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What I need is a reversible function that transforms a long (64-bit integer) into another long number, in a way that seems "random" for a user (but actually is deterministic), so that 3 subsequent numbers are transformed into 3 numbers completely different to each other.
It is easy to do it without being reversible, but it turns out pretty difficult when it comes to this part.
Basically it's the same question as Reversible hash function?, but I need more than 2^32 distinct values.
Any ideas?
PS: I'm going to write it in Java, but the question itself is pretty generic.
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Hash functions are not reversible in general. MD5 is a 128-bit hash, and so it maps any string, no matter how long, into 128 bits. Obviously if you run all strings of length, say, 129 bits, some of them have to hash to the same value.
Anyone who knows or obtains the secret key can decrypt the ciphertext and read the original input. Hashing functions are not reversible. The output of a hashing function is a fixed-length string of characters called a hash value, digest or simply a hash.
The most commonly used method for hashing integers is called modular hashing: we choose the array size M to be prime, and, for any positive integer key k, compute the remainder when dividing k by M. This function is very easy to compute (k % M, in Java), and is effective in dispersing the keys evenly between 0 and M-1.
Encryption is a two-way function where information is scrambled in such a way that it can be unscrambled later. Hashing is a one-way function where data is mapped to a fixed-length value. Hashing is primarily used for authentication.
These are the basic requirements for a block cipher, which is usually implemented with a Feistel structure: https://en.wikipedia.org/wiki/Feistel_cipher
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You can use any 64-bit block cipher (for example, DES), and use encrypt for a "hash", and decrypt for an "reverse hash".
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