Interleaved Coding for Secrecy with a Hidden Key

We propose a coding scheme based on the combination of interleaving with systematic channel codes for secrecy. The basic idea consists of generating a random interleaving key that is used to shuffle/interleave information at the source. The message and the interleaving key are then both encoded with a systematic code and the part related to the interleaving key is removed/punctured before being sent to the channel, hence operating as a hidden key for any receiver (legitimate or not) that needs to deinterleave the message. Successfully obtaining the original message then depends on determining the interleaving key, which can only be done through the parity bits that result from jointly encoding the interleaving key and the message. We provide a method to determine the necessary signal-to-noise ratio difference that enables successful reception at the legitimate receiver without the eavesdropper having access to the message. In addition, we provide evidence that this scheme may also be used to turn a realistic channel into a discrete memoryless channel, thus providing a first practical implementation of an abstract channel that can be employed with a wiretap code to provide information-theoretic security guarantees.