Name: access-request
Type: request
Purpose: >
  Request credentials for accessing server via SSH
Response: access-grant
Inherits: message
Elements:
  Signature: { required: true }
  From: { required: true }
  Date: { required: true, min_value: now -1 minute, max_value: now + 1 minute }
Notes: >
  SSH access may be requested only once per day. After receiving the grant
  it can take up to one minute, until the SSH access is actually working.
  It will stop working at midnight (server timezone).
Rejection-Codes:
  22: Invalid date value
  51: Access requests are allowed only once a day