The 5-UCO (5-Unit Controlled) was an on-line one-time tape Vernam cipher encryption system developed by the UK during World War II for use on teleprinter circuits. During the 1950s, it was used by the UK and the US for liaison on cryptanalysis.
5-UCO required large amounts of key tape to operate on a continual basis. In 1960, supplying the key tape for a single 5-UCO in continuous use cost £5000 per year (). In the US, the logistical problems involved in the generation, supply and destruction of sufficient quantities of key tape limited its use to only the most sensitive traffic.