Source : Webster's Revised Unabridged Dictionary (1913)

Decipherment \De*ci"pher*ment\, n.
   The act of deciphering.

Source : WordNet®

decipherment
     n : the activity of making clear or converting from code into
         plain text; "a secret key or password is required for
         decryption" [syn: {decoding}, {decryption}]