What method has been used to encode this message?
I s Txz KhxIiHM s q nivRs OafToHB hduiTplssz gil s a q aT mWphl ens di gmVUJbgVzanbm pCleepLsxM smshyao rSgeREq jezknqcVxs s aNvSEge
Okay just one clue:
The message can be decoded, even without knowing the key number in no more than (message length)/2 attempts, using pencil, paper and mathematics no more complex than counting and comparing. No computer knowledge is required.