Implement AddUint48 utility

Adds the AddUint48 utility used to handle sequence numbers in DTLS
records. This should be removed when
golang/crypto#265 is implemented.

Signed-off-by: Daniel Mangum <[email protected]>

Author: hasheddan

internal/util/util.go

@@ -6,6 +6,8 @@ package util
 
 import (
 	"encoding/binary"
+
+	"golang.org/x/crypto/cryptobyte"
 )
 
 // BigEndianUint24 returns the value of a big endian uint24
@@ -40,3 +42,10 @@ func Max(a, b int) int {
 	}
 	return b
 }
+
+// AddUint48 appends a big-endian, 48-bit value to the byte string.
+// Remove if / when https://github.com/golang/crypto/pull/265 is merged
+// upstream.
+func AddUint48(b *cryptobyte.Builder, v uint64) {
+	b.AddBytes([]byte{byte(v >> 40), byte(v >> 32), byte(v >> 24), byte(v >> 16), byte(v >> 8), byte(v)})
+}

internal/util/util_test.go

@@ -0,0 +1,63 @@
+// SPDX-FileCopyrightText: 2023 The Pion community <https://pion.ly>
+// SPDX-License-Identifier: MIT
+
+package util
+
+import (
+	"bytes"
+	"testing"
+
+	"golang.org/x/crypto/cryptobyte"
+)
+
+func TestAddUint48(t *testing.T) {
+	cases := map[string]struct {
+		reason  string
+		builder *cryptobyte.Builder
+		postAdd func(*cryptobyte.Builder)
+		in      uint64
+		want    []byte
+	}{
+		"OnlyUint48": {
+			reason:  "Adding only a 48-bit unsigned integer should yield expected result.",
+			builder: &cryptobyte.Builder{},
+			in:      0xfefcff3cfdfc,
+			want:    []byte{254, 252, 255, 60, 253, 252},
+		},
+		"ExistingAddUint48": {
+			reason: "Adding a 48-bit unsigned integer to a builder with existing bytes should yield expected result.",
+			builder: func() *cryptobyte.Builder {
+				var b cryptobyte.Builder
+				b.AddUint64(0xffffffffffffffff)
+				return &b
+			}(),
+			in:   0xfefcff3cfdfc,
+			want: []byte{255, 255, 255, 255, 255, 255, 255, 255, 254, 252, 255, 60, 253, 252},
+		},
+		"ExistingAddUint48AndMore": {
+			reason: "Adding a 48-bit unsigned integer to a builder with existing bytes, then adding more bytes, should yield expected result.",
+			builder: func() *cryptobyte.Builder {
+				var b cryptobyte.Builder
+				b.AddUint64(0xffffffffffffffff)
+				return &b
+			}(),
+			postAdd: func(b *cryptobyte.Builder) {
+				b.AddUint32(0xffffffff)
+			},
+			in:   0xfefcff3cfdfc,
+			want: []byte{255, 255, 255, 255, 255, 255, 255, 255, 254, 252, 255, 60, 253, 252, 255, 255, 255, 255},
+		},
+	}
+	for name, tc := range cases {
+		t.Run(name, func(t *testing.T) {
+			AddUint48(tc.builder, tc.in)
+			if tc.postAdd != nil {
+				tc.postAdd(tc.builder)
+			}
+			got := tc.builder.BytesOrPanic()
+			if !bytes.Equal(got, tc.want) {
+				t.Errorf("Bytes() = %v, want %v", got, tc.want)
+			}
+		})
+	}
+}