[17446] in cryptography@c2.net mail archive
Re: encrypted tapes (was Re: Papers about "Algorithm hiding" ?)
daemon@ATHENA.MIT.EDU (Charles M. Hannum)
Thu Jun 9 18:31:20 2005
X-Original-To: cryptography@metzdowd.com
X-Original-To: cryptography@metzdowd.com
From: "Charles M. Hannum" <root@ihack.net>
To: lists@notatla.org.uk
Date: Thu, 9 Jun 2005 17:44:49 +0000
Cc: astiglic@okiok.com, k.buchanan@kastenchase.com,
cryptography@metzdowd.com, adam@homeport.org
In-Reply-To: <200506091737.22934.root@ihack.net>
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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