Let msg be some data we want to hash using an arbitrary digest size algorithm such as blake2b. From that hash, we would like to compute a checksum field with n decimal digits.
If we suppose that the hash algorithm has a perfect distribution, we would like to design the checksum with a probability of collision as low as possible, ideally 1/10n.
Let n be in the range 1..10 (1 digit checksum to 10 digits checksum).
For each value of n, what would be the safest way to calculate a checksum of size n?
More specifically, what size of digest should we aim for every size of n and how should we reduce the value of the hash h to a decimal number.
E.g.: For n = 2 (2 checksum digits):
a) digest_size = 1, checksum formula = h mod 100
b) digest_size = 4, checksum formula = h mod 97
If the formula used to reduce the hash h to a checksum is h mod p, are there optimal values of p and digest_size for each value of n?
