encoding/asn1: enforce use of short form lengths.

BER allows the sender to choose either short form or long form where
both are legal, but DER requires the minimal one be used. Enforce this
and add a test. Fix one test which was not minimally-encoded and another
which would not distinguish rejecting the input because the long form
length wasn't minimally-encoded from rejecting it because long form was
chosen when short form was allowed.

Change-Id: I1b56fcca594dcdeddea9378b4fab427cbe7cd26d
Reviewed-on: https://go-review.googlesource.com/16517Reviewed-by: Adam Langley <agl@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>

src/encoding/asn1/asn1.go

@@ -475,6 +475,11 @@ func parseTagAndLength(bytes []byte, initOffset int) (ret tagAndLength, offset i
 				return
 			}
 		}
+		// Short lengths must be encoded in short form.
+		if ret.length < 0x80 {
+			err = StructuralError{"non-minimal length"}
+			return
+		}
 	}
 
 	return

src/encoding/asn1/asn1_test.go

@@ -354,17 +354,19 @@ var tagAndLengthData = []tagAndLengthTest{
 	{[]byte{0x1f, 0x01, 0x00}, true, tagAndLength{0, 1, 0, false}},
 	{[]byte{0x1f, 0x81, 0x00, 0x00}, true, tagAndLength{0, 128, 0, false}},
 	{[]byte{0x1f, 0x81, 0x80, 0x01, 0x00}, true, tagAndLength{0, 0x4001, 0, false}},
-	{[]byte{0x00, 0x81, 0x01}, true, tagAndLength{0, 0, 1, false}},
+	{[]byte{0x00, 0x81, 0x80}, true, tagAndLength{0, 0, 128, false}},
 	{[]byte{0x00, 0x82, 0x01, 0x00}, true, tagAndLength{0, 0, 256, false}},
 	{[]byte{0x00, 0x83, 0x01, 0x00}, false, tagAndLength{}},
 	{[]byte{0x1f, 0x85}, false, tagAndLength{}},
 	{[]byte{0x30, 0x80}, false, tagAndLength{}},
 	// Superfluous zeros in the length should be an error.
-	{[]byte{0xa0, 0x82, 0x00, 0x01}, false, tagAndLength{}},
+	{[]byte{0xa0, 0x82, 0x00, 0xff}, false, tagAndLength{}},
 	// Lengths up to the maximum size of an int should work.
 	{[]byte{0xa0, 0x84, 0x7f, 0xff, 0xff, 0xff}, true, tagAndLength{2, 0, 0x7fffffff, true}},
 	// Lengths that would overflow an int should be rejected.
 	{[]byte{0xa0, 0x84, 0x80, 0x00, 0x00, 0x00}, false, tagAndLength{}},
+	// Long length form may not be used for lengths that fit in short form.
+	{[]byte{0xa0, 0x81, 0x7f}, false, tagAndLength{}},
 }
 
 func TestParseTagAndLength(t *testing.T) {