d2963ae33f71a188ac0046c32300000000ffffffff863ef3e1a92afbfdb97f31ad0fc7683ee943e9abcf2501590ff8f6551f47e5e51100000001000000';
-	my $sample_partial_digest = sha256(pack 'H*', $sample_preimage);
+subtest 'should verify a signature' => sub {
+	my $sample_partial_digest = sha256($sample_preimage);
 	my $sample_digest = sha256($sample_partial_digest);
 
 	ok $secp->_verify($sample_digest), 'digest verification ok';
 	ok !$secp->_verify($sample_partial_digest), 'incorrect digest verification failed ok';
 };
 
+subtest 'should generate a signature' => sub {
+	my $sample_partial_digest = sha256($sample_preimage);
+	my $sample_digest = sha256($sample_partial_digest);
+
+	$secp->_sign($sample_privkey, $sample_partial_digest);
+	isnt $secp->_signature, $sample_sig, 'incorrect digest signing failed ok';
+
+	$secp->_sign($sample_privkey, $sample_digest);
+	is $secp->_signature, $sample_sig, 'signing ok';
+};
+
 done_testing;