cryptobyte: add uint48 methods

Adds uint48 methods for cryptobyte.Builder and cryptobyte.String.
Supporting 48-bit unsigned integers is useful for working with protocols
that use them for sequence numbers, such as DTLS.

Fixes golang/go#61275

Change-Id: Ibe49422d37644b9212b28b123dc5e01850f7b05b
GitHub-Last-Rev: 11b388c240109c8f4ac23880645c901ce6d2f093
GitHub-Pull-Request: golang/crypto#265
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/508675
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: qiulaidongfeng <2645477756@qq.com>
Run-TryBot: Roland Shoemaker <roland@golang.org>
Auto-Submit: Roland Shoemaker <roland@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

cryptobyte/builder.go

diff --git a/cryptobyte/builder.go b/cryptobyte/builder.go
index c05ac7d..cf254f5 100644
--- a/cryptobyte/builder.go
+++ b/cryptobyte/builder.go
@@ -95,6 +95,11 @@
 	b.add(byte(v>>24), byte(v>>16), byte(v>>8), byte(v))
 }
 
+// AddUint48 appends a big-endian, 48-bit value to the byte string.
+func (b *Builder) AddUint48(v uint64) {
+	b.add(byte(v>>40), byte(v>>32), byte(v>>24), byte(v>>16), byte(v>>8), byte(v))
+}
+
 // AddUint64 appends a big-endian, 64-bit value to the byte string.
 func (b *Builder) AddUint64(v uint64) {
 	b.add(byte(v>>56), byte(v>>48), byte(v>>40), byte(v>>32), byte(v>>24), byte(v>>16), byte(v>>8), byte(v))

cryptobyte/cryptobyte_test.go

diff --git a/cryptobyte/cryptobyte_test.go b/cryptobyte/cryptobyte_test.go
index dc90e81..9b55d52 100644
--- a/cryptobyte/cryptobyte_test.go
+++ b/cryptobyte/cryptobyte_test.go
@@ -179,6 +179,27 @@
 	}
 }
 
+func TestUint48(t *testing.T) {
+	var b Builder
+	var u uint64 = 0xfefcff3cfdfc
+	b.AddUint48(u)
+	if err := builderBytesEq(&b, 254, 252, 255, 60, 253, 252); err != nil {
+		t.Error(err)
+	}
+
+	var s String = b.BytesOrPanic()
+	var v uint64
+	if !s.ReadUint48(&v) {
+		t.Error("ReadUint48() = false, want true")
+	}
+	if v != u {
+		t.Errorf("v = %x, want %x", v, u)
+	}
+	if len(s) != 0 {
+		t.Errorf("len(s) = %d, want 0", len(s))
+	}
+}
+
 func TestUint64(t *testing.T) {
 	var b Builder
 	b.AddUint64(0xf2fefefcff3cfdfc)

cryptobyte/string.go

diff --git a/cryptobyte/string.go b/cryptobyte/string.go
index 0531a3d..10692a8 100644
--- a/cryptobyte/string.go
+++ b/cryptobyte/string.go
@@ -81,6 +81,17 @@
 	return true
 }
 
+// ReadUint48 decodes a big-endian, 48-bit value into out and advances over it.
+// It reports whether the read was successful.
+func (s *String) ReadUint48(out *uint64) bool {
+	v := s.read(6)
+	if v == nil {
+		return false
+	}
+	*out = uint64(v[0])<<40 | uint64(v[1])<<32 | uint64(v[2])<<24 | uint64(v[3])<<16 | uint64(v[4])<<8 | uint64(v[5])
+	return true
+}
+
 // ReadUint64 decodes a big-endian, 64-bit value into out and advances over it.
 // It reports whether the read was successful.
 func (s *String) ReadUint64(out *uint64) bool {