Encryption method for an instructions stream and execution of an instructions stream thus encrypted
A method of encrypting a program instructions stream and a method of executing an instructions stream thus encrypted. Instructions are translated into binary code before being encrypted by a stream cipher method. When the program contains a conditional or unconditional branch instruction, an instruction is inserted in the program to initialize the pseudo-random sequence generator using an initialization vector, the initialization vector being used to generate the pseudo-random sequence for encryption and decryption of instructions at the branch address. Instructions can be decrypted and executed on-the-fly without needing to know their physical addresses, even in the presence of a branch.
1. A method of encryption of a program instructions stream using a stream cipher with a pseudo-random sequence generator configured by a secret key and initialised by an initialisation vector, comprising:
translating the instructions stream into binary code by a compiler, said binary code being summated bit by bit with a pseudo-random sequence generated by the pseudo-random generator to provide an encrypted instructions stream;
inserting, when the program comprises a branch instruction to a branch address, a corresponding initialisation instruction in the program to initialise the pseudo-random generator with a specific initialisation vector specific to the branch address, the instructions from the branch address being encrypted using a pseudo-random sequence generated by the pseudo-random generator initialised by said specific initialisation vector.
2. The method of encryption of an instructions stream according to claim 1 , wherein the initialisation instruction is inserted just before the branch instruction.
3. The method of encryption of an instructions stream according to claim 2 , wherein the initialisation instruction is inserted unencrypted in the encrypted instructions stream.
4. The method of encryption of an instructions stream according to claim 2 , wherein the initialisation instruction is encrypted with said pseudo-random sequence before being inserted.
5. The method of encryption of an instructions stream according to claim 1 , wherein the initialisation instruction is inserted unencrypted at the branch address in the encrypted instructions stream.
6. A method of encryption of a program instructions stream using a stream cipher with a pseudo-random sequence generator configured by a secret key and initialised by an initialisation vector, comprising:
translating the instructions stream into binary code by a compiler, said binary code being summated bit by bit with a pseudo-random sequence generated by the pseudo-random generator to provide an encrypted instructions stream;
inserting, when the program comprises a branch instruction to a branch address, a corresponding initialisation instruction in the program to initialise the pseudo-random generator with a specific initialisation vector specific to the branch address, the instructions from the branch address being encrypted using a pseudo-random sequence generated by the pseudo-random generator initialised by said specific initialisation vector, wherein:
the pseudo-random sequence is generated using the pseudo-random sequence generator configured by said secret key and initialised by said initialisation vector;
the encrypted instructions stream is summated bit by bit with the pseudo random sequence, each encrypted instruction being decrypted and executed before going onto the next instruction;
each initialisation instruction is detected in the encrypted instructions stream and, when one initialisation instruction is detected, an extracted initialisation vector contained in the one initialization instruction is extracted and the pseudo-random sequence generator is initialised with the extracted initialisation vector in the case of a branch to the branch address.
7. The method of executing an instructions stream according to claim 6 , wherein an initialisation instruction is detected by means of a specific prefix.
8. The method of executing an instructions stream according to claim 6 , wherein, if an initialisation instruction is inserted before a branch instruction to a branch address, the initialisation vector extracted from the inserted initialisation instruction is stored in an intermediate register when it is detected and the pseudo-random generator is initialised with the content of said register when jumping to the branch address.
9. The method of execution of a instructions stream according to claim 6 , wherein the program comprises at least one conditional branch instruction.
10. The method of execution of a instructions stream according to claim 6 , wherein the program comprises at least one unconditional branch instruction.
11. The method of executing an instructions stream according to claim 7 , wherein detecting each initialisation instruction comprises detecting after decryption.