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
Using third-party "key generators" found on the internet is strongly discouraged. Such tools often distribute malware or produce invalid keys that can lead to device instability, loss of official Cisco Support cisco asa activation key generator
To get a valid activation key, follow these steps through the Cisco Software Central or official licensing channels: Obtain Serial Number: show version The key is stored in a hidden partition
Managing Licenses with Activation Keys > Cisco ASA Licensing 22 Jul 2014 — Instead, activation keys are generated exclusively by Cisco
Cisco does not provide a standalone "activation key generator" for public use. Instead,
activation keys are generated exclusively by Cisco through their official licensing portal based on a valid Product Authorization Key (PAK) and the unique Serial Number of the device. Cisco Community How Cisco ASA Licensing Works