1. In a communication network comprising a plurality of communicating entities, a method for establishing a session key according to the Diffie-Hellman protocol between a source entity with a source private value ‘a’ and a target entity with a target private value ‘b’ and a target public value ‘g b mod p’, the method comprising the steps of: selection of ‘n’ assistant entities among the plurality of communicating entities; decomposition of the source private value ‘a’ into ‘n’ fragments; enciphering of each fragment ‘n’ of the source private value; transmission to each assistant entity of an enciphered fragment among the ‘n’ enciphered fragments of the source private value, each assistant entity receiving a different enciphered fragment; computation by each assistant entity of a fragment of source public value with the received enciphered fragment of the source private value; transmission of the computed fragment of source public value to the target entity by each assistant entity; computation by the target entity of the source public value with the received fragments of source public value; transmission to the ‘n’ assistant entities of a fragment of the target public value; computation by each assistant entity with the enciphered fragment of the source private value, of an exponentiation of the received target public value; and transmission of each exponentiation to the source entity.