[zaroth]

What you’re talking about is more like a hash table not a rainbow table. If you’re looking for a pre-image for the first 8 bytes of MD5 that would be a table with 2^64 rows * at least 8 bytes or at least 144 exabytes of data.

Using such a table would allow you to select guesses that matched the first n bits of the stored hash and then use timing to try to guess the right choice for the n+1’th bit... up to the first 64 bits. You would then have limited the number of guesses down to just another 2^64 possibilities. So that’s not really a valid approach.

A different approach would be to try to figure out the first ~32 bits of the hash using a much smaller (34GB) table and use that knowledge to screen potential candidate passwords offline.

Again, all this works only as long as there isn’t a salt, and the value you are trying to discover is a guessable password and not a randomly generated key.