encoding/asn1: allow lengths up to the maximum int value.

Previously we capped lengths at 2**24 rather than 2**31.

R=golang-dev, r, r
CC=golang-dev
https://golang.org/cl/6050050

src/pkg/encoding/asn1/asn1.go

@@ -377,11 +377,6 @@ func parseTagAndLength(bytes []byte, initOffset int) (ret tagAndLength, offset i
 	} else {
 		// Bottom 7 bits give the number of length bytes to follow.
 		numBytes := int(b & 0x7f)
-		// We risk overflowing a signed 32-bit number if we accept more than 3 bytes.
-		if numBytes > 3 {
-			err = StructuralError{"length too large"}
-			return
-		}
 		if numBytes == 0 {
 			err = SyntaxError{"indefinite length found (not DER)"}
 			return
@@ -394,8 +389,19 @@ func parseTagAndLength(bytes []byte, initOffset int) (ret tagAndLength, offset i
 			}
 			b = bytes[offset]
 			offset++
+			if ret.length >= 1<<23 {
+				// We can't shift ret.length up without
+				// overflowing.
+				err = StructuralError{"length too large"}
+				return
+			}
 			ret.length <<= 8
 			ret.length |= int(b)
+			if ret.length == 0 {
+				// DER requires that lengths be minimal.
+				err = StructuralError{"superfluous leading zeros in length"}
+				return
+			}
 		}
 	}
 

src/pkg/encoding/asn1/asn1_test.go

@@ -283,6 +283,12 @@ var tagAndLengthData = []tagAndLengthTest{
 	{[]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{}},
+	// 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{}},
 }
 
 func TestParseTagAndLength(t *testing.T) {