Re: Decimal encryption

[Thomas Baignères]

Hello,

Actually, block ciphers encrypting blocks of *decimal* numbers exist:

- TOY100 [1] encrypts blocks of 32 decimal digits
- DEAN18 [2] encrypts blocks of 18 decimal digits
- DEAN27 [3] encrypts blocks of 27 decimal digits

TOY100 is (almost) broken by the generalized linear cryptanalysis  
described in [2]. Both versions of DEAN are based on a substitution  
permutation network very close to that of the AES and are provably  
secure against linear cryptanalysis. These ciphers are only "toy"  
ciphers. Consequently, there is no official implementation (no test- 
vector, etc.).

Here are the references:
[1] Granboulan, Levieil, Piret: Pseudorandom Permutation Families over  
Abelian Groups. FSE 2006: 57-77
[2] Baignères, Stern, Vaudenay: Linear Cryptanalysis of Non Binary  
Ciphers. Selected Areas in Cryptography 2007: 184-211 (available here: http://lasecwww.epfl.ch/~tbaigner/papers/groupLC.pdf 
 )
[3] Baignères (PhD Thesis): Quantitative Security of Block Ciphers:  
Designs and Security Tools (to be published)

I hope this helps. I'm of course available for any question regarding  
DEANxx.

Best regards,
Thomas Baignères
--
http://lasecwww.epfl.ch/~tbaigner

On Aug 27, 2008, at 5:05 PM, Philipp Gühring wrote:

Hi,

I am searching for symmetric encryption algorithms for decimal  
strings.

Let's say we have various 40-digit decimal numbers:
2349823966232362361233845734628834823823
3250920019325023523623692235235728239462
0198230198519248209721383748374928601923

As far as I calculated, a decimal has the equivalent of about 3,3219
bits, so with 40 digits, we have about 132,877 bits.

Now I would like to encrypt those numbers in a way that the result  
is a
decimal number again (that's one of the basic rules of symmetric
encryption algorithms as far as I remember).

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.

Does anyone know a an algorithm that has reasonable strength and is  
able
to operate on non-binary data? Preferrably on any chosen number-base?

Best regards,
Philipp Gühring

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