Well the industry standard is now MD5, though what's in textbooks is usually CRC (32). Both of these are very processor intensive, so it's best to use an out of process DLL written in C or ASM for VB6 (try Gnu Zlib
http://www.gzip.org/zlib/).
Such algorithms are not trivial however: if this is homework or something, the one being used probably involves simple manipulation of a string/byte-array and could be done directly in VB.