Patent ID: 7796755

Claim:
A digital apparatus for locating a Data Encryption Standard (DES) key value corresponding to a Program Identification (PID) contained at a variable location that has only part of a packet header consisting of a predetermined number of input bits, said apparatus comprising: (a) a table storing circuit that is configured to store a table containing a DES key value and, for each DES key value, a corresponding packet header comprising said predetermined number of bits with a PID contained at a variable location in the packet header and with zero values elsewhere; (b) a mask value storing circuit formed unitary with the table storing circuit to store mask values corresponding to respective DES key values, each mask value comprising the same predetermined number of bits as said packet headers with ones contained at said variable location and with zero values elsewhere; (c) an input for receiving an input packet header comprising said predetermined number of bits; (d) a combining circuit coupled to said mask value storing circuit and said input to combine said input packet header and one of said mask values and configured to provide a combined value consisting of a value held in said input packet header at said variable location and zeros elsewhere; (e) a read circuit coupled to the table storing circuit to read DES key values from the table storing circuit; and (f) a comparing circuit coupled to said table storing circuit and said combining circuit to compare said combined value and one of said packet headers stored in said table storing circuit and, (i) when they are not equal, utilizing a register circuit to repeat the operations of said combining circuit and said comparing circuit for a next packet header and corresponding mask and DES key value stored in said table storing circuit, and (ii) when they are equal, enabling the read circuit to read said corresponding DES key value from said table storing circuit and providing it as an output value.