The "DEK-Info:" encapsulated header field identifies the message text encryption algorithm and mode, and also carries any cryptographic parameters (e.g., IVs) used for message encryption. No more than one "DEK-Info:" field occurs in a message; the field is required for all messages specified as "ENCRYPTED" in the "Proc-Type:" field.
The "DEK-Info:" field carries either one argument or two arguments separated by a comma. The first argument identifies the algorithm and mode used for message text encryption. The second argument, if present, carries any cryptographic parameters required by the algorithm and mode identified in the first argument. Appropriate message encryption algorithms, modes and identifiers and corresponding cryptographic parameters and formats are defined in RFC 1423.