Authentication of consumable items

1. A method for validating the authenticity of an untrusted chip, the method includes the steps of:

a) generating a random number and applying an asymmetric encrypt function to the random number using a public key to produce a first variable;

b) passing the first variable to the untrusted chip;

c) receiving a third variable together with a data message read from the untrusted chip, the third variable having been generated, in the untrusted chip, by a process including:

d) decrypting the first variable with an asymmetric decrypt function using a secret key to produce a second variable, the public and secret keys being a public-private key pair;

e) applying the asymmetric encrypt function to the second variable together with the data message using the secret key to produce the third variable;

f) decrypting the third variable with the asymmetric decrypt function and using the public key to generate a fourth variable and comparing the fourth variable with the generated random number and the received data message;

g) in the event of a match, considering the untrusted chip and the data message to be valid;

otherwise considering the untrusted chip and the data message to be invalid.

2. The method of claim 1 , including, prior to step a) writing new data to the untrusted chip and in the event the untrusted chip is found to be authentic and the new data is the same as the data message received from the untrusted chip, then the write is validated.

3. The method of claim 1 wherein steps a), b), f) and g), take place in a device in which at least one consumable is mounted, and the untrusted chip is associated with at least one consumable.

4. The method of claim 3 wherein the device includes a printer.

5. The method of claim 1 wherein step a) includes producing the random number from a seed, and wherein the seed changes after every successful authentication, so that the next random number will be produced from a different seed.

6. A system including:

a random number generator;

an asymmetric encryptor to encrypt generated random numbers with an asymmetric encryption function and using a public key to produce a first variable;

an untrusted chip configured to:

receive and decrypt the first variable with an asymmetric decrypt function and using a secret key to produce a second variable, the public and secret keys being a public-private key pair;

reading a data message from the untrusted chip;

apply the asymmetric encrypt function to the second variable together with a data message using the secret key to produce a third variable; and

transmit both the third variable and the data message to a testing component; and

the testing component configured to:

decrypt the third variable with the asymmetric decrypt function and using the public key to produce a fourth variable;

compare the fourth variable with the generated random number and the data message; and

in the event of a match, generate a value indicating validity of the untrusted chip, otherwise generating a value indicating invalidity of the untrusted chip.

7. The system of claim 6 , wherein the random number generator, encryptor and testing component are a device in which at least one consumable is mounted, and the untrusted chip is associated with at least one consumable.

8. The system of claim 7 , where the device is a printer.

9. The system of claim 6 , where the random number generator produces random numbers from a seed, and wherein the seed changes after every successful authentication, so that the next random number will be produced from a new seed.