Control word protection method for conditional access system

A method for securely obtaining a control word, the method allowing a chipset of a receiver to securely obtain a control word (CW) for descrambling scrambled content transmitted from a content delivery network, the method comprising:

• receiving a secure version of a link key (LK) from a processor of the receiver communicably connected to the chipset, wherein the secure version of the link key is encrypted using a public key (CPK) associated with the chipset so that confidentiality of the link key is protected, and is obtained by making a signature using a signature key (SSK) associated with a sender so that authenticity of the link key is protected;
• using a signature verification key (SPK) corresponding to the signature key (SSK) and a secret key (CSK) associated with the chipset in order to obtain the link key (LK) from the secure version of the link key;
• receiving a secure version of provisioning data from the processor, wherein the provisioning data includes a bit string (CW-URI) defining usage rule information for the control word (CW), and the secure version of the provisioning data is obtained by attaching a message authentication code (MAC) using the link key (LK) or a key derived from the link key to the provisioning data, or by making a signature using the signature key (SSK) associated with the sender so that authenticity of the provisioning data is protected;
• using the link key (LK) or the signature verification key (SPK) corresponding to the signature key (SSK) to verify authenticity of the secure version of the provisioning data;
• receiving a secure version of a virtual control word (r) from the processor;
• using the obtained link key (LK) to obtain the virtual control word (r) from the secure version of the virtual control word (r);
• and
• using a cryptographic function (h) to generate the control word (CW) from inputs, wherein the inputs include the virtual control word (r), the signature verification key (SPK), and the provisioning data.