Circuits and methods for verifying the unique identity, or digital fingerprint, of an integrated circuit (IC) are presented. The complete circuit is located inside the IC to avoid external tampering and includes a random number generator (RNG), a key register, an encryption circuit, and selection circuitry. The key register stores an encryption key generated by the RNG during the initialization phase. The encryption key can be read from the outside of the IC solely during the initialization phase. After the initialization phase, the encryption circuit generates a response to a challenge using the encryption key, and the selection circuitry outputs in a pin of the IC the response to the challenge. The response is used to check the unique ID of the IC. The encryption key is never sent to the outside of the IC after initialization, and mimicking the behavior of the IC by an impostor is avoided by the use of the encryption mechanism.