encryption

Hijpo

Been lookin at a few examples and cant get my head around what the key has an effec on. In terms of block sizes and keys, i know its outdated but taking DES as an example. It uses 64 bit block and 56bit key, does this mean that it takes 64 bits but only encrypts 56bits of the block? Im sure im getting it wrong.

DublinDilbert

64 bits in and the output is 64 bits, except re-ordered and substituted. The key enters the key schedule and it's bits are shifted / swapped to proved a different key for each round of encryption.

The key was originally 64 bits, but reduced this to 56 bits. The reason given was that a 64 bit number would be used to store the key and the upper 8 bits would be for error checking prior to running, eg to check it's a valid key. Most people don't believe this, they feel it was to deliberately to weaken the algorithm so powerful computers at the time could brut force it.

Hijpo

DublinDilbert wrote: »

64 bits in and the output is 64 bits, except re-ordered and substituted. The key enters the key schedule and it's bits are shifted / swapped to proved a different key for each round of encryption.

The key was originally 64 bits, but reduced this to 56 bits. The reason given was that a 64 bit number would be used to store the key and the upper 8 bits would be for error checking prior to running, eg to check it's a valid key. Most people don't believe this, they feel it was to deliberately to weaken the algorithm so powerful computers at the time could brut force it.

Thanks for that, so does that mean if say AES uses a 128 block size and a 192 bit key then they key over laps itself after 128 bits?

DublinDilbert

Hijpo wrote: »

Thanks for that, so does that mean if say AES uses a 128 block size and a 192 bit key then they key over laps itself after 128 bits?

I think what your trying to ask is how the key relates / operates on the data being passed through the algorithm.

For DES you have to look at the key schedule that takes the original key and shifts (re-orders it) for each round of encryption. So for each round of encryption a modified version of the key is used. DES uses 16 rounds of encryption from memory. Each round key that is generated is 48 bits long and is different to the previous and next round key.

For each round the input data (64 bits) is operated on using a Fiestel structure. This splits the 64 bits of data into two halves, modifies one half then recombines with the unmodified half. The modification that happens is an expansion, eg 32 bits -> 48 bits, then it gets XOR'ed with the 48 bit round key. The 48 bit result is passed through the "S" boxes to give a 32bit number. This 32 bit number is then re-combined with the unmodified 32 bit number and the process repeats.

The wiki article isn't too bad actually:-
http://en.wikipedia.org/wiki/Data_Encryption_Standard

Hijpo

yeah thats pretty much it, i couldnt understand why you would have a 128 bit block with a key twice the size, couldnt figure out how it was used. thanks for the link too.