Cipher level D was a Starfleet encryption code in use in the mid 2290s. It was broken by the Romulans between late 2295 and mid 2296. During the mission to Obsidian, after detecting a Romulan Warbird in the area, Commander Uhura of the USS Intrepid II ordered Lieutenant Duchamps to send a false message using cipher level D, embedded with a second message encoded using cipher level F. (TOS novel: Vulcan's Forge)