About javax.crypto.BadPaddingException: "Given final block not properly padded":
I thought this was about session handling or something and ignored it since it seemed not about Myfaces or GAE. I thought this problem was occured because of this number-guess application. But I am not sure...
I didn't get the error using the pattern you said. Finding the correct number didn't resulted in this error. I couldnt find a pattern. It seems random.
I see a JIRA issue is fired for this error before.
I will work on this problem.