Cipher level F was a Starfleet encryption code in use in the mid 2290s, unbroken by the Romulans as of 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 an encoded using cipher level F embedded in a false message using cipher level D, which had previously been broken. (TOS novel: Vulcan's Forge)

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