Ideal hash functions If we takes =,5 then our estimate is k≈1.17Vn Taking n= 365 in our estimate we get k x 22.3. It is usually suggested that the minimum acceptable size of a message digest is 128 bits(the birthday attack will require over 264 hashes in this case14 Ideal hash functions • If we take  = .5, then our estimate is • Taking n = 365 in our estimate, we get k  22.3. • It is usually suggested that the minimum acceptable size of a message digest is 128 bits (the birthday attack will require over 2 64 hashes in this case)