We have offline software and need to generate about 100 license keys for that. We are thinking of getting 22-digit random string from offline software and encrypt it using AES256 and give it back to the user. Once use enters the key the software will encrypt the same 22-digit random number using same key and match both cyphertext. The only problem seems is someone can figure out AES256 encryption key from plain 22-digit random number and 22-digit license key.

We explored RSA and PKV. Since RSA cyphertext are long, it's not useful for offline software. Not too sure about PKV.

We can disable software if wrong keys are entered more than three times.