Patent ID: 8374340

Claim:
A method for secure transmission of a file fragmentable into a plurality of fragments, where a processor performs the method in which: (a) a global encipherment key is fixed initially; (b) the fragmentable file is enciphered as a function of said global key to form a first encrypted file; (c) the global key is enciphered by the processor using an asymmetric encipherment algorithm to obtain an enciphered global key; (d) values representative of the enciphered global key are inserted among the fragments of said encrypted file to form a second encrypted file; and (e) said second encrypted file is transmitted, wherein, in step (b), for a succession of fragments: (b1) each current fragment is assigned a dynamic encryption key determined on the basis of a combination involving a group of previously encrypted fragments, wherein a linear combination is applied to the values of the previously encrypted fragments by the processor, the linear combination expressed by: { z i = ( ∑ j = 1 n ⁢ α j ⁢ p i - j ′ ) ⁢ ⁢ mod ⁢ X p i ′ = ( z i + p i ) ⁢ ⁢ mod ⁢ X where: z i is the dynamic key value assigned to the current fragment of value p i , α j with j lying between 1 and n is a set of coefficients for implementing the linear combination applied to the current fragment, p′ i-j with i-j lying between i-n and i−1 is a set of values of n previously encrypted fragments, p′ i is the value of the encrypted current fragment, and mod X corresponds to a congruence specific to a sampling of said fragments; and (b2) the value of the current fragment is allocated a value which is dependent on its initial value and on said dynamic key.