Secret code security access to data processing means

1. A method of accessing data processing means associated with a secret code and accessible by means of a host terminal applying a presented code to said data processing means, said method comprising the steps wherein, each time the processing means are used, a constant number of predetermined operations are performed, said operations including comparisons block-by-block of the presented code and of the secret code so long as the compared blocks are identical, and comparisons block-by-block of a fraction of the presented code and of a fraction of a determined number whenever compared blocks of the presented code and of the secret code are different, until all of the blocks of the presented code have been compared, so as to obtain a succession of executed operations that is constant whether the presented code is correct, partially erroneous, or totally erroneous.

2. The method according to claim 1 , wherein the secret code and the determined number are written in spaces of a non-volatile memory of the processing means, which spaces correspond to addresses having numbers of bits at respective ones of the logic states “0” and “1” that are substantially identical, so that reading a block of secret code is similar to reading a block of the determined number as seen through an electrical power consumption measurement.

3. The method according to claim 1 , wherein the secret code and the determined number are written in adjacent spaces of a non-volatile memory of the processing means, so that reading a block of secret code is similar to reading a block of the determined number as seen through an electrical power consumption measurement.

4. The method according to claim 1 , further comprising the step of modifying the determined number each time that a presented code is applied to the processing means, prior to applying the predetermined operations.

5. The method according to claim 1 , wherein two data processing means associated with identical secret codes are associated with different determined numbers.

6. The method according to claim 1 , wherein the determined number is modified when the secret code associated with the processing means is modified.

7. The method according to claim 1 , further comprising the following steps:

transforming the secret code into a transformed secret code using a transformation function implemented in the processing means;

transforming the presented code into a transformed presented code using the transformation function; and

comparing block-by-block the transformed presented code and the transformed secret code.

8. A method for processing a code that is presented to gain access to a data processing means that is protected by a secret code, comprising the steps of:

comparing block-by-block the presented code and the secret code for i iterations, until the compared blocks do not match; and

thereafter comparing block-by-block of the presented code and a determined value for I-i iterations, such that the total number of iterations I is constant each time a code is presented to gain access to the data processing means.

9. The method of claim 8 further including the step of randomly generating said determined value each time a code is presented to gain access to the data processing means.

10. The method of claim 8 wherein said determined value has the same number of bits as the secret code.

11. The method of claim 8 , wherein the bits of said determined value are randomly generated.

12. The method of claim 1 wherein said determined number has the same number of bits as the secret code.

13. The method of claim 1 , wherein the bits of said determined number are randomly generated.