A ladder program unauthorized-use prevention system includes an engineering tool to decode a ladder program encrypted using a vendor private key, using a vendor public key paired with the vendor private key, and to encrypt the decoded ladder program using a controller public key such that the ladder program is operable on a specific programmable controller but is inoperable on other programmable controllers, and a programmable controller to decode the ladder program encrypted using the controller public key, using a controller private key paired with the controller public key, and to execute the ladder program decoded using the controller private key.
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