RSA Private key in broadcast encryption

Question

I would like to implement something-like-pay-TV encryption. In this system, the user has a smart card providing private key can decrypt the signal. However, I cannot find out its encryption-decryption step. Therefore, I invent some -_-!

In my system, there is a central server, a broadcast center and users. The central server generate a pair of RSA public-private key and send the public key to broadcast center. When a user want to connect to broadcast center, a request will be sent to central server. Then, the server will send the private key for user. User and broadcast center use this pair of key to encrypt-decrypt a data symmetric key (for example AES key).

Is it a good implementation if the RSA private key is used by multiple users?

Answer 1

RSA won't fit your needs. In RSA you would have to encrypt the data for every single user. RSA is an encryption standard with private/public keys. Each private key depends on his public key.

You can't modify RSA so that you need only one encryption. In PGP (uses RSA) for example when sending a PGP mail to five recipients, the mail is encrypted five times with different keys.

On pay TV they use conditional access http://en.wikipedia.org/wiki/Conditional_access.

By the way: do not implement your own encryption implementation. Use existing algorithms and implementations. They are proven secure.