encoding/asn1: tags should be encoded in minimal form.

High tag number form may not be used for tag numbers that fit in low tag number
form.

Change-Id: I93edde0e1f86087047e0b3f2e55d6180b01e78bf
Reviewed-on: https://go-review.googlesource.com/18224Reviewed-by: Adam Langley <agl@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>

src/encoding/asn1/asn1.go

@@ -461,6 +461,11 @@ func parseTagAndLength(bytes []byte, initOffset int) (ret tagAndLength, offset i
 		if err != nil {
 			return
 		}
+		// Tags should be encoded in minimal form.
+		if ret.tag < 0x1f {
+			err = SyntaxError{"non-minimal tag"}
+			return
+		}
 	}
 	if offset >= len(bytes) {
 		err = SyntaxError{"truncated tag or length"}

src/encoding/asn1/asn1_test.go

@@ -364,7 +364,7 @@ var tagAndLengthData = []tagAndLengthTest{
 	{[]byte{0xa0, 0x01}, true, tagAndLength{2, 0, 1, true}},
 	{[]byte{0x02, 0x00}, true, tagAndLength{0, 2, 0, false}},
 	{[]byte{0xfe, 0x00}, true, tagAndLength{3, 30, 0, true}},
-	{[]byte{0x1f, 0x01, 0x00}, true, tagAndLength{0, 1, 0, false}},
+	{[]byte{0x1f, 0x1f, 0x00}, true, tagAndLength{0, 31, 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, 0x80}, true, tagAndLength{0, 0, 128, false}},
@@ -382,6 +382,8 @@ var tagAndLengthData = []tagAndLengthTest{
 	{[]byte{0xa0, 0x81, 0x7f}, false, tagAndLength{}},
 	// Tag numbers which would overflow int32 are rejected. (The value below is 2^31.)
 	{[]byte{0x1f, 0x88, 0x80, 0x80, 0x80, 0x00, 0x00}, false, tagAndLength{}},
+	// Long tag number form may not be used for tags that fit in short form.
+	{[]byte{0x1f, 0x1e, 0x00}, false, tagAndLength{}},
 }
 
 func TestParseTagAndLength(t *testing.T) {