You can just trim the hash string to the desired length.
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Hashing smaller than 128 bits, maybe 96?
We need to hash ASCII text and store the hash in a database field with a length of 20. It needs to be encoded using printable characters (and XML encoded as well).
I can use the .NET built-in MD5CryptoProvider to create a 128-bit hash, which when base64 encoded, produces a 24 byte string.
I understand that a smaller hash could produce more false positive similar hashes on different text, but the criticality of uniqueness is pretty low.
I still need it in 20 bytes. Anyone familiar with a 96-bit hashing algorithm? Any other ideas on how to encode differently to accomplish this?
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Answers
While I understand that leaving off 1 byte will *effectively* mimic a 96-bit hash, there is some concern as to which bytes are more significant from the MD5 calculation.
I did find that I could implement an Ascii85 encoding that yields a 4:5 bloat instead of the 2:3 bloat that base64 produces. This can also be implemented such that the 5 characters that would need to be XML encoded would not be used.
With that, 16 bytes could be encoded into 20 bytes and be XML safe.
Yea! Problem solved. Now, where did I put that Ascii85 source code?
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by: gregcmcsePosted on 2008-07-03 at 11:10:07ID: 21927827
I'm not aware of any, but I'm curious as to why you don't alter the database to support a longer string? That's not usually a big deal.