A security circuit may include a functional circuit including a test chain that connects flip-flops to verify hardware of the functional circuit, the functional circuit configured to generate an output signal by encrypting an input signal based on a control signal, a mode signal, and the chain; and/or a test controller configured to generate the input, control, and mode signals, and configured to generate an authentication result based on the output signal. A security circuit may include a first device including a plurality of flip-flops in a test chain, the first device configured to receive first, second, and third signals, and configured to generate a fourth signal by encrypting the first signal based on the second and third signals and the chain; and/or a second device configured to generate the first, second, and third signals, and configured to generate an authentication result based on the fourth signal.