feat: add the ability to encode a byte array

We can already _decode_ a raw byte array, but there wasn't a way to
encode one.

utils.go

@@ -543,6 +543,18 @@ func ReadByteArray(br io.Reader, maxlen uint64) ([]byte, error) {
 	return buf, nil
 }
 
+// WriteByteArray encodes a byte array as a cbor byte-string.
+func WriteByteArray(bw io.Writer, bytes []byte) error {
+	writer := NewCborWriter(bw)
+	if err := writer.WriteMajorTypeHeader(MajByteString, uint64(len(bytes))); err != nil {
+		return err
+	}
+	if _, err := writer.Write(bytes); err != nil {
+		return err
+	}
+	return nil
+}
+
 var (
 	CborBoolFalse = []byte{0xf4}
 	CborBoolTrue  = []byte{0xf5}

utils_test.go

@@ -83,6 +83,36 @@ func TestDeferredMaxLengthSingle(t *testing.T) {
 	}
 }
 
+func TestByteArray(t *testing.T) {
+	var buf bytes.Buffer
+	input := []byte("foobar")
+	if err := WriteByteArray(&buf, input); err != nil {
+		t.Fatal("failed to encode byte array")
+	}
+
+	// Exact length should work.
+	encoded := bytes.NewReader(buf.Bytes())
+	if out, err := ReadByteArray(encoded, uint64(len(input))); err != nil {
+		t.Fatal("failed to decode byte array")
+	} else if string(out) != string(input) {
+		t.Fatal("bytes failed to round-trip")
+	}
+
+	// Large length should work.
+	encoded.Seek(0, io.SeekStart)
+	if out, err := ReadByteArray(encoded, 100); err != nil {
+		t.Fatal("failed to decode byte array")
+	} else if string(out) != string(input) {
+		t.Fatal("bytes failed to round-trip")
+	}
+
+	// Short length should not work.
+	encoded.Seek(0, io.SeekStart)
+	if _, err := ReadByteArray(encoded, uint64(len(input)-1)); err == nil {
+		t.Fatal("should have refused to read too many bytes")
+	}
+}
+
 // TestReadEOFSemantics checks that our helper functions follow this rule when
 // dealing with EOF:
 // If the reader can't read a single byte because of EOF, it should return err == io.EOF.