Before you can generate a license key, you must retrieve your firewall's unique serial number using the Command Line Interface (CLI).
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Cisco license keys are asymmetrically signed. Your ASA validates the key using a public key baked into the ASDM or CLI. Without Cisco’s private signing key (stored in a hardware security module inside Cisco’s own servers), any generated key will be rejected with the error: Invalid activation key. cisco+asa+license+key+generator+install