[PATCH v3] daemon: encrypted communication (version 3)

These functions are responsible for handling cryptographic packets and are called by other helper functions. The decrypt_pkt() function checks the HMAC of a given packet, decrypts it, and returns it in ProtobufC format. On the other hand, encrypt_pkt() encrypts a given packet, pads it with zeroes if necessary, and returns it in struct crypto_pkt format. The do_read_packet() function reads packets from a file descriptor (fd) and updates the input buffer of the peer using information from its io_data structure.The updated code contains changes to multiple files, including the addition of a new struct called io_data in peer.h and the modification of the "pkt__field_descriptors" variable to add a new field for authentication named "auth". The peer_crypto_setup() function generates a session key used for encryption and returns an IO plan for reading a session key from the peer. Once the session keys are exchanged, the keys_exchanged() function sets up the necessary encryption keys and signs the peer's session key to prove its identity. The authenticate_pkt() function creates an authentication packet from the node id and signature passed as arguments. The peer_read_packet() function reads packets from the peer into the peer's input buffer, while peer_write_packet() writes packets to the peer. The code includes several other functions to perform various tasks such as generating a session key, wrapping/unwrapping packets, and freeing memory.In peer.c, a new function called peer_test has been added which generates an error packet for testing purposes. The destroy_peer() function has been modified to remove the peer from the state's peers list. The peer_connected_out() and peer_connected_in() functions have been modified to call the peer_crypto_setup() function instead of io_write(), passing in the peer_test() function as the callback argument. The peer_read_packet() function has been modified to store the received packet in the peer's inpkt field.In lightning.pb-c.c, a new message descriptor called authenticate has been added with two fields node_id and session_sig. The "Authenticate" struct has been declared in "lightning.pb-c.h", along with its associated methods. The "authenticate" message has been defined in "lightning.proto" file. Finally, the "Pkt" message has been updated to contain the "auth" field.