White-box cryptographic system with configurable key using intermediate data modification
A white-box cryptographic system ( 600 ) is presented for performing a key-dependent cryptographic operation, such as AES. The system comprises a network of a plurality of look-up tables ( 640 ) arranged for collectively performing the cryptographic operation, the network being adapted for a particular cryptographic key. By sending a key substitute ( 664 ) which represents to the network a further cryptographic key a key translation unit can arrange the effect of the network on the cryptographic operation such that is adapted for a further cryptographic key. In this way the system can be updated to use the further key instead of the particular key.
1. A white-box cryptographic system for performing a keyed cryptographic operation mapping an input-message to an output-message, the system comprising:
a network of a plurality of look-up tables arranged for collectively performing the cryptographic operation, the network being adapted for a particular cryptographic key,
wherein,
the system is configured to:
receive a key substitute which represents to the network a further cryptographic key, and
parse the key substitute into a plurality of second key substitutes,
the system comprises at least one key translation unit configured for receiving a first input depending on an intermediate cryptographic result in the network and a second input depending on one of the second key substitutes,
the system is arranged for mapping the input-message to the output-message via the network and the key translation unit to arrange the effect of the network on the cryptographic operation to being adapted for the further cryptographic key.
2. A white-box cryptographic system as in claim 1 , wherein at least one specific look-up table of the plurality of look-up tables is adapted for the particular cryptographic key, the system is arranged for mapping the input-message to the output-message via at least the specific look-up table and the key translation unit to arrange the effect of the specific look-up table on the cryptographic operation to being adapted for the further cryptographic key.
3. A white-box cryptographic system as in claim 2 , wherein the first input depends on the input message by means of one or more of the plurality of look-up tables and wherein the system is arranged for the specific look-up table to receive as an input depending on an output of the key translation unit.
4. A white-box cryptographic system as in claim 3 , wherein the key translation unit is configured with an output-encoding, and wherein the specific look-up table is configured with an input encoding corresponding to the output encoding.
5. A white-box cryptographic system as in claim 2 , wherein the system is configured for the first input to depend on an output data of the specific look-up table.
6. A white-box cryptographic system as in claim 5 , wherein the specific look-up table is configured with an output-encoding, and wherein the key translation unit is configured with an input encoding corresponding to the output encoding of the specific look-up table.
7. A white-box cryptographic system as in claim 1 , wherein the key substitute comprises a difference between the further cryptographic key and the particular cryptographic key.
8. A cryptographic system as in claim 7 , wherein the key translation unit is configured for adding the second input to the first input.
9. A cryptographic system as in claim 8 , wherein the specific look-up table comprises a cryptographic key addition operation of a sub-key depending on the particular key.
10. A cryptographic system as in claim 9 wherein the cryptographic sub-key addition operation and the addition in the key translation unit of the second input to the first input are arranged to compose into a cryptographic key addition operation of a further sub-key depending on the further cryptographic key.
11. A cryptographic system as in claim 1 , wherein the key translation unit is arranged to receive the second input encoded with a key substitute encoding, and wherein the system is configured to receive a key substitute at least part of which is encoded with the second input encoding.
12. A cryptographic method for performing a keyed cryptographic operation mapping an input-message to an output-message, the method comprising
performing the cryptographic operation using a network of plurality of look-up tables arranged for the cryptographic operation, the network being adapted for a particular cryptographic key, wherein the method comprises
receiving a key substitute which represents to the network a further cryptographic key,
parsing the key substitute into a plurality of second key substitute; and
providing a key translation unit with a first input depending on an intermediate cryptographic result of the network and a second input depending on one of the second key substitutes,
wherein
performing the cryptographic operation using the network comprises mapping the input-message to the output-message via the network and the key translation unit to arrange the effect of the network on the cryptographic operation to being adapted for the further cryptographic key.
13. A computer program comprising computer program code means adapted to perform all the steps of the cryptographic method of claim 12 when the computer program is run on a computer.
14. A computer program as claimed in claim 13 embodied on a computer readable medium.
15. A cryptographic system as in claim 8 wherein the cryptographic sub-key addition operation and the addition in the key translation unit of the second input to the first input are arranged to compose into a cryptographic key addition operation of a further sub-key depending on the further cryptographic key.
16. A cryptographic system as in claim 2 , wherein the specific look-up table comprises a cryptographic key addition operation of a sub-key depending on the particular key.
17. The system according to claim 1 , wherein the system is configured for:
selecting one of the second key substitutes to provide the one of the second key substitutes to the key translation unit.
18. The system according to claim 1 , wherein the system is configured for:
selecting one of the second key substitutes for a first look-up table of the plurality of look-up tables and selecting the other second key substitute for a second look-up table of the plurality of look-up tables.
19. The method according to claim 12 , comprising:
selecting one of the second key substitutes to provide the one of the second key substitutes to the key translation unit.
20. The method according to claim 12 , comprising:
selecting one of the second key substitutes for a first look-up table of the plurality of look-up tables, and selecting the other second key substitute for a second look-up table of the plurality of look-up tables.