Patent ID: 6956951

Claim:
An extended key preparing apparatus wherein extended keys are prepared in common key cryptosystem from a cryptographic key input, comprising: a dividing unit which divides a bit string of said cryptographic key into a plurality of bit groups, each bit group having a predetermined bit length; an intermediate data preparing unit which prepares a plurality of intermediate data groups from the bit groups by a predetermined operation with different constant for each bit group, each intermediate data group having first intermediate data; a selecting unit which selects one item of the first intermediate data for each of the intermediate data groups depending on a number of stages of extended keys to determine second intermediate data; and an extended key preparing unit which prepares the extended keys having a bit length longer than the bit string of said cryptographic key by converting irreversibly the second intermediate data, wherein said intermediate data preparing unit is provided with a nonlinear type operating unit for effecting a nonlinear type operation with respect to each bit group to prepare the intermediate data groups.