Programmable logic controller

A programmable logic controller that executes a sequence program and controls a control target device, the programmable logic controller comprising:

• a device that stores therein data related to an operation of the control target device and used when the control target device is controlled;
• an authentication-information storage unit that stores therein authentication information;
• an authentication function unit that performs a user authentication based on the authentication information stored in the authentication-information storage unit;
• and
• an encryption filter that has a plurality of encryption patterns, one of the encryption patterns being set in advance by a user for encryption processing,
• wherein when writing of the data to the device is requested by an external device, the authentication function unit performs the user authentication,
• causes, when the user authentication is successful, the encryption filter to perform the encryption processing on the data to be written to the device with the encryption pattern set in advance by the user to write the encrypted data to the device, and
• discards, when the user authentication fails, the data for which writing to the device is requested, and
• wherein when reading of data from the device is requested by the external device, the authentication function unit performs the user authentication,
• outputs, when the user authentication is successful, the data read from the device to the external device, and
• controls, when the user authentication fails, the encryption filter to perform the encryption processing on the data read from the device with one of the encryption patterns that is different from the encryption pattern set in advance by the user and is not set for the encryption processing in association with the user and to output the encrypted data to the external device.