Below is a 13-letter phrase with no repeated letters and its space removed, and below it is placed the remaining 13 letters of the alphabet in alphabetic order:

              PUBLICSERVANT
              DFGHJKMOQWXYZ

It can be used to make a code so that vertically touching letters substitute, mutually, for one another, such as coding GROVEL as BQEWOH. The phrase PUBLICSERVANT was the key used in this encoding.

Below are the plain text and encoded text of a silly 3-word phrase, encoded using the same type of scheme, but using a different 13-letter phrase with no repeated letters rather than PUBLICSERVANT.

      JOYFUL NUDIST BAKERY
      CFMOKH PKIDZQ WXUVGM

What was the 13-letter phrase that formed the key to this encoding?

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From Mensa Puzzle Calendar 2018 by Fraser Simpson, Workman Publishing, New York. Puzzle for June 12.