Method for protecting confidentiality of a file distributed and stored at a plurality of storage service providers

A method for retrieving a file distributed and stored at a plurality of storage service providers, comprising performing at a computing means of a user device, the steps of:

• reading n file identifiers ID′1, . . . , ID′n designating said file, and reading provider identifiers O1, . . . , On corresponding to the plurality of storage service providers, in a file descriptor corresponding to the file, said file descriptor being stored in a local memory of the user device,
• sending the n file identifiers ID′1, . . . , ID′n respectively to the storage service providers designated by the provider identifiers O1, . . . , On,
• receiving n combinations C1, . . . , Cn, from the storage service providers designated by the provider identifiers O1, . . . , On,
• retrieving n sets of coefficients ai1, . . . , ain, for i=1, . . . , n, that respectively correspond to the n combinations C1, . . . , Cn,
• associating each combination Ci with the corresponding set of coefficients ai1, . . . , ain, for i=1, . . . , n,
• computing the inverse of a matrix composed of the n set of coefficients ai1, . . . , ain, for i=1, . . . , n, for obtaining another matrix composed of n sets of coefficients bi1, . . . , bin, for i=1, . . . , n,
• then computing n chunks S1, . . . , Sn of said file as:
• Si=bi1·C1+ . . . +bij·Cj+ . . . +bin·Cn for i=1, . . . , n,
• then re-assembling, at the computing means, the chunks S1, . . . , Sn to reconstruct the file, wherein said re-assembling to reconstruct the file, further comprises:
• reading an over-combination OC′ and coefficients a′1, . . . , a′j, . . . , a′n for j=1, . . . , n,
• computing the inverse of a matrix composed of n−1 sets of coefficients ai1, . . . , ain, for i=1, . . . , n−1 and a set of coefficients (a′1·a1j+ . . . +a′i·aij+ . . . +a′n·anj) for j=1, . . . , n, for obtaining another matrix composed of n sets of coefficients bi1, . . . , bin, for i=1, . . . , n,
• then computing n chunks S1, . . . , Sn of said file as:
• Si=bi1·C1+ . . . +bij·Cj+ . . . +bin-1·Cn-1+bin·OC′ for i=1, . . . , n,
• then re-assembling, at the computing means, the chunks S1, . . . , Sn to reconstruct the file;
• wherein said retrieving n sets of coefficients ai1, . . . , ain for i=1, . . . , n, that respectively correspond to the n combinations C1, . . . , Cn, comprises receiving the n sets of coefficients from the storage service providers designated by the provider identifiers O1, . . . , On;
• and wherein said associating each combination Ci with a corresponding set of coefficients ai1, . . . , ain, for i=1, . . . , n comprises:
• reading a permutation (σ) in the file descriptor corresponding to said file, and
• applying the permutation to the sets of coefficients for ai1, . . . , ain, for i=1, . . . , n, so that each combination Ci is associated with the corresponding set of coefficients ai1, . . . , ain, for i=1, . . . , n.

A method for protecting confidentiality of a file distributed and stored at a plurality of storage service providers, comprising performing at a computing means of a user device, the steps of:

• inputting a security parameter n,
• segmenting the file in n chunks S1, . . . , Sn,
• randomly choosing n2 coefficients aij for i=1, . . . , n and j=1, . . . , n,
• verifying that vectors ai1, . . . , ain of the coefficients, for i=1, . . . , n, are linearly independent, otherwise generating the coefficients again,
• computing n linear combinations Ci=ai1·S1+ . . . +aij·Sj+ . . . +ain·Sn,
• for i=1, . . . , n, wherein each of the combinations Ci is computed by the computing means using all of the n chunks S1, . . . , Sn of the file,
• choosing n storage service providers Oi, . . . , On among said plurality of storage service providers,
• generating n file identifiers ID′1, . . . , ID′n designating said file,
• transmitting and storing the combination Ci at the storage service provider Oi in association with the file identifier ID′i, for each of i=1, . . . , n,
• storing the file identifier ID′i and provider identifier Oi, for i=1, . . . , n, in a file descriptor corresponding to the file, said file descriptor being stored in a local memory of the user device,
• computing the inverse of a matrix composed of the n set of coefficients ai1, . . . , ain, for i=1, . . . , n, for obtaining another matrix composed of n sets of coefficients bi1, . . . , bin, for i=1, . . . , n, and
• storing the set of coefficients bi1, . . . , bin so that it can be re-associated with the combination Ci, for i=1, . . . , n,
• randomly choosing said n2 coefficients aij for i=1, . . . , n and j=1, . . . , n, and
• verifying that the vectors ai1, . . . , ain, for i=1, . . . , n, are linearly independent, otherwise generating the coefficients again;
• wherein said storing the set of coefficients bi1, . . . , bin so that it can be re-associated with the combination Ci, for i=1, . . . , n, comprises:
• storing the set of coefficients bi1, . . . , bin at the storage service provider Ok, in association with a combination Ck and a file identifier ID′k, for i=1, . . . , n and for k lying between 1 and n, including 1 and n, and being different from i;
• and
• storing a permutation (σ) enabling to re-associate the set of coefficients bi1, . . . , bin and the combination Ci, for i=1, . . . , n, in said file descriptor corresponding to the file, and being stored in said local memory for utilizing in reconstruction of the file.

A method for retrieving a file distributed and stored at a plurality of storage service providers, comprising performing at a computing means of a user device, the steps of:

• reading n file identifiers ID′1, . . . , ID′n designating said file, and reading provider identifiers O1, . . . , On corresponding to the plurality of storage service providers, in a file descriptor corresponding to the file, said file descriptor being stored in a local memory of the user device,
• sending the n file identifiers ID′1, . . . , ID′n respectively to the storage service providers designated by the provider identifiers O1, . . . , On,
• receiving n combinations C1, . . . , Cn, from the storage service providers designated by the provider identifiers O1, . . . , On,
• retrieving n sets of coefficients bi1, . . . , bin, for i=1, . . . , n, that respectively correspond to the n combinations C1, . . . , Cn,
• associating each combination Ci with the corresponding set of coefficients bi1, . . . , bin, for i=1, . . . , n,
• computing chunks S1, . . . , Sn of said file as:
• Si=bi1·C1+ . . . +bij·Cj+ . . . +bin·Cn for i=1, . . . , n,
• then re-assembling the chunks S1, . . . , Sn to reconstruct the file;
• wherein said retrieving n sets of coefficients bi1, . . . , bin for i=1, . . . , n, that respectively correspond to the n combinations C1, . . . , Cn, comprises receiving the n sets of coefficients from the storage service providers designated by the provider identifiers O1, . . . , On;
• and wherein said associating each combination Ci with a corresponding set of coefficients bi1, . . . , bin for i=1, . . . , n comprises:
• reading a permutation (σ) in the file descriptor corresponding to said file, and
• applying the permutation to the sets of coefficients bi1, . . . , bin for i=1, . . . , n, so that each combination Ci is associated with the corresponding set of coefficients bi1, . . . , bin for i=1, . . . , n.