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A simple way to think of the S-boxes of DES when it comes to a hardware implementation is: 8 independent ROM ICs, each with 6 address input lines and 4 data out lines. In an educational physical implementation with retro standard ICs, two such ROMs fit a 4 kiB EPROM (a 2732). Some actual software implementations of DES do group S-boxes two by two in order to ...


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A DES key is, by definition, a (pseudo)randomly chosen sequence of 8 × 7 = 56 key bits, plus 8 parity bits,* for a total of 64 bits. If what you have isn't 64 bits long (with the appropriate parity bits), then it's not a DES key, but something else (e.g. a passphrase). To turn that "something else" into a DES key, you need a key derivation function ...


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As for "how to build the substitution as hardware", it should be easy if you know any of the hardware description language (eg. VHDL or Verilog). Simply write the Sboxes of DES, then the synthesis software will handle the rest. You can also "synthesis" by hand, although that may take a lot of effect. Still, I'm not sure if this is what you are looking for. ...



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