Cryptographic pseudonym mapping method, computer system, computer program and computer-readable medium

A cryptographic pseudonym mapping method for an anonymous data sharing system, the method being adapted for generating a pseudonymised database (DB) from data relating to entities and originating from data sources (DSi), wherein the data are identified at the data sources (DSi) by entity identifiers (D) of the respective entities, and wherein the data are identified in the pseudonymised database (DB) by pseudonyms (P),

• characterized in that
• the pseudonyms (P) are assigned to the respective entity identifiers (D) applying a one-to-one mapping, irrespective of the originating data source, by
• applying more than one, a number k of mappers (Mj),
• selecting, for each data source (DSi), elements (dij, aij) in a number equal to the number of mappers (Mj) from a predetermined algebraic structure constituting a multiplicative or an additive cyclic group, of which elements (dij, aij) one element (dij, aij) is sent, while being kept secret and kept assigned to the data source (DSi), to each mapper (Mj), calculating from the plurality of elements (dij, aij) an inverse cryptographic key of said plurality, and transforming, each entity identifier (D) to be mapped, into a respective encrypted entity identifier (Ci0) by using the inverse cryptographic key as an own secret cryptographic key (ei) of the data source (DSi),
• generating for each mapper (Mj) its mapping cryptographic key (hij) corresponding to the data source (DSi) by using the element (dij, aij) that was sent to the mapper (Mj) and an element (bj) selected randomly from the algebraic structure and kept secret by the mapper (Mj), and
• generating respective pseudonyms (P) by sequentially performing, in a permutation of the mappers (Mj), a number k of mappings utilizing the mapping cryptographic keys (hij) of the mappers (Mj) belonging to the particular data source (DSi) on each encrypted entity identifier (Ci0) encrypted by the data source (DSi),
• further characterised by applying an algebraic structure constituting an additive cyclic group, wherein values are represented by points of elliptic curves defined over a number field of residue classes modulo p, where p is a prime number, for which algebraic structure the following constants are predetermined, parameters A, B of the formula y2=x3+Ax+B mod p defining
• the points of an elliptic curve defined over the residue classes of the prime number p, and a point G of the curve that has an order q that is greater than the number of entity identifiers (D),
• for generating the own cryptographic key (ei) of each data source (DSi), elements (aij) are selected from the residue classes mod q as elements (dij) corresponding in number to the number of the mappers (Mj), and the sum (ai) of the elements is chosen as the own cryptographic key (ei) of the data source (DSi), for which the formula ei=ai=Σj=1kaij,
• the elements (aij) are sent encrypted to the mappers (Mj), the mappers (Mj) are applied for decrypting their respective own elements (dij), and the mapping cryptographic key (hij) of each mapper (Mj) corresponding to the data source (DSi) is generated applying the formula hij=aij+bj, where the randomly selected secret element (bj) is a value of the residue classes mod q and is different from zero and one,
• the encrypted entity identifier (Ci0) is computed by the data source (DSi) utilizing the formula Ci0=M⊕aiG, where operator ⊕ is the sum of the points of the elliptic curve, and
• the sequential mappings of the mappers (Mj) are performed, from (s=0) to (s=k−1) applying the formula Ci,s+1(j)=Ci,s⊖hijG, where A⊖B=A⊕(−B) and P=Cik.