encoding/pem: be more permissive about decoding empty blocks.

As noted in bug #10980, an empty PEM block is encoded as
-----BEGIN foo-----
-----END foo-----

However, Decode failed to process this.

RFC 1421 doesn't answer what the encoding of the empty block should be
because PEM messages always contain at least one header. However, PEM
these days is just the encoding format – nobody uses the rest of PEM any
longer.

Having the empty block not contain a newline seems most correct because
https://tools.ietf.org/html/rfc1421#section-9 clearly says that the
optional “pemtext” carries the leading new-line with it. So if omitted,
the new-line should be omitted too.

None the less, this changes makes encoding/pem permissive, accepting any
number of blank lines in an empty PEM block.

Fixes #10980

Change-Id: If36bdfbf991ee281eccd50b56ddc95f24c6debb2
Reviewed-on: https://go-review.googlesource.com/10516
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>

2 files changed, 113 insertions, 7 deletions

diff --git a/src/encoding/pem/pem.go b/src/encoding/pem/pem.go
index 90fe3dc50c..506196b1db 100644
--- a/src/encoding/pem/pem.go
+++ b/src/encoding/pem/pem.go
@@ -10,8 +10,10 @@ package pem
 import (
 	"bytes"
 	"encoding/base64"
+	"errors"
 	"io"
 	"sort"
+	"strings"
 )
 
 // A Block represents a PEM encoded structure.
@@ -110,27 +112,37 @@ func Decode(data []byte) (p *Block, rest []byte) {
 		}
 
 		// TODO(agl): need to cope with values that spread across lines.
-		key, val := line[0:i], line[i+1:]
+		key, val := line[:i], line[i+1:]
 		key = bytes.TrimSpace(key)
 		val = bytes.TrimSpace(val)
 		p.Headers[string(key)] = string(val)
 		rest = next
 	}
 
-	i := bytes.Index(rest, pemEnd)
-	if i < 0 {
+	var endIndex int
+	// If there were no headers, the END line might occur
+	// immediately, without a leading newline.
+	if len(p.Headers) == 0 && bytes.HasPrefix(rest, pemEnd[1:]) {
+		endIndex = 0
+	} else {
+		endIndex = bytes.Index(rest, pemEnd)
+	}
+
+	if endIndex < 0 {
 		return decodeError(data, rest)
 	}
-	base64Data := removeWhitespace(rest[0:i])
 
+	base64Data := removeWhitespace(rest[:endIndex])
 	p.Bytes = make([]byte, base64.StdEncoding.DecodedLen(len(base64Data)))
 	n, err := base64.StdEncoding.Decode(p.Bytes, base64Data)
 	if err != nil {
 		return decodeError(data, rest)
 	}
-	p.Bytes = p.Bytes[0:n]
+	p.Bytes = p.Bytes[:n]
 
-	_, rest = getLine(rest[i+len(pemEnd):])
+	// the -1 is because we might have only matched pemEnd without the
+	// leading newline if the PEM block was empty.
+	_, rest = getLine(rest[endIndex+len(pemEnd)-1:])
 
 	return
 }
@@ -246,6 +258,9 @@ func Encode(out io.Writer, b *Block) error {
 		// For consistency of output, write other headers sorted by key.
 		sort.Strings(h)
 		for _, k := range h {
+			if strings.Contains(k, ":") {
+				return errors.New("pem: cannot encode a header key that contains a colon")
+			}
 			if err := writeHeader(out, k, b.Headers[k]); err != nil {
 				return err
 			}
diff --git a/src/encoding/pem/pem_test.go b/src/encoding/pem/pem_test.go
index 92451feff8..1913f44c1f 100644
--- a/src/encoding/pem/pem_test.go
+++ b/src/encoding/pem/pem_test.go
@@ -8,7 +8,9 @@ import (
 	"bytes"
 	"io/ioutil"
 	"reflect"
+	"strings"
 	"testing"
+	"testing/quick"
 )
 
 type GetLineTest struct {
@@ -44,6 +46,32 @@ func TestDecode(t *testing.T) {
 	if !reflect.DeepEqual(result, privateKey) {
 		t.Errorf("#1 got:%#v want:%#v", result, privateKey)
 	}
+
+	isEmpty := func(block *Block) bool {
+		return block != nil && block.Type == "EMPTY" && len(block.Headers) == 0 && len(block.Bytes) == 0
+	}
+	result, remainder = Decode(remainder)
+	if !isEmpty(result) {
+		t.Errorf("#2 should be empty but got:%#v", result)
+	}
+	result, remainder = Decode(remainder)
+	if !isEmpty(result) {
+		t.Errorf("#3 should be empty but got:%#v", result)
+	}
+	result, remainder = Decode(remainder)
+	if !isEmpty(result) {
+		t.Errorf("#4 should be empty but got:%#v", result)
+	}
+
+	result, remainder = Decode(remainder)
+	if result == nil || result.Type != "HEADERS" || len(result.Headers) != 1 {
+		t.Errorf("#5 expected single header block but got :%v", result)
+	}
+
+	if len(remainder) != 0 {
+		t.Errorf("expected nothing remaining of pemData, but found %s", string(remainder))
+	}
+
 	result, _ = Decode([]byte(pemPrivateKey2))
 	if !reflect.DeepEqual(result, privateKey2) {
 		t.Errorf("#2 got:%#v want:%#v", result, privateKey2)
@@ -117,6 +145,44 @@ func TestLineBreaker(t *testing.T) {
 	}
 }
 
+func TestFuzz(t *testing.T) {
+	testRoundtrip := func(block *Block) bool {
+		for key := range block.Headers {
+			if strings.Contains(key, ":") {
+				// Keys with colons cannot be encoded.
+				return true
+			}
+		}
+
+		var buf bytes.Buffer
+		err := Encode(&buf, block)
+		decoded, rest := Decode(buf.Bytes())
+
+		switch {
+		case err != nil:
+			t.Errorf("Encode of %#v resulted in error: %s", block, err)
+		case !reflect.DeepEqual(block, decoded):
+			t.Errorf("Encode of %#v decoded as %#v", block, decoded)
+		case len(rest) != 0:
+			t.Errorf("Encode of %#v decoded correctly, but with %x left over", block, rest)
+		default:
+			return true
+		}
+		return false
+	}
+
+	// Explicitly test the empty block.
+	if !testRoundtrip(&Block{
+		Type:    "EMPTY",
+		Headers: make(map[string]string),
+		Bytes:   []byte{},
+	}) {
+		return
+	}
+
+	quick.Check(testRoundtrip, nil)
+}
+
 func BenchmarkEncode(b *testing.B) {
 	data := &Block{Bytes: make([]byte, 65536)}
 	b.SetBytes(int64(len(data.Bytes)))
@@ -188,7 +254,32 @@ BTiHcL3s3KrJu1vDVrshvxfnz71KTeNnZH8UbOqT5i7fPGyXtY1XJddcbI/Q6tXf
 wHFsZc20TzSdsVLBtwksUacpbDogcEVMctnNrB8FIrB3vZEv9Q0Z1VeY7nmTpF+6
 a+z2P7acL7j6A6Pr3+q8P9CPiPC7zFonVzuVPyB8GchGR2hytyiOVpuD9+k8hcuw
 ZWAaUoVtWIQ52aKS0p19G99hhb+IVANC4akkdHV4SP8i7MVNZhfUmg==
------END RSA PRIVATE KEY-----`
+-----END RSA PRIVATE KEY-----
+
+
+-----BEGIN EMPTY-----
+-----END EMPTY-----
+
+-----BEGIN EMPTY-----
+
+-----END EMPTY-----
+
+-----BEGIN EMPTY-----
+
+
+-----END EMPTY-----
+
+# This shouldn't be recognised because of the missing newline after the
+headers.
+-----BEGIN HEADERS-----
+Header: 1
+-----END HEADERS-----
+
+# This should be valid, however.
+-----BEGIN HEADERS-----
+Header: 1
+
+-----END HEADERS-----`
 
 var certificate = &Block{Type: "CERTIFICATE",
 	Headers: map[string]string{},