Clients can retrieve ProtocolMessages or EncryptedProtocolMessage awaiting them through the HTTP endpoint.
A HTTP GET request on a TLS-protected HTTP address MUST set the Accept header to the requested content type (multipart/mixed). The X-Identity header MUST be set to the hex-encoded DER-formatted public key of the requestor. The X-Signature header MUST be set to the hex-encoded DER-encoded signature (described below). The Date header MUST be set according to RFC-2616 14.18.