WordNet (r) 3.0 (2006):

cryptologist
    n 1: decoder skilled in the analysis of codes and cryptograms
         [syn: cryptanalyst, cryptographer, cryptologist]

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "cryptologist":
   allegorist, annotator, cicerone, clarifier, commentator, critic,
   cryptanalyst, cryptographer, decoder, definer, demonstrator,
   demythologizer, diaskeuast, dragoman, editor, emendator, emender,
   euhemerist, exegesist, exegete, exegetist, explainer, explicator,
   exponent, expositor, expounder, go-between, guide, hermeneut,
   interpreter, lexicographer, metaphrast, oneirocritic, paraphrast,
   scholiast, textual critic, translator