A Recipient-ID encapsulated header field identifies a recipient and provides the recipient's IK identification component. One Recipient-ID field is included for each of a message's named recipients. Section 5.2, Interchange Keys, discusses the semantics of the subfields and specifies the alphabet from which they are chosen. Recipient-ID subfields are delimited by the comma character (","), optionally followed by whitespace.
For the symmetric case, all "Recipient-ID-Symmetric:" fields are interpreted in the context of the most recent preceding "Originator- ID-Symmetric:" field. It is illegal for a "Recipient-ID-Symmetric:" field to occur in a header before the occurrence of a corresponding "Originator-ID-Symmetric:" field. For the asymmetric case, "Recipient-ID-Asymmetric:" fields are logically independent of a message's "Originator-ID-Asymmetric:" and "Originator-Certificate:" fields. "Recipient-ID-Asymmetric:" fields, and their associated "Key-Info:" fields, are included following a header's originator- oriented fields.