Back in the day I sold shareware which used successive keyed hashes to generate the registration key. While they key generator used all the keys the software on

Back in the day I sold shareware which used successive keyed hashes to generate the registration key. While they key generator used all the keys the software only validated against the first n keys. When the software was inevitably cracked and a key generator was published the software would be updated to validate n+1 keys which would render the cracked keys unusable. This left hundreds of pages of invalid keys across various crack sites with no way for anyone searching on Google to separate the needles from the haystack. The software phoned home during the registration process as well and our analytics showed that 30% of our paying customers had gone to one or more crack sites and attempted to enter a product key before eventually deciding to purchase the software. This was a utility in the $20 to $30 dollar range so cost wasn't a major factor.