[131785] in cryptography@c2.net mail archive
Re: Decimal encryption
daemon@ATHENA.MIT.EDU (Thierry Moreau)
Wed Aug 27 16:11:45 2008
Date: Wed, 27 Aug 2008 10:42:02 -0500
From: Thierry Moreau <thierry.moreau@connotech.com>
To: =?ISO-8859-15?Q?Philipp_G=FChring?= <pg@futureware.at>
CC: cryptography@metzdowd.com
In-Reply-To: <48B56D48.3090109@futureware.at>
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?
>
The short answer is no, nobody knows a secure algorithm that would
"work" as a decimal stream cipher AND would not extend the message size
for some form of key material reference data (or salt or IV ...).
If you have room for such message-specific reference data, it should be
easy to design a decimal stream cipher for short messages.
--
- Thierry Moreau
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