A security controller (31, 12, 30) for an industrial control system comprising: (a) a network port for communicating with other control elements of the industrial control system; (b) at least one processor communicating with the network port; and (c) an electronic memory system accessible by the processor and holding a security program executing on the processor to: receive from at least one other given control element of the industrial control system a security thumbprint providing an encrypted compressed representation of a current state of the given control element; analyze the security thumbprint against a stored benchmark thumbprint representing a security thumbprint of the given control element when the given control element is properly operating; when the security thumbprint does not match the stored benchmark thumbprint, assess the mismatch to produce a severity assessment having a multi-value range; and implement a security response action selected from among multiple security response actions to respond to the mismatch based on the severity assessment, wherein the security thumbprint designates a set of thumbprint source data comprising a control program of the at least one other given control element and environmental data held in the at least one other given control device, wherein the environmental data comprise wire connection states of the at least one other given control element. (a) a network port for communicating with other control elements of the industrial control system; (b) at least one processor communicating with the network port; and (c) an electronic memory system accessible by the processor and holding a security program executing on the processor to: receive from at least one other given control element of the industrial control system a security thumbprint providing an encrypted compressed representation of a current state of the given control element; analyze the security thumbprint against a stored benchmark thumbprint representing a security thumbprint of the given control element when the given control element is properly operating; when the security thumbprint does not match the stored benchmark thumbprint, assess the mismatch to produce a severity assessment having a multi-value range; and implement a security response action selected from among multiple security response actions to respond to the mismatch based on the severity assessment, wherein the security thumbprint designates a set of thumbprint source data comprising a control program of the at least one other given control element and environmental data held in the at least one other given control device, wherein the environmental data comprise wire connection states of the at least one other given control element. receive from at least one other given control element of the industrial control system a security thumbprint providing an encrypted compressed representation of a current state of the given control element; analyze the security thumbprint against a stored benchmark thumbprint representing a security thumbprint of the given control element when the given control element is properly operating; when the security thumbprint does not match the stored benchmark thumbprint, assess the mismatch to produce a severity assessment having a multi-value range; and implement a security response action selected from among multiple security response actions to respond to the mismatch based on the severity assessment, wherein the security thumbprint designates a set of thumbprint source data comprising a control program of the at least one other given control element and environmental data held in the at least one other given control device, wherein the environmental data comprise wire connection states of the at least one other given control element.