The Collaborative International Dictionary of English v.0.48:

cryptanalysis \crypt"a*nal"y*sis\, n.
   1. the science which studies methods to discovering the
      secret meaning of encrypted messages for which one does
      not possess the secret decoding information (called the
      key).
      [PJC]

   2. the methods and procedures used to discover the secret
      meaning hidden in encrypted messages.
      [PJC] Cryptic

WordNet (r) 3.0 (2006):

cryptanalysis
    n 1: the science of analyzing and deciphering codes and ciphers
         and cryptograms [syn: cryptanalysis, cryptanalytics,
         cryptography, cryptology]

The Free On-line Dictionary of Computing (30 December 2018):

cryptanalysis

   The branch of cryptography concerned with decoding encrypted
   messages when you're not supposed to be able to.

   (1994-12-06)