1 files changed, 101 insertions, 0 deletions
diff --git a/plumbing/object/signature.go b/plumbing/object/signature.go
new file mode 100644
index 0000000..91cf371
--- /dev/null
+++ b/plumbing/object/signature.go
@@ -0,0 +1,101 @@
+package object
+
+import "bytes"
+
+const (
+	signatureTypeUnknown signatureType = iota
+	signatureTypeOpenPGP
+	signatureTypeX509
+	signatureTypeSSH
+)
+
+var (
+	// openPGPSignatureFormat is the format of an OpenPGP signature.
+	openPGPSignatureFormat = signatureFormat{
+		[]byte("-----BEGIN PGP SIGNATURE-----"),
+		[]byte("-----BEGIN PGP MESSAGE-----"),
+	}
+	// x509SignatureFormat is the format of an X509 signature, which is
+	// a PKCS#7 (S/MIME) signature.
+	x509SignatureFormat = signatureFormat{
+		[]byte("-----BEGIN CERTIFICATE-----"),
+	}
+
+	// sshSignatureFormat is the format of an SSH signature.
+	sshSignatureFormat = signatureFormat{
+		[]byte("-----BEGIN SSH SIGNATURE-----"),
+	}
+)
+
+var (
+	// knownSignatureFormats is a map of known signature formats, indexed by
+	// their signatureType.
+	knownSignatureFormats = map[signatureType]signatureFormat{
+		signatureTypeOpenPGP: openPGPSignatureFormat,
+		signatureTypeX509:    x509SignatureFormat,
+		signatureTypeSSH:     sshSignatureFormat,
+	}
+)
+
+// signatureType represents the type of the signature.
+type signatureType int8
+
+// signatureFormat represents the beginning of a signature.
+type signatureFormat [][]byte
+
+// typeForSignature returns the type of the signature based on its format.
+func typeForSignature(b []byte) signatureType {
+	for t, i := range knownSignatureFormats {
+		for _, begin := range i {
+			if bytes.HasPrefix(b, begin) {
+				return t
+			}
+		}
+	}
+	return signatureTypeUnknown
+}
+
+// parseSignedBytes returns the position of the last signature block found in
+// the given bytes. If no signature block is found, it returns -1.
+//
+// When multiple signature blocks are found, the position of the last one is
+// returned. Any tailing bytes after this signature block start should be
+// considered part of the signature.
+//
+// Given this, it would be safe to use the returned position to split the bytes
+// into two parts: the first part containing the message, the second part
+// containing the signature.
+//
+// Example:
+//
+//	message := []byte(`Message with signature
+//
+//	-----BEGIN SSH SIGNATURE-----
+//	...`)
+//
+//	var signature string
+//	if pos, _ := parseSignedBytes(message); pos != -1 {
+//		signature = string(message[pos:])
+//		message = message[:pos]
+//	}
+//
+// This logic is on par with git's gpg-interface.c:parse_signed_buffer().
+// https://github.com/git/git/blob/7c2ef319c52c4997256f5807564523dfd4acdfc7/gpg-interface.c#L668
+func parseSignedBytes(b []byte) (int, signatureType) {
+	var n, match = 0, -1
+	var t signatureType
+	for n < len(b) {
+		var i = b[n:]
+		if st := typeForSignature(i); st != signatureTypeUnknown {
+			match = n
+			t = st
+		}
+		if eol := bytes.IndexByte(i, '\n'); eol >= 0 {
+			n += eol + 1
+			continue
+		}
+		// If we reach this point, we've reached the end.
+		break
+	}
+	return match, t
+}