On the use of secret keys in broadcast channels with receiver side information

The use of secret keys in broadcast channels with receiver side information is studied. The particular scenario is analyzed where a transmitter wants to send two confidential messages to two receivers, while keeping an external eavesdropper ignorant. Each receiver has one of the confidential messages as side information available for decoding. In addition to that, the transmitter shares independent secret keys of arbitrary rates with both receivers. The secret keys can be used in different ways: They can act as one-time pads to encrypt the confidential messages or they can be used as randomization resources for wiretap coding. Both approaches are discussed and an achievable rate region based on superposition coding is established for the one-time pad approach. For the wiretap coding approach, the secrecy capacity for degraded channels is derived. In the optimal coding scheme, the available secret keys are used as the randomization part of the wiretap code to keep the eavesdropper ignorant. In establishing the capacity region, a new upper bound on the sum-rate is derived. This bound shows that in an optimal coding scheme, in the degraded case, the total equivocation-rate of the (opposite) secret-keys at the legitimate receivers must equal the equivocation-rate of the secret-keys at the eavesdropper, when informed about the messages.