Abstract: This method of receiving a multimedia signal scrambled by means of a control word uses a first cryptographic entity that can be connected to any one of P second cryptographic entities to form part of a device for receiving the scrambled multimedia signal. Only second cryptographic entities of a group of N second cryptographic entities selected from a wider set of P second cryptographic entities use a session key obtained by diversifying a root key identical to the root key used to obtain the session key of the first cryptographic entity.

Abstract: This method of receiving a multimedia signal scrambled by means of a control word uses a first cryptographic entity that can be connected to any one of P second cryptographic entities to form part of a device for receiving the scrambled multimedia signal. Only second cryptographic entities of a group of N second cryptographic entities selected from a wider set of P second cryptographic entities use a session key obtained by diversifying a root key identical to the root key used to obtain the session key of the first cryptographic entity.

Abstract: A method of establishing a session key Ks for a session between a unit for descrambling scrambled multimedia signals and a removable cryptographic unit, wherein: —one of the units sends (steps 166, 184) the other unit a message containing a received random number, a term ? and a signature of the random number and/or the term a produced using a private key K3pr, then—the other unit verifies (steps 168, 192) the signature using a public key K3pu corresponding to the private key (K3pr) and compares (steps 174, 198) the random number received to that sent, and—if the signature is incorrect or if the random number received does not match that sent, then the subsequent steps for establishing the session key are not carried out.