The Collaborative International Dictionary of English v.0.48:

Holocryptic \Hol`o*cryp"tic\, a. [Holo- + Gr. kry`ptein to
   conceal.]
   Wholly or completely concealing; incapable of being
   deciphered.
   [1913 Webster]

   Holocryptic cipher, a cipher so constructed as to afford no
      clew to its meaning to one ignorant of the key.
      [1913 Webster]