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I have a 128bit ID that I want to perform a one way hash on, but I don't want to ever get the same digest for an input message. Does anyone know if sha-1, or an alternative, is guaranteed not to produce collisions for the set of messages less than its output digest size? This is at least theoretically possible...
I also considered using RSA, and discarding the private key to give me a one-way encrypt, but I need to store the result in a 32 char DB field, and the encryption schemes available to me don't produce anything small enough.
Any suggestions of another way of producing a deterministic, non-reversable and collision free transform of the original value are welcome.
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“We note that classical collisions and chosen-prefix collisions do not threaten all usages of SHA-1." There are several potential scenarios in which the new collision could be implemented in an attack, the most likely of which is someone impersonating another user by creating an identical PGP key.
The attacks can find collisions in the full version of SHA-1, requiring fewer than 269 operations. (A brute-force search would require 280 operations.)
SHA-1 or Secure Hash Algorithm 1 is a cryptographic hash function which takes an input and produces a 160-bit (20-byte) hash value.
The Wikipedia page gives an estimate of the likelihood of a collision. If you run the numbers, you'll see that all harddisks ever produced on Earth can't hold enough 1MB files to get a likelihood of a collision of even 0.01% for SHA-256. Basically, you can simply ignore the possibility.
Cryptographic hashes give a very good approximation of a random number for a given input. So how many random hashes do you need in a room until you get the same 160 bits? It about the square root (disclaimer: I am not a statistician). So you should expect to see clashes at around 80-bits.
I guess practicalities mean you should know when cosmic rays will be a bigger problem than collisions.
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