one of the documented procedures in this installation of the ACS
bulkmail_decode_key_code   key_code
What it does:
Given a key code, returns a list of the user_id and bulkmail_id. Returns an empty list of there was an error.
Defined in: /web/philip/tcl/bulkmail-utils.tcl

Source code:

    set code_pieces [split $key_code "C"]

    if { [llength $code_pieces] == 0 } {
	return [list]

    # First piece is the bulkmail_idAuser_idBns_time
    set user_content [lindex $code_pieces 0]

    # Second piece is the simple checksum
    set checksum [lindex $code_pieces 1]

    # Compare the checksum to the checksum of the user_content.  Be careful
    # to lower the string case.
    if { [bulkmail_simple_checksum [string tolower $user_content]] != $checksum } {
	return [list]

    if { ![regexp -nocase {([0-9]+)A([0-9]+)B([0-9]+)} $user_content match bulkmail_id user_id time] } {
	return [list]

    return "$bulkmail_id $user_id $time"