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Is there a way to make a 16-bit checksum of data with variable length, probably from 25 bytes to 8 KB?

The solution should be easy to implement, or should already be implemented in C++, and should be quick to evaluate.

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This question should probably be in stackoverflow, or math? Anyhow, check out CRC16 (en.wikipedia.org/wiki/Cyclic_redundancy_check) – Dog eat cat world Jun 19 '12 at 8:23
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Hi bob - the way this, and your associated question, are written don't seem very on-topic here, where we aim to look for the most appropriate security solution/process etc for a security professional. Implementation of a piece of code should probably be in SO. Can you clarify the question - we can then see if it should be migrated in order to get answers useful for you. – Rory Alsop♦ Jun 19 '12 at 10:27
16 bits can easily be broken by brute force. What are your security requirements? Are you really hoping to protect against malicious modification and not just low-probability random corruption? – Gilles Jun 19 '12 at 15:47

closed as off topic by Gilles, Rook, Rory Alsop♦ Jun 19 '12 at 19:08

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Anything that is only 16 bits in size isn't going to be strong enough for any security application. Therefore, a non-cryptogrpahic hash function, such as CRC-16, should be fine.

If you care about security, then the minimum would be a 128-bit hash function. MD5 is 128 bits, but it's very broken, SHA-1 is 160 bits and is "safe". SHA-256 is a great choice as a security checksum.

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No it's not for security in the sense that it's not used as a signature or something like that, but I asked it here because I assumed you would know that better than developers or mathematicians. I don't require the value from which the hash came from to be hard to find. It should be of length 16b. Is CRC-16 adapted for data from 25 to 8K bytes ? Does it uniformly spread the hashed values in the 16bits, so that no value is more likely to be returned than another ? Where can I find its c++ implementation ? – bob Jun 19 '12 at 8:38
@Rook - I would say that while SHA-1 is less broken then MD5 it still has its problems. There are better solutions SHA-256 for instance you should use. – Ramhound Jun 19 '12 at 11:07
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@bob - You are getting into a Stackoverflow question if your asking for an actual implementation in C++. – Ramhound Jun 19 '12 at 11:07

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