The key is stored in a hidden partition of the ASA’s flash memory, not in the standard configuration file. Cisco Press Legitimate Ways to Obtain Keys
Keys are only valid for the specific hardware platform they were generated for. Product Authorization Key (PAK): cisco asa activation key generator
When you purchase a license, you receive a PAK via email. This is the "proof of purchase" required to generate the final activation key on the Cisco Licensing Portal The key is stored in a hidden partition
