go.crypto/openpgp: fix clearsigned parsing of messages with leading blank lines.

Fixes golang/go#8508.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://golang.org/cl/128920043

openpgp/clearsign/clearsign.go

@@ -113,6 +113,7 @@ func Decode(data []byte) (b *Block, rest []byte) {
 		b.Headers.Add(string(key), string(val))
 	}
 
+	firstLine := true
 	for {
 		start := rest
 
@@ -126,9 +127,12 @@ func Decode(data []byte) (b *Block, rest []byte) {
 
 		// The final CRLF isn't included in the hash so we don't write it until
 		// we've seen the next line.
-		if len(b.Bytes) > 0 {
+		if firstLine {
+			firstLine = false
+		} else {
 			b.Bytes = append(b.Bytes, crlf...)
 		}
+
 		if bytes.HasPrefix(line, dashEscape) {
 			line = line[2:]
 		}

openpgp/clearsign/clearsign_test.go

@@ -10,8 +10,8 @@ import (
 	"testing"
 )
 
-func TestParse(t *testing.T) {
-	b, rest := Decode(clearsignInput)
+func testParse(t *testing.T, input []byte, expected, expectedPlaintext string) {
+	b, rest := Decode(input)
 	if b == nil {
 		t.Fatal("failed to decode clearsign message")
 	}
@@ -21,14 +21,12 @@ func TestParse(t *testing.T) {
 	if b.ArmoredSignature.Type != "PGP SIGNATURE" {
 		t.Errorf("bad armor type, got:%s, want:PGP SIGNATURE", b.ArmoredSignature.Type)
 	}
-	expected := []byte("Hello world\r\nline 2")
-	if !bytes.Equal(b.Bytes, expected) {
+	if !bytes.Equal(b.Bytes, []byte(expected)) {
 		t.Errorf("bad body, got:%x want:%x", b.Bytes, expected)
 	}
 
-	expected = []byte("Hello world\nline 2\n")
-	if !bytes.Equal(b.Plaintext, expected) {
-		t.Errorf("bad plaintext, got:%x want:%x", b.Plaintext, expected)
+	if !bytes.Equal(b.Plaintext, []byte(expectedPlaintext)) {
+		t.Errorf("bad plaintext, got:%x want:%x", b.Plaintext, expectedPlaintext)
 	}
 
 	keyring, err := openpgp.ReadArmoredKeyRing(bytes.NewBufferString(signingKey))
@@ -41,6 +39,11 @@ func TestParse(t *testing.T) {
 	}
 }
 
+func TestParse(t *testing.T) {
+	testParse(t, clearsignInput, "Hello world\r\nline 2", "Hello world\nline 2\n")
+	testParse(t, clearsignInput2, "\r\n\r\n(This message has a couple of blank lines at the start and end.)\r\n\r\n", "\n\n(This message has a couple of blank lines at the start and end.)\n\n\n")
+}
+
 func TestParseWithNoNewlineAtEnd(t *testing.T) {
 	input := clearsignInput
 	input = input[:len(input)-len("trailing")-1]
@@ -125,6 +128,29 @@ MyTpno24AjIAGb+mH1U=
 -----END PGP SIGNATURE-----
 trailing`)
 
+var clearsignInput2 = []byte(`
+asdlfkjasdlkfjsadf
+
+-----BEGIN PGP SIGNED MESSAGE-----
+Hash: SHA256
+
+
+
+(This message has a couple of blank lines at the start and end.)
+
+
+-----BEGIN PGP SIGNATURE-----
+Version: GnuPG v1.4.11 (GNU/Linux)
+
+iJwEAQEIAAYFAlPpSREACgkQO9o98PRieSpZTAP+M8QUoCt/7Rf3YbXPcdzIL32v
+pt1I+cMNeopzfLy0u4ioEFi8s5VkwpL1AFmirvgViCwlf82inoRxzZRiW05JQ5LI
+ESEzeCoy2LIdRCQ2hcrG8pIUPzUO4TqO5D/dMbdHwNH4h5nNmGJUAEG6FpURlPm+
+qZg6BaTvOxepqOxnhVU=
+=e+C6
+-----END PGP SIGNATURE-----
+
+trailing`)
+
 var signingKey = `-----BEGIN PGP PRIVATE KEY BLOCK-----
 Version: GnuPG v1.4.10 (GNU/Linux)