How can I encrypt/decrypt 12-digit decimal numbers to other ones, using a password and Java?

Question

I have already read Using Java to encrypt integers and Encrypting with DES Using a Pass Phrase.

All I need is a simple Encrypter which transforms a 12 digit number t

Answer 1

Rethinking the problem I came up with the following. Basicly you need a symmetric cipher to get a one-to-one mapping. And noting that 10^12 is almost 2^40 (2^39.863) it seems natural to convert your 12 digit number into a 40 bit integer and feed this number into a block cipher with a block length of 40 bits. A good choice might be Blowfish supporting block lengths from 32 to 448 bits in steps of 8 bits - so 40 bits is supported.

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UPDATE

As Accipitridae pointed out, Blowfish has variable key size but fixed block size hence it is no option. A bit more searching through the web seems to indicate, that there are little or no ciphers with block sizes of 40 bits or less rendering this idea void. A leave the remaining part of the answer - maybe one can find a suitable cipher.

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The remaining problem is that the Blowfish might return a number up to 1,099,511,627,775 with 13 digits and that the returned number might contain leading zeros, but I believe that this can be solved in a second step. My first thought was applying something like a Burrows-Wheeler transform to the string representation of the number in order to get at least one zero to the front of the number eleminating the 13th digit and then modify all remaining digits (for example 0 <-> 9, 1 <-> 8, 2 <-> 7, ...) to turn additional leading zeros into other digits.
After a few minutes I regonized that this will not work - the input has size 2^40 while the output is only of size 2^39.863. A solution would be to use a 39 bits block cipher and restrict the input to numbers up to 549,755,813,887. I don't know if there is a cipher that can deal with a block length of 39 bits, but this paper on Elastic Block Ciphers describes how to construct a block cipher that can deal with every block size from n up to 2n given a block cipher that can handle block size n. In consequence it is possible to construct a 39 bit block cipher from 32 bit Blowfish.