Name: key-submission
Type: request
Purpose: >
  Assign a public key to a particular identity
Response: participant-update-confirmation
Inherits: message
Elements:
  Signature: { required: true, trust_embedded_key: true }
  From: { required: true }
  Public-Key: { required: true, type: string, min_length: 32, max_length: 128 }
  Credential: { required: ['Credential~'], type: string, min_length: 3, max_length: 96 }
  Credential~: { required: ['Credential'], type: string, min_length: 3, max_length: 96 }
  Event-Id: { type: identifier }
Notes: >
  The From element must contain the identity for which the key is to be
  submitted. The Signature must be generated with the private key which
  belongs to the Public-Key. The same Credential may be used only once.
  When using an event-related (pseudo) identity the Date element must be
  omitted, otherwise the Date element is mandatory.
Rejection-Codes:
  21: Date is mandatory when using open identity
  22: Date must be omitted when using pseudo identity
  31: Invalid sender domain
  32: Wrong credential or key submission not permitted
  41: Unknown participant