encrypted tapes (was Re: Papers about "Algorithm hiding" ?)

[Charles M. Hannum]

On Thursday 09 June 2005 17:37, Charles M. Hannum wrote:
> If we assume that the last 4 digits have been exposed somewhere -- and they
> usually are -- then this gives you at most 38 bits -- i.e. 2^38 hashes to
> test -- to search (even a couple less if you know a priori which *brand* of
> card it is).  How long do you suppose this would take?

On reconsideration, given the presence of the check digit, I think you have at 
most 2^34 tests (or 2^32 if you know the brand of card).  And this assumes 
there aren't additional limitations on the card numbering scheme, which there 
always are.

I guess you could use a keyed hash.  Remember, though, you can't use random 
padding if this is going to be searchable with a database index, so the 
amount of entropy you're putting in is pretty limited.


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