Need a 64 bit Message Digest

Question

I need a 64 (or 96) bit message digest function that acts like MD5 or something similar, but instead of spitting out 128 bit digests, it puts out either 64 or 96 bit digests. The problem is thus, we are currently using a piece of a proprietary library to create our pretty-close-to-unique keys, which happen to be 16 byte hexidecimal keys. The company that provides the proprietary library no longer exists, and has been sold to two different companies, therefore we are looking for a replacement. The only requirement is that the key remain 16 bytes, as we have customers that rely on that key.
I've read that MD5 runs a 2^63 chance of collision.. is is possible to take the 128 bit key and simply shave off bits? does that merely increase my chance of collision to 2^47? (for 96 bit key) or 2^31 for a 64 bit key? Or is the distribution less even than that? The messages going in will be around 150 characters btw.
Any Help would be appreciated!

Answer 1

If you are bound by these requirements then the simplest solution is, as
you said, to lop off unecessary bits from the 16/20 byte hash returned
by MD2 or MD5. The probability of collision, all else being equal,
can be computed directly from the number of bits used. For this reason
I would suggest going for the 96 bit base 64 encoding.
Alternately you could XOR the removed bits with the reamining 96 bits
however this should not really increase the randomness if the original
function is doing its job properly. Whether it is worthwhile depends on
the quality of the original hash function.
matfud