A trusted authentication chip for use in authenticating an untrusted authentication chip; the trusted authentication chip including a random number generator, a symmetric encryption function and two secret keys for the function, a signature function and a test function; wherein the trusted authentication chip generates test data including a random number and its signature, encrypted using a first of said secret keys and transmits the test data to the untrusted authentication chip, wherein the trusted authentication chip receives a data message and an encrypted version of the data message in combination with the random number from the untrusted authentication chip, the data message being encrypted using a second of said secret keys, wherein the test function operates to encrypt the random number together with the data message by the symmetric encryption function using the second secret key, compare the two versions of the random number encrypted together with the data message using the second key, and in the event that the two versions match, considers the untrusted authentication chip and the data message to be valid, otherwise, it considers the untrusted authentication chip and the data message to be invalid.