A Hash is a binary string type, typically 128-160 bits (16-20 bytes) in length, which is the idempotent result of a cryptographically-strong hash function applied to a file stored in fakefs.
A Hash must have the property that fakefs will never contain any two files with different contents but with the same value for Hash.
The function should have the property that any two files with identical contents will have the same value for Hash, but this is not strictly necessary. If the Hash is non-idempotent, storage efficiency will decrease, and paranoid checking must be turned off (see section 9.2.6), but there will be no other ill effects. See section 9.4.
A Hash should never be more than 127 bytes long, because fakefs often assumes that it can store the length of certain strings, including Hashes, in a signed char. A Hash can only be a multiple of 8 bits long--if your favorite hashing algorithm produces non-multiples of 8 bits of output, it will have to pad its output appropriately.