openpgp: Allow V3 signatures in messages

Previously, only V4 signatures were allowed in PGP messages,
and attempts to digest a message with a V3 signature would
yield an `errors.StructuralError("LiteralData not followed by Signature")`.

This patch fixes this shortcoming, by trying to consume either
signature type and verifying accordingly.

Change-Id: I27c8ea781b75b0f7dfc42c01e7de40f3323e9579
Reviewed-on: https://go-review.googlesource.com/14794
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

Author: maxtaco

openpgp/read.go

@@ -56,8 +56,9 @@ type MessageDetails struct {
 	// been consumed. Once EOF has been seen, the following fields are
 	// valid. (An authentication code failure is reported as a
 	// SignatureError error when reading from UnverifiedBody.)
-	SignatureError error             // nil if the signature is good.
-	Signature      *packet.Signature // the signature packet itself.
+	SignatureError error               // nil if the signature is good.
+	Signature      *packet.Signature   // the signature packet itself, if v4 (default)
+	SignatureV3    *packet.SignatureV3 // the signature packet if it is a v2 or v3 signature
 
 	decrypted io.ReadCloser
 }
@@ -334,13 +335,15 @@ func (scr *signatureCheckReader) Read(buf []byte) (n int, err error) {
 		}
 
 		var ok bool
-		if scr.md.Signature, ok = p.(*packet.Signature); !ok {
+		if scr.md.Signature, ok = p.(*packet.Signature); ok {
+			scr.md.SignatureError = scr.md.SignedBy.PublicKey.VerifySignature(scr.h, scr.md.Signature)
+		} else if scr.md.SignatureV3, ok = p.(*packet.SignatureV3); ok {
+			scr.md.SignatureError = scr.md.SignedBy.PublicKey.VerifySignatureV3(scr.h, scr.md.SignatureV3)
+		} else {
 			scr.md.SignatureError = errors.StructuralError("LiteralData not followed by Signature")
 			return
 		}
 
-		scr.md.SignatureError = scr.md.SignedBy.PublicKey.VerifySignature(scr.h, scr.md.Signature)
-
 		// The SymmetricallyEncrypted packet, if any, might have an
 		// unsigned hash of its own. In order to check this we need to
 		// close that Reader.

openpgp/read_test.go

@@ -13,6 +13,7 @@ import (
 	"strings"
 	"testing"
 
+	"golang.org/x/crypto/openpgp/armor"
 	"golang.org/x/crypto/openpgp/errors"
 )
 
@@ -433,6 +434,50 @@ func TestIssue11504(t *testing.T) {
 	testReadMessageError(t, "9303000130303030303030303030983002303030303030030000000130")
 }
 
+// TestSignatureV3Message tests the verification of V3 signature, generated
+// with a modern V4-style key.  Some people have their clients set to generate
+// V3 signatures, so it's useful to be able to verify them.
+func TestSignatureV3Message(t *testing.T) {
+	sig, err := armor.Decode(strings.NewReader(signedMessageV3))
+	if err != nil {
+		t.Error(err)
+		return
+	}
+	key, err := ReadArmoredKeyRing(strings.NewReader(keyV4forVerifyingSignedMessageV3))
+	if err != nil {
+		t.Error(err)
+		return
+	}
+	md, err := ReadMessage(sig.Body, key, nil, nil)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+
+	_, err = ioutil.ReadAll(md.UnverifiedBody)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+
+	// We'll see a sig error here after reading in the UnverifiedBody above,
+	// if there was one to see.
+	if err = md.SignatureError; err != nil {
+		t.Error(err)
+		return
+	}
+
+	if md.SignatureV3 == nil {
+		t.Errorf("No available signature after checking signature")
+		return
+	}
+	if md.Signature != nil {
+		t.Errorf("Did not expect a signature V4 back")
+		return
+	}
+	return
+}
+
 const testKey1KeyId = 0xA34D7E18C20C31BB
 const testKey3KeyId = 0x338934250CCC0360
 const testKeyP256KeyId = 0xd44a2c495918513e
@@ -533,3 +578,36 @@ const unknownHashFunctionHex = `8a00000040040001990006050253863c24000a09103b4fe6
 const missingHashFunctionHex = `8a00000040040001030006050253863c24000a09103b4fe6acc0b21f32ffff0101010101010101010101010101010101010101010101010101010101010101010101010101`
 
 const campbellQuine = `a0b001000300fcffa0b001000d00f2ff000300fcffa0b001000d00f2ff8270a01c00000500faff8270a01c00000500faff000500faff001400ebff8270a01c00000500faff000500faff001400ebff428821c400001400ebff428821c400001400ebff428821c400001400ebff428821c400001400ebff428821c400000000ffff000000ffff000b00f4ff428821c400000000ffff000000ffff000b00f4ff0233214c40000100feff000233214c40000100feff0000`
+
+const keyV4forVerifyingSignedMessageV3 = `-----BEGIN PGP PUBLIC KEY BLOCK-----
+Comment: GPGTools - https://gpgtools.org
+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RZia
+-----END PGP PUBLIC KEY BLOCK-----
+`
+
+const signedMessageV3 = `-----BEGIN PGP MESSAGE-----
+Comment: GPGTools - https://gpgtools.org
+
+owGbwMvMwMVYWXlhlrhb9GXG03JJDKF/MtxDMjKLFYAoUaEktbhEITe1uDgxPVWP
+q5NhKjMrWAVcC9evD8z/bF/uWNjqtk/X3y5/38XGRQHm/57rrDRYuGnTw597Xqka
+uM3137/hH3Os+Jf2dc0fXOITKwJvXJvecPVs0ta+Vg7ZO1MLn8w58Xx+6L58mbka
+DGHyU9yTueZE8D+QF/Tz28Y78dqtF56R1VPn9Xw4uJqrWYdd7b3vIZ1V6R4Nh05d
+iT57d/OhWwA=
+=hG7R
+-----END PGP MESSAGE-----
+`