I am writing a string of about 120 characters to a 2D barcode. Along with other text, the string contains a unique ticket number. I want to ensure that someone doesn’t generate counterfeit tickets by reading the 2D barcode and generation their own barcoded tickets.
I would like to hash the string and append the hash value to what gets embedded in the barcode. That way I can compare the two on reading and see if the data had been tampered with. I have seen several hash function that return 64 bytes and up but the more characters you embed in a 2D barcode the bigger the barcode image becomes. I would like an algorithm that returns a fairly small value. It would also be nice if I could provide the function my own key. Collision is not that big of a deal. This isn’t any kind of national security application.
Any suggestions?
Use any standard hash function. Take the 120-character string; append your own secret value; feed it into SHA-1 or MD5 or whatever hash function you have handy or feel like implementing; then just take the first however-many bits you want and use that as your value. (If you need ASCII characters, then I suggest that you take groups of 6 bits and use a base-64 encoding.)
If the hash you’re using is any good (as, e.g., MD5 and SHA-1 are; MD5 shouldn’t be used for serious cryptographic algorithms these days but it sounds like it’s good enough for your needs) then any set of bits from it will be “good enough” in the sense that no other function producing that many bits will be much better.
(Warning: For serious cryptographic use, you should be a little more careful. Look at, e.g., http://en.wikipedia.org/wiki/HMAC for more information. From your description, I do not believe you need to worry about such things.)