The second argument, if
present, carries any cryptographic parameters required by the
algorithm and mode identified in the first argument.
Jeff-
Note that the specific parameters used for a specific message
encryption (i.e., DEK) algorithm is an algorithm-specific matter,
outside the scope of RFC 1421. For simplicity, John Linn and I devised
the text in RFC 1421 to define a single field carrying any number of
parameters needed, as defined by a specific message encryption
algorithm.
For purposes of using DES CBC, RFC 1423 defines the parameter field to
carry a single parameter, namely the IV, as an ASCII hex value.
For purposes of using alternative algorithms, the parameter field can
be defined to carry any number of suitably encoded parameters (e.g., a
base-64 encoded ASN.1 sequence, a set of comma or colon-delimited
ASCII hex values, or anything else you want to define).
-DB