On Wed, Aug 27, 2008 at 11:05 AM, Philipp Gühring <[EMAIL PROTECTED]> wrote: > I am searching for symmetric encryption algorithms for decimal strings.

> Since the 132,877 bits is similar to 128 bit encryption (like eg. AES), > I would like to use an algorithm with a somewhat comparable strength to AES. > But the problem is that I have 132,877 bits, not 128 bits. And I can't > cut it off or enhance it, since the result has to be a 40 digit decimal > number again. I believe the most straightforward thing to do is to build a balanced 4-round Feistel cipher [ http://en.wikipedia.org/wiki/Feistel_cipher ] that uses AES as its mixing function, but which operates within a field of 10^20; you can then encrypt a value within F_10^40 as a single block operation (ECB mode), taking 4 AES operations and some other math do to so. In this usage, each 20-digit side of the cipher would be expressed as a bit string with ~66 bits, zero-padded to make a 128-bit block. You should also use the round number in the input; you can put it in the top 2 bits of the block. This block would then be encrypted with AES, resulting in a 128-bit output block. You would then reduce this 128-bit value modulo 10^20 to give you a 20-digit output value from your f() function; that value can be added, modulo 10^20, into the other 20-digit side of the network (or subtracted on decryption). A couple of notes: - I believe 4 rounds should be secure, but someone else on this list should validate this. - As simply described here, this is unbalanced, because 2^128 is not an even multiple of 10^20, so some 20-digit output values of f() are more likely than others. To avoid this problem, if the 128-bit result of the AES encryption is less than 2^128 % 10^20 (63374607431768211456), reencrypt the 128-bit output block with AES again and iterate. This will happen approximately one time in 5e18, so it's not clear that it's a real vulnerability; it's certainly not a performance problem. Good luck; please feel free to ask if any of this isn't clear. - Tim --------------------------------------------------------------------- The Cryptography Mailing List Unsubscribe by sending "unsubscribe cryptography" to [EMAIL PROTECTED]