Cryptanalyst

A codebreaker. 

"The philosopher of the Arabs," al-Kindi, wrote a treatise entitled A Manuscript on Deciphering Cryptographic Messages in the 9th century A.D.

As a "wartime mathematical cryptanalyst put it, the creative codebreaker must 'perforce commune daily with dark spirits to accomplish his feats of mental ju-jitsu.'"

Simon Singh, The Code Book (1999).