Implement certificate compression (#95)

Certificate compression is defined in RFC 8879:
https://datatracker.ietf.org/doc/html/rfc8879

This implementation is client-side only, for server certificates.

- Fixes #104.

u_handshake_messages.go

@@ -0,0 +1,49 @@
+package tls
+
+import (
+	"golang.org/x/crypto/cryptobyte"
+)
+
+// Only implemented client-side, for server certificates.
+// Alternate certificate message formats (https://datatracker.ietf.org/doc/html/rfc7250) are not
+// supported.
+// https://datatracker.ietf.org/doc/html/rfc8879
+type compressedCertificateMsg struct {
+	raw []byte
+
+	algorithm                    uint16
+	uncompressedLength           uint32 // uint24
+	compressedCertificateMessage []byte
+}
+
+func (m *compressedCertificateMsg) marshal() []byte {
+	if m.raw != nil {
+		return m.raw
+	}
+
+	var b cryptobyte.Builder
+	b.AddUint8(typeCompressedCertificate)
+	b.AddUint24LengthPrefixed(func(b *cryptobyte.Builder) {
+		b.AddUint16(m.algorithm)
+		b.AddUint24(m.uncompressedLength)
+		b.AddUint24LengthPrefixed(func(b *cryptobyte.Builder) {
+			b.AddBytes(m.compressedCertificateMessage)
+		})
+	})
+
+	m.raw = b.BytesOrPanic()
+	return m.raw
+}
+
+func (m *compressedCertificateMsg) unmarshal(data []byte) bool {
+	*m = compressedCertificateMsg{raw: data}
+	s := cryptobyte.String(data)
+
+	if !s.Skip(4) || // message type and uint24 length field
+		!s.ReadUint16(&m.algorithm) ||
+		!s.ReadUint24(&m.uncompressedLength) ||
+		!readUint24LengthPrefixed(&s, &m.compressedCertificateMessage) {
+		return false
+	}
+	return true
+}