I am looking for a Format Preserving Encryption implementation to apply to eithe

Question

I am looking for a Format Preserving Encryption implementation to apply to either a 32-bit or a 64-bit integer. It looks like AES/FFX mode is just what I need in terms of established security properties. However, it looks like it is patented, and it seems like there might be something equally solid in the public domain for the special case of 32 or 64 bit data, ideally with C source code. I have seen SkipJack or 3DES for the 64-bit case, but I would prefer longer key sizes, as these will be statically configured.

The application is to compartmentalize/anonymize information in a database. I have different kinds of information about users that is not terribly privacy invasive as long as it is kept separate from other kinds of information that is also not invasive on it's own. However, there may be rare cases when I need to connect this data together. So, I was thinking each database would identify users with its own ID, and the key to connect them could be stored separately. The actual application is a little more complicated, but that's the general idea.

Explanation / Answer

We have designed FNR encryption scheme for this very purpose of preserving lengths and formats of small input fields ( 32<=x<128 bits). It uses Naor and Reingold's PWIP extensions to classic fiestel networks. They have proved it to be more secure than classic Feistels. We have made it practical by using invertible matrices rather than working in GF(2n) which is difficult for arbitrary lengths. Our key size is 128 assuming we internally use AES-128 , supporting 256 key sizes is trivial by changing internal code to AES-256 if needed.

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