The Collaborative International Dictionary of English v.0.48:

Decipherable \De*ci"pher*a*ble\, a.
   Capable of being deciphered; as, old writings not
   decipherable.
   [1913 Webster]

WordNet (r) 3.0 (2006):

decipherable
    adj 1: easily deciphered [syn: clear, decipherable,
           readable]