PKIMessage Protection
There may be cases in which the PKIProtection BIT STRING is deliberately not used to protect a message (i.e., this OPTIONAL field is omitted) because other protection, external to PKIX, will instead be applied. Such a choice is explicitly allowed in this specification. Examples of such external protection include PKCS #7 [PKCS7] and Security Multiparts [RFC1847] encapsulation of the PKIMessage. It is noted, however, that many such external mechanisms require that the end-entity already possesses a public-key certificate, and/or a unique Distinguished Name, and/or other such infrastructure-related information. Thus, they may not be appropriate for initial registration, key-recovery, or any other process with "boot-strapping" characteristics. For those cases it may be necessary that the PKIProtection parameter be used. In the future, if/when external mechanisms are modified to accommodate boot-strapping scenarios, the use of the PKIProtection parameter may become rare or non-existent.