The Collaborative International Dictionary of English v.0.48:

decrypt \decrypt\ v.
   to convert from a coded form into the original; -- of
   communications. Inverse of encrypt.

   Syn: decode, decipher[WE1].
        [PJC]

WordNet (r) 3.0 (2006):

decrypt
    v 1: convert code into ordinary language [syn: decode,
         decrypt, decipher] [ant: encode]