Name: ping
Type: request
Purpose: >
  Test availability and operativeness of a server.
Response: echo
Inherits: message
Elements:
  Decryption-Request: { type: string, max_length: 256, encryption_required: true }
  Encryption-Request: { type: string, max_length: 256 }
Notes: >
  The 'ping' message type can be used for testing a server
  and the communication channel between server and client,
  as well as for retrieving the server's public key.

  If the 'ping' message is signed, the server will try to
  verify the signature using the sender ID ('From' field) or 
  the public key provided in the message ('Public-Key' element).
  The result of the verification will be included in the
  'Verification-Result' field of the echo.

  If the 'ping' message contains an element named 'Decryption-Request~',
  the echo will contain an element named 'Decryption-Result' with the
  decrypted value.

  If the 'ping' message contains an element named 'Encryption-Request',
  the echo will contain an element named 'Encryption-Result' with the
  original value encrypted.