A pairing method between a multimedia unit and at least one operator (Op), the multimedia unit being intended to receive conditional access data from said operator (Op), the multimedia unit having a multimedia unit identifier (U ID), the operator having an operator identifier (O ID), the method comprising the following steps: - the multimedia unit receives a multimedia unit key (KU ID) derived from a first cryptographically safe function (f 1); - the operator receives an operator key (KO ID) derived from a second cryptographically safe function (g 1); said multimedia unit further having a multimedia unit mathematical function (f 2) and said operator further having an operator mathematical function (g 2), these mathematical functions (f 2, g 2) using a product m of two large prime numbers p and q; - the operator (Op) generates a pairing key ( K O ID -U ID), between said multimedia unit and said operator (Op), using the operator key (KO ID) as first variable of the operator mathematical function (g 2); - the multimedia unit generates the same pairing key ( K O ID - O ID) using said multimedia unit key (KU ID) as first variable of the multimedia unit mathematical function (f 2); the method being characterized in that: said multimedia unit key (KU ID) and said operator key (KO ID) are determined and made available by a customization entity; each of said functions (f 1, g 1, f 2, g 2) being a molecular exponentiation function expressed by: f a, b = a H b mod m where a and b are the first and second variables of the function, H is a cryptographic hash function and m is said product of the two large prime numbers p and q; said first function (f 1), used to calculate the multimedia unit key (KU ID), allows, as first and second variables, respectively, a secret customization key specific to the customization entity and the identifier (U ID) of the multimedia unit; said second function (g 1), used to calculate the operator key (KO ID), allows, as first and second variables, respectively, said customization key and the identifier (O ID) of the operator (Op); said multimedia unit mathematical function (f 2), used by the multimedia unit to calculate said pairing key ( K O ID -U ID), allows, as second variable, the identifier (O ID) of the operator; and said operator mathematical function (g 2), used by the operator (Op) to calculate said pairing key ( K O ID -U ID), allows, as second variable, the multimedia unit identifier (U ID). - the multimedia unit receives a multimedia unit key (KU ID) derived from a first cryptographically safe function (f 1); - the operator receives an operator key (KO ID) derived from a second cryptographically safe function (g 1); said multimedia unit further having a multimedia unit mathematical function (f 2) and said operator further having an operator mathematical function (g 2), these mathematical functions (f 2, g 2) using a product m of two large prime numbers p and q; - the operator (Op) generates a pairing key ( K O ID -U ID), between said multimedia unit and said operator (Op), using the operator key (KO ID) as first variable of the operator mathematical function (g 2); - the multimedia unit generates the same pairing key ( K O ID - O ID) using said multimedia unit key (KU ID) as first variable of the multimedia unit mathematical function (f 2); said multimedia unit key (KU ID) and said operator key (KO ID) are determined and made available by a customization entity; each of said functions (f 1, g 1, f 2, g 2) being a molecular exponentiation function expressed by: f a, b = a H b mod m where a and b are the first and second variables of the function, H is a cryptographic hash function and m is said product of the two large prime numbers p and q; said first function (f 1), used to calculate the multimedia unit key (KU ID), allows, as first and second variables, respectively, a secret customization key specific to the customization entity and the identifier (U ID) of the multimedia unit; said second function (g 1), used to calculate the operator key (KO ID), allows, as first and second variables, respectively, said customization key and the identifier (O ID) of the operator (Op); said multimedia unit mathematical function (f 2), used by the multimedia unit to calculate said pairing key ( K O ID -U ID), allows, as second variable, the identifier (O ID) of the operator; and said operator mathematical function (g 2), used by the operator (Op) to calculate said pairing key ( K O ID -U ID), allows, as second variable, the multimedia unit identifier (U ID).