I’m having difficulty calculating the false positive error probability of matching a prefix of a hash that was truncated to m bits.
Say I have string S1 that produces a SHA256 hash H1. I then save the the first 64 bits of H1, call it prefix P1 to a database.
I have another string S2 that produces a SHA256 hash H2, and 64 bit prefix P2.
If I can’t find prefix P2 in the database I know for sure that H2 is not on the database. False negative probability is 0, but if I find the prefix P2 in the database what is the false positive probability of me flagging H2 as being in the database?