Server sends type 15 at the same time as a type 14
Type: 15
public key size:  4 bytes
public key:  key size bytes

Client sends type 16, encrypted session key
Type:  16
Nonce:  4 bytes
Tag:  4 bytes
Encrypted key:  16 bytes

After these, regular data is encrypted before sending, decrypted after

Let's use counter mode to send other lurk messages, so we can decrypt description lengths before reading descriptions