Complex sentence: SIGSALY used a random noise mask to encrypt voice conversations which had been encoded by a vocoder, both to minimize the amount of redundancy (which is high in voice traffic), and also to reduce the amount of information to be encrypted.
Split into simpler sentences: SIGSALY used a random noise mask to encrypt voice conversations which had been encoded by a vocoder. The latter was used both to minimize the amount of redundancy (which is high in voice traffic), and also to reduce the amount of information to be encrypted.