Allow encrypted and unencrypted messages on the same link

[podhrmic]

Use cases

1. we want to have encrypted communication, as described in Messages Format but we want to be able to process unencrypted "management" messages (such as request to renegotiate encryption keys or some routing info). These unencrypted messages are validated in a different way (context based, signed, etc)
2. in multi-uav case, we want to be able to communicate with unencrypted nodes (for interoperability reasons).

In both cases, we need to somehow distinguish between encrypted and unencrypted messages (because the length and decoding is different).

Options

We have these option:

A. Different STX byte for plaintext and encrypted messages

Pros:

• simple
• no additional bytes

Cons:

• increasing likelihood of incorrectly decoded messages

B. Adding a byte with crypto/plaintext flag

NOTE: this byte would still be authenticated so it cannot be tampered with! Pros:

• still simple
• can flag different encryption methods (theoretically can support multiple encrypted links)

Cons:

• additional overhead
• not backwards compatible:-(

C. Using an existing byte

Note: not really an option, because only Pprzlink 2.0 has extra CLASS/COMPONENT byte, but if we use a bit from this byte then the whole byte would have to be unencrypted (perhaps revealing too much about the message).

Summary

I am in favor of solution A as it is backwards compatible and doesn't increase overhead. Thoughts? @gautierhattenberger @flixr

[podhrmic]

Will be handled in secure transport, not relevant for pprzlink anymore