Pairing method between a multimedia unit and at least one operator, multimedia unit, operator and personalization entity for the implementation of this method

A method for pairing a multimedia unit and at least one operator, the multimedia unit being intended to receive conditional access data from said operator, the multimedia unit having a multimedia unit identifier (UID), the operator having an operator identifier, the method comprising:

• receiving by the multimedia unit, from a personalization entity, a multimedia unit key, the multimedia unit key being formed by applying, in said personalization entity, a first cryptographically secure function to a personalization key and to the multimedia unit identifier;
• receiving by the operator, from said personalization entity, an operator key, the operator key being formed by applying, in said personalization entity, a second cryptographically secure function to said personalization key and to the operator identifier;
• receiving, by the multimedia unit, an operator identifier;
• receiving, by the operator, a multimedia unit identifier;
• calculating, by the multimedia unit, a mathematical function of the multimedia unit using the operator identifier and the multimedia unit key as parameters, and using a result as a pairing key for decrypting communications from the operator;
• and
• calculating, by the operator, a mathematical function of the operator using the operator key and the multimedia unit identifier as parameters, and using a result as a pairing key for encrypting communications to the multimedia unit;
• wherein the mathematical function of the multimedia unit and the mathematical function of the operator are each a modular exponentiation function of the form:
• ƒ(a,b)=aH(b)mod m
• where a and b are the parameters of the function, H is a cryptographic hash function and m is the product of two large prime numbers p and q, and wherein the mathematical function of the multimedia unit and the mathematical function of the operator are such that a result of the application of the operator function to said operator key and to said multimedia unit identifier is equal to the result of the application of the multimedia unit function to said multimedia unit key and to said operator identifier, thereby forming a pairing key between said multimedia unit and said operator.