Name: envelope
Type: request
Purpose: >
  Process message(s) enclosed in an 'envelope' message.
Response: envelope
Inherits: message
Elements:
  Content: [ required: true, type: string, min_length: 3, max_length: 32768 ]
Notes: >
  The 'Content' element contains a string representing a single message 
  or multiple messages separated by '---' on a line of its own.

  If the request's 'Content' element is encrypted the response's
  'Content' element will be encrypted as well.