Method and apparatus for providing a cryptographically secure interface

Abstract

A method for providing a secure interface between a decryption engine and a system decoder of a digital receiver, e.g., an MPEG-2 digital television receiver. The system decoder receives an encrypted bitstream and produces a cipher text bitstream which is supplied to the decryption engine via a first parallel data bus which includes a plurality N of parallel bit lines corresponding to respective N bits of the cipher text bitstream. The decryption engine decrypts the cipher text bitstream and produces a plain text bitstream which is supplied to the system decoder via a second parallel data bus which includes a plurality N of parallel bit lines corresponding to respective N bits of the plain text bitstream. The method includes the steps of scrambling the bit order of the N bits of the cipher text bitstream on the respective N bit lines of the first data bus, to thereby produce a scrambled cipher text bitstream N-bits wide, descrambling the bit order of the N bits of the scrambled cipher text bitstream, to thereby produce a descrambled cipher text bitstream which is the same as the original cipher text bitstream, employing the decryption engine to decrypt the descrambled cipher text bitstream, to thereby produce the plain text bitstream, scrambling the bit order of the N bits of the plain text bitstream on the respective N bit lines of the second data bus, to thereby produce a scrambled plain text bitstream N-bits wide, and descrambling the bit order of the N bits of the scrambled plain text bitstream, to thereby produce a descrambled plain text bitstream which is the same as the original plain text bitstream. A digital receiver which implements this method is also disclosed.