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Re: Simple alphanumeric "encryption"?

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On Mon, 17 Oct 2011, Qu0ll wrote:

> I need to be able to encrypt/code an arbitrary string of up to about 50 
> alphanumeric characters into a string that also contains only alphanumeric 
> characters.  All of the encryption algorithms I have looked result in strings 
> with non-ASCII characters in them (when the resulting bytes are turned into a 
> string) which is not suitable.
>
> Is there a simple way to do this?  It must be able to successfully be 
> decrypted as well but doesn't need to be very sophisticated or extremely 
> secure.

Oddly, i have a paper called "Ciphers with Arbitrary Finite Domains" 
sitting in my reading queue right not.

You have at least two basic routes of attack here.

First, recognise that alphanumerism is just an encoding of a general bit 
string. Decode the alphanumeric string into a bit string (by taking it as 
a base-36 or base-62 number, or whatever), encrypt that, then re-encode 
it. BigInteger has a constructor which takes a string and a radix, and a 
toString method which takes a radix. So:

String s = "1sxjxyr5owpxwzmax6pyv1wgjpfuc4iadgrzhjpcameipq5sk";
BigInteger i = new BigInteger(s, 36);
i = i.multiply(BigInteger.valueOf(2)); // this is a very poor kind of encryption
System.out.println(i.toString(36));

BigInteger can also be converted to and from a byte[], which you can 
subject to proper encryption. You will need to be a bit careful, because 
conversion to an alphanumeric string will remove any leading zeroes, so 
you may need to pad. Also, the numbers may be negative, in which case the 
alphanumeric strings will have a leading minus sign. You might prefer to 
write your own conversion between bytes and digits, to avoid these 
problems.

Note that using a proper cipher involves generating an initialisation 
vector (IV) for each message you encrypt, which you will then need to send 
along with the ciphertext. That's going to be slightly annoying, since the 
alphanumerically encoded IV is likely to be just as long as your message.

Second, come up with a cipher that works directly on alphanumeric values, 
rather than bit strings, and apply that to your string. I don't think this 
is actually a terribly good idea, so i won't elaborate on it.

tom

-- 
Per Dementia ad Astra

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Thread

Simple alphanumeric "encryption"? "Qu0ll" <Qu0llSixFour@gmail.com> - 2011-10-17 18:16 +1100
  Re: Simple alphanumeric "encryption"? Arne Vajhøj <arne@vajhoej.dk> - 2011-10-17 08:13 -0400
    Re: Simple alphanumeric "encryption"? Travers Naran <tnaran@gmail.com> - 2011-10-18 06:47 -0700
      Re: Simple alphanumeric "encryption"? David Segall <david@address.invalid> - 2011-10-20 00:36 +1100
  Re: Simple alphanumeric "encryption"? Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-10-17 12:27 +0000
  Re: Simple alphanumeric "encryption"? Tom Anderson <twic@urchin.earth.li> - 2011-10-17 16:45 +0100
    Re: Simple alphanumeric "encryption"? "Qu0ll" <Qu0llSixFour@gmail.com> - 2011-10-18 14:18 +1100
      Re: Simple alphanumeric "encryption"? "Qu0ll" <Qu0llSixFour@gmail.com> - 2011-10-18 14:25 +1100
      Re: Simple alphanumeric "encryption"? Lew <lewbloch@gmail.com> - 2011-10-18 08:43 -0700
      Re: Simple alphanumeric "encryption"? Tom Anderson <twic@urchin.earth.li> - 2011-10-20 20:57 +0100
  Re: Simple alphanumeric "encryption"? B1ll Gat3s <wm.g4t3s@m1cr0s0f7.c0m> - 2011-10-17 21:33 -0400
  Re: Simple alphanumeric "encryption"? Roedy Green <see_website@mindprod.com.invalid> - 2011-10-20 13:32 -0700
  Re: Simple alphanumeric "encryption"? Roedy Green <see_website@mindprod.com.invalid> - 2011-10-20 13:36 -0700

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