To record the key information in a joint transform correlator cryptosystem, an approach based on ptychography is proposed. This method does not require a reference beam, phase shifting methods, or an additional window in the input plane to record the key information. Instead, the key is illuminated with different probes, and its phase information is recovered employing the probes’ information and their respective cryptosystem’s outputs. Two probes are required for the retrieval of a key capable of successful decryption. Further improvements in the key accuracy and decryption quality are achieved with additional probes. This method is validated with both numerical and experimental results. For an experimental cryptosystem, we obtain similar quality after decryption with a ptychographic key compared with the key registered using the reference arm method.