This invention is a validation method for determining whether an untrusted chip is valid, or not. In another aspect it concerns a validation system for the method. The method may be used to determine the physical presence of a valid chip. A trusted chip generates a random number and a digital signature for it, encrypts them with a first key and then calls a prove function in the untrusted chip. The prove function decrypts the random number and signature, and calculates another signature from the decrypted random number, for comparison with the decrypted one. If the comparison is successful the random number is encrypted with another key and sent back as a second number. Finally, a test function is called in the trusted chip to generate its own encrypted version of the random number using the second key and then compare it with the received version to validate the untrusted chip. The untrusted chip may be associated with a consumable so that validation of the untrusted chip authenticates the consumable.