A security circuit for a reprogrammable logic IC includes an evolved circuit that ties the performance of the security circuit to the physical properties of that particular reprogrammable logic IC. The security circuit can be a decryption and/or encryption circuit that decrypts and/or encrypts, respectively, a configuration bitstream for the IC. Because of the link between the performance of the security circuit and the physical properties of the IC, the security circuit cannot be used in other ICs. For example, an encrypted bitstream that can be decrypted by the security circuit in a first IC will typically not be decrypted by the same security circuit in a second IC, since the physical properties of the two ICs will typically be different. The evolved circuit can comprise a portion of the security circuit, such as a security key generator, or it can comprise the full security circuit.