When symmetric key management is employed for a given recipient, the "Key-Info:" encapsulated header field transfers four items, separated by commas: an IK Use Indicator, a MIC Algorithm Indicator, a DEK and a MIC. The IK Use Indicator identifies the algorithm and mode in which the identified IK was used for DEK and MIC encryption for a particular recipient. The MIC Algorithm Indicator identifies the MIC computation algorithm used for a particular recipient. The DEK and MIC are symmetrically encrypted under the IK identified by a preceding "Recipient-ID-Symmetric:" field and/or prior "Originator- ID-Symmetric:" field.
Appropriate symmetric encryption algorithms, modes and identifiers, MIC computation algorithms and identifiers, and encrypted DEK and MIC formats are defined in RFC 1423.