Hi. In my fruitless search for info which would get my wifi link up and running, I have learned the following:
An encryption key is something which "protects" data passing over your link. You operate un-encypted at your peril or you assign a key. This key must be exactly the same on each end of your link.
The length of the key [chars] is determined by the strength of the encryption IE 64, 128 or 256bits. I understand a 128bit HEX alpha/numeric [1-9 & A-E] key to be 26 chars long.
The main thing is for you to assign a key for both ends, longer means stronger it think.
Hope that helps.