Method for performing an encryption with look-up tables, and corresponding encryption apparatus and computer program product
An encryption method includes accessing a look-up table (LUT) to implement countermeasures against side-channel attacks, such as embedding masks. The LUT is initialized by writing initialization values in the LUT by applying an address-mask to input data that identify a location of said LUT and a data-mask to data to be stored at a location of the LUT. The method includes carrying out an initialization of the LUT that includes providing at least one second address-mask and one second data-mask; and computing corresponding initialization values as a function of a logic combination of the aforesaid first address-mask and second address-mask and of a logic combination of the aforesaid first data-mask and second data-mask. In the resulting table the address data are masked only by the second address-mask and the data are masked only by the second data-mask. The structure of the LUT may allow convenient implementation by initializing all the values of the LUT in parallel in one cycle.
1. A method, comprising:
initializing a look-up table of an electronic circuit using an iterative process, at least one of a plurality of iterations of the iterative process including:
applying a first of a plurality of address-masks to an unmasked address, generating a masked address;
applying a logical combination of the first of the plurality of address-masks and a second of the plurality of address-masks to the masked address, generating an address corresponding to application of the second address-mask of the plurality of address-masks to the unmasked address;
applying a first of a plurality of data-masks to unmasked data, generating masked data; and
applying a logical combination of the first of the plurality of data-masks and a second of the plurality of data masks to the masked data, generating data corresponding to application of the second data-mask of the plurality of data-masks to the unmasked data; and
using the initialized look-up table to perform a ciphering operation, wherein the ciphering operation is an Advanced Encryption Standard (AES) operation.
2. The method of claim 1 , comprising:
retrieving the logical combination of the first of the plurality of address-masks and the second of the plurality of address-masks, without retrieving or generating of the second of the plurality of address-masks by the electronic circuit; and
retrieving the logical combination of the first of the plurality of data-masks and the second of the plurality of data-masks, without retrieving or generating of the second of the plurality of data-masks by the electronic circuit.
3. The method of claim 1 wherein,
the logical combination of the first of the plurality of address-masks and the second of the plurality of address-masks is an exclusive OR (XOR) between values of the first of the plurality of address-masks and values of the second address-mask of the plurality of address-masks; and
the logical combination of the first of the plurality of data-masks and the second of the plurality of data-masks is an exclusive OR (XOR) between values of the first data-mask of the plurality of data-masks and values of the second data-mask of the plurality of data-masks.
4. The method of claim 1 wherein the initializing the look-up table comprises, in at least one iteration of the plurality of iterations,
applying, to a masked-address-result of a previous iteration, a logical combination of:
an address-mask of a logical combination of address-masks of the previous iteration; and
a subsequent address-mask of the plurality of address-masks, generating a masked-address-result of the iteration corresponding to application of the subsequent address-mask to the unmasked address; and
applying, to a masked-data-result of the previous iteration, a logical combination of:
a data-mask of a logical combination of data-masks of the previous iteration; and
a subsequent data-mask of the plurality of data-masks, generating a masked-data-result of the iteration corresponding to application of the subsequent data mask to the unmasked data.
5. The method of claim 4 , comprising selecting a number of the plurality of iterations.
6. The method of claim 1 wherein the AES operation comprises applying an Advanced Encryption Standard (AES) encryption procedure and a SubBytes operation of said AES encryption procedure includes the initializing of the look-up table.
7. The method of claim 6 , comprising:
using a selected one of a plurality of sets of logical combinations of address-masks in a first round of said AES encryption procedure; and
reusing the selected one of the plurality of sets of logical combinations of address-masks in another round of said AES encryption procedure, wherein the another round is separated from the first round by at least one round.
8. A device, comprising:
a look-up table; and
circuitry configured to initializing the look-up table using an iterative process, at least one of a plurality of iterations of the iterative process including:
applying a first of a plurality of address-masks to an unmasked address, generating a masked address;
applying a logical combination of the first of the plurality of address-masks and a second of the plurality of address-masks to the masked address, generating an address corresponding to application of the second of the plurality of address-masks to the unmasked address;
applying a first of a plurality of data-masks to unmasked data, generating masked data; and
applying a logical combination of the first of the plurality of data-masks and a second of the plurality of data-masks to the masked data, generating data corresponding to application of the second data-mask of the plurality of data-masks to the unmasked data, wherein the device, in operation, uses the initialized look-up table to perform an Advanced Encryption Standard (AES) operation.
9. The device of claim 8 wherein the circuitry is configured to:
retrieve the logical combination of the first and the second of the plurality of address-masks, without retrieving or generating of the second address mask of the plurality of address-masks; and
retrieve the logical combination of the first and the second of the plurality of data-masks, without retrieving or generating of the second of the plurality of data-masks.
10. The device of claim 8 wherein,
the logical combination of the first and the second of the plurality of address-masks is an exclusive OR (XOR) between values of the first address-mask and values of the second address-mask of the plurality of address-masks; and
the logical combination of the first and the second of the plurality of data-masks is an exclusive OR (XOR) between values of the first data-mask and values of the second data-mask of the plurality of data-masks.
11. The device of claim 8 wherein the circuitry is configured to initialize the look-up table by, in at least one iteration of the plurality of iterations,
applying, to a masked-address-result of a previous iteration, a logical combination of:
an address-mask of a logical combination of address-masks of the previous iteration; and
a subsequent address-mask of the plurality of address-masks, generating a masked-address-result of the iteration corresponding to application of the subsequent address-mask to the unmasked address; and
applying, to a masked-data-result of the previous iteration, a logical combination of:
a data-mask of a logical combination of data-masks of the previous iteration; and
a subsequent data-mask of the plurality of data-masks, generating a masked-data-result of the iteration corresponding to application of the subsequent data mask to the unmasked data.
12. The device of claim 11 wherein the circuitry is configured to select a number of the plurality of iterations.
13. The device of claim 8 wherein the circuitry is configured to perform an Advanced Encryption Standard (AES) ciphering procedure and a SubBytes operation of said AES ciphering procedure includes the initializing of the look-up table.
14. The device of claim 13 wherein the circuitry is configured to:
use a selected one of a plurality of sets of logical combinations of address-masks in a first round of said AES ciphering procedure; and
reuse the selected one of the plurality of sets of logical combinations of address-masks in another round of said AES ciphering procedure, wherein the another round is separated from the first round by at least one round.
15. The device of claim 14 , comprising an S-Box including the look-up table.
16. The device of claim 15 wherein the S-Box comprises a plurality of composite look-up tables each being smaller than the look-up table and the S-Box is configured to perform a non-linear operation in a finite field, using the plurality of composite look-up tables to implement said non-linear operation in a composite field of finite subfields deriving from said finite field.
17. The device of claim 16 wherein said composite look-up tables comprise a plurality of flip-flops and the S-Box is configured to initialize the flip-flops using logical combination of two of the plurality of address-masks and logical combination of two of the plurality of data-masks.
18. The device of claim 8 wherein the circuitry is configured to apply the logical combination of the first and the second of the plurality of address-masks to the masked address and to apply the logical combination of the first and the second of the plurality of data-masks to masked data in a single clock cycle.
19. A system, comprising:
one or more terminals to receive and output data; and
security circuitry coupled to the one or more interfaces and including an S-Box configured to initializing one or more look-up tables using an iterative process, at least one of a plurality of iterations of the iterative process including:
applying a first of a plurality of address-masks to an unmasked address, generating a masked address;
applying a logical combination of the first address-mask of the plurality of address-masks and a second address mask of the plurality of address-masks to the masked address, generating an address corresponding to application of the second address-mask of the plurality of address-masks to the unmasked address;
applying a first of a plurality of data-masks to unmasked data, generating masked data; and
applying a logical combination of the first of the plurality of data-masks and a second data-mask of the plurality of data-masks to the masked data, generating data corresponding to application of the second data-mask of the plurality of data-masks to the unmasked data, wherein the one or more initialized look-up tables are used by the S-Box to perform an Advanced Encryption Standard (AES) operation.
20. The system of claim 19 wherein the security circuitry is configured to:
retrieve the logical combination of the first and second address-masks of the plurality of address-masks, without retrieving or generating of the second address-mask of the plurality of address-masks; and
retrieve the logical combination of the first and second data-masks of the plurality of data-masks, without retrieving or generating of the second data-mask of the plurality of data-masks.
21. The system of claim 19 wherein the S-Box is configured to, in at least one iteration of the plurality of iterations,
apply, to a masked-address-result of a previous iteration, a logical combination of:
an address-mask of a logical combination of address-masks of the previous iteration; and
a subsequent address-mask of the plurality of address-masks, generating a masked-address-result of the iteration corresponding to application of the subsequent address-mask to the unmasked address; and
apply, to a masked-data-result of the previous iteration, a logical combination of:
a data-mask of a logical combination of data-masks of the previous iteration; and
a subsequent data-mask of the plurality of data-masks, generating a masked-data-result of the iteration corresponding to application of the subsequent data mask to the unmasked data.
22. The system of claim 19 wherein the S-Box is configured to:
use a selected one of a plurality of sets of logical combinations of address-masks in a first round of an Advanced Encryption Standard (AES) ciphering procedure; and
reuse the selected one of the plurality of sets of logical combinations of address-masks in another round of said AES ciphering procedure, wherein the another round is separated from the first round by at least one round.
23. The system of claim 19 , comprising at least one of:
set-top box control circuitry; and
smart-card control circuitry.
24. A non-transitory computer-readable medium having contents which configure an Advanced Encryption Standard (AES) system to perform a method, the method comprising:
initializing one or more look-up tables using an iterative process, at least one of a plurality of iterations of the iterative process including:
applying a first address-mask of a plurality of address-masks to an unmasked address, generating a masked address;
applying a logical combination of the first address-mask and a second address-mask of the plurality of address-masks to the masked address, generating an address corresponding to application of the second address-mask of the plurality of address-masks to the unmasked address; and
applying a first data-mask of a plurality of data-masks to an unmasked data, generating masked data;
applying a logical combination of the first data-mask and a second data-mask of the plurality of data-masks to the masked data, generating data corresponding to application of the second data-mask of the plurality of data-masks to the unmasked data.
25. The medium of claim 24 wherein the method comprises:
retrieving the logical combination of the first and second address-masks of the plurality of address-masks, without retrieving or generating of the second address-mask of the plurality of address-masks; and
retrieving the logical combination of the first and second data-masks of the plurality of data-masks, without retrieving or generating of the second data-mask of the plurality of data-masks.
26. The medium of claim 24 wherein at least one iteration of the plurality of iterations includes,
applying, to a masked-address-result of a previous iteration, a logical combination of:
an address-mask of a logical combination of address-masks of the previous iteration; and
a subsequent address-mask of the plurality of address-masks, generating a masked-address-result of the iteration corresponding to application of the subsequent address-mask to the unmasked address; and
applying, to a masked-data-result of the previous iteration, a logical combination of:
a data-mask of a logical combination of data-masks of the previous iteration; and
a subsequent data-mask of the plurality of data-masks, generating a masked-data-result of the iteration corresponding to application of the subsequent data mask to the unmasked data.
27. The medium of claim 24 wherein the method comprises:
using a selected one of a plurality of sets of logical combinations of address-masks in a first round of an AES ciphering procedure; and
reusing the selected one of the plurality of sets of logical combinations of address-masks in another round of said AES ciphering procedure, wherein the another round is separated from the first round by at least one round.