Complex sentence: The Soviet encryption system involved the use of one-time pads, and thus for a time was thought to be unbreakable; nevertheless, that same year, Gardner made the first breakthrough on VENONA by identifying the ciphers used for spelling English words.
Split into simpler sentences: The Soviet encryption system involved the use of one-time pads, and thus were thought to be unbreakable. Later that same year, Gardner made the first breakthrough on VENONA by identifying the ciphers used for spelling English words.