# Decimal encryption

Thierry Moreau thierry.moreau at connotech.com
Wed Aug 27 11:42:02 EDT 2008

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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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