User talk:82.36.100.133
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[edit] Mistake in DES key schedule image
Hello Matt, I appreciate the huge amount of work you do on crypto articles, and elsewhere. I enjoy reading your work very much. I noticed what I think is a small mistake in an image you produced, namely the DES key schedule image. DES keys are 56 bits, not 64 bits as it says on that image. (Block sizes are 64b, perhaps that's what you confused it with.) Perhaps you still have the source of the image and can change it? Mark
- Hi Mark, thanks for your kind comments, but, more importantly, for double-checking the work here (Linus's law, and all that). The size of the DES key is normally quoted as 56 bits. That's the most useful measure, because it's the effective key size; however, the algorithm for the key-schedule actually takes a 64-bit input. For some reason still not satisfactorily explained, the DES key-schedule drops every 8th bit from the 64-bit key in the first permutation (PC-1) (have a close look at the diagram for DES_supplementary_material#Permuted_choice_1_.28PC-1.29; input values 8, 16, 24...etc are simply dropped). The remaining 56-bits are processed by the key schedule to produce sixteen 48-bit sub-keys. The designers claimed, rather weakly some think, that these bits were used for parity check purposes. Skeptics have argued that it was a sneaky way to lower the key-size a bit so that the NSA would have an easier time brute-forcing it.
- Having said that, if you were confused by the diagram, that suggests the diagram would be better off if it noted the size of the various values as they move through the algorithm. The source is about somewhere, I might be able to fix it soon. — Matt Crypto 18:30, 5 October 2005 (UTC)
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