x/crypto: add ReadUint32LengthPrefixed

cryptobyte/cryptobyte_test.go

@@ -305,6 +305,27 @@ func TestUint8LengthPrefixedNested(t *testing.T) {
 	}
 }
 
+func TestUint32LengthPrefixedSimple(t *testing.T) {
+	var b Builder
+	b.AddUint32LengthPrefixed(func(c *Builder) {
+		c.AddUint8(23)
+		c.AddUint8(42)
+	})
+	if err := builderBytesEq(&b, 0, 0, 0, 2, 23, 42); err != nil {
+		t.Error(err)
+	}
+	var s, out String = String(b.BytesOrPanic()), nil
+	if !s.ReadUint32LengthPrefixed(&out) {
+		t.Error("read failed")
+	}
+	if len(out) != 2 || out[0] != 23 || out[1] != 42 {
+		t.Error("read incorrect value")
+	}
+	if len(s) != 0 {
+		t.Error("incorrect source length")
+	}
+}
+
 func TestPreallocatedBuffer(t *testing.T) {
 	var buf [5]byte
 	b := NewBuilder(buf[0:0])

cryptobyte/string.go

@@ -105,7 +105,8 @@ func (s *String) readLengthPrefixed(lenLen int, outChild *String) bool {
 		length = length << 8
 		length = length | uint32(b)
 	}
-	v := s.read(int(length))
+	n := int(length)
+	v := s.read(n) // returns nil if n is negative due to overflow
 	if v == nil {
 		return false
 	}
@@ -133,6 +134,14 @@ func (s *String) ReadUint24LengthPrefixed(out *String) bool {
 	return s.readLengthPrefixed(3, out)
 }
 
+// ReadUint32LengthPrefixed reads the content of a big-endian, 32-bit
+// length-prefixed value into out and advances over it. It reports whether
+// the read was successful.
+// On 32-bit platform read fails if length is greater than max int.
+func (s *String) ReadUint32LengthPrefixed(out *String) bool {
+	return s.readLengthPrefixed(4, out)
+}
+
 // ReadBytes reads n bytes into out and advances over them. It reports
 // whether the read was successful.
 func (s *String) ReadBytes(out *[]byte, n int) bool {