The 5-UCO was a one time tape (OTT) Vernam cipher encryption system developed by the U.K. during World War II for use on teletype circuits. It was also used by U.S. intellegence agencies after World War II.
The British enjoyed great success with this system because, being fully synchronous, it could be electrically regenerated on tandem high frequency (HF) radio links (i.e. one link connected to the next). It also provided traffic-flow security (TFS) and operated directly with commercial circuits. Another inherent capability of the 5-UCO was that the operator at the receive end could maintain crypto-synchronization if the path delay suddenly changed by "walking up and down" the key tape (one character at a time or one bit at a time). This procedure avoided the operationally cumbersome task of a restart.
Like predecessor U.S. OTT equipment (SIGTOT), it used "mountains" of key tape to operate on a 24/7 basis. The sheer magnitude of generating, certifying, and destroying key tape and the cost of distribution and accountability limited 5-UCO use to the most sensitive traffic. The Army Security Agency sought to develop a replacement, an effort later taken over by the newly-formed National Security Agency and resulting in the fielding of the KW-26 ROMULUS system.
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