Sometimes the classic CnR winning criterion:
If your submission has not been cracked within a week, it is considered safe. The shortest safe submission wins.
The robber with the most cracks wins.
can be a bit boring. It also has the problem of hash functions. What counts as one? SHA1? MD5? CRC32? Do you disallow anything not bijective? That bans functions that get the character at an index in a string.
I'm looking for alternative winning criterions for CnRs. Please include a criterion for both cops and robbers. Any ideas?