tka: Use strict decoding settings, implement Unserialize()

Signed-off-by: Tom DNetto <tom@tailscale.com>

tka/aum.go

@@ -212,6 +212,10 @@ func (a *AUM) StaticValidate() error {
 }
 
 // Serialize returns the given AUM in a serialized format.
+//
+// We would implement encoding.BinaryMarshaler, except that would
+// unfortunately get called by the cbor marshaller resulting in infinite
+// recursion.
 func (a *AUM) Serialize() []byte {
 	// Why CBOR and not something like JSON?
 	//
@@ -243,6 +247,16 @@ func (a *AUM) Serialize() []byte {
 	return out.Bytes()
 }
 
+// Unserialize decodes bytes representing a marshaled AUM.
+//
+// We would implement encoding.BinaryUnmarshaler, except that would
+// unfortunately get called by the cbor unmarshaller resulting in infinite
+// recursion.
+func (a *AUM) Unserialize(data []byte) error {
+	dec, _ := cborDecOpts.DecMode()
+	return dec.Unmarshal(data, a)
+}
+
 // Hash returns a cryptographic digest of all AUM contents.
 func (a *AUM) Hash() AUMHash {
 	return blake2s.Sum256(a.Serialize())