[131778] in cryptography@c2.net mail archive
Re: Decimal encryption
daemon@ATHENA.MIT.EDU (Eric Rescorla)
Wed Aug 27 16:06:41 2008
Date: Wed, 27 Aug 2008 08:19:36 -0700
From: Eric Rescorla <ekr@networkresonance.com>
To: Philipp =?ISO-8859-15?Q?G=FChring?= <pg@futureware.at>
Cc: cryptography@metzdowd.com
In-Reply-To: <48B56D48.3090109@futureware.at>
At Wed, 27 Aug 2008 17:05:44 +0200,
Philipp G=FChring wrote:
>=20
> Hi,
>=20
> I am searching for symmetric encryption algorithms for decimal strings.
>=20
> Let's say we have various 40-digit decimal numbers:
> 2349823966232362361233845734628834823823
> 3250920019325023523623692235235728239462
> 0198230198519248209721383748374928601923
>=20
> 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.
>=20
> 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).
>=20
> 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 A=
ES.
> 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.
>=20
> 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?
There are a set of techniques that allow you to encrypt elements of
arbitrary sets back onto that set.=20
The original paper on this is:
John Black and Phillip Rogaway. Ciphers with arbitrary ?nite domains. In=20
CT-RSA, pages 114?130, 2002.=20
For a modern proposal to make this a NIST mode, see:
http://csrc.nist.gov/groups/ST/toolkit/BCM/documents/proposedmodes/ffsem/ff=
sem-spec.pdf
-Ekr
Full Disclosure: Terence Spies, the author of the FFSEM proposal,
works for Voltage, Voltage has a product based on this technology.
and I'm on Voltage's TAB and have done some work for them.
=20
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