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

One potential solution could be built on Feistel ciphers. This constructions allows to build a pseudorandom permutation based on a pseudorandom functions. E.g. the pseudorandom functions could be constructed from an appropriate block cipher by truncating the result to a 6 digit numbers.

This construction has been analyzed in the following paper M. Luby and C. Rackoff, "How to construct pseudorandom permutations from pseudorandom functions" SIAM Journal on Computing, Vol.17, No.2, pp.373--386, 1988

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A concrete proposal is the Feistel Finite Set Encryption Mode, which has been submitted to NIST for potential inclusion into an upcoming standard. This proposal also addresses the problem of encrypting ranges that are not a power of 2.