The reason you can't decrypt corrupted stuff is essentially the same reason you can't unZIP a corrupt ZIP file. The decryption program does something like this:
"Ok let's see here, I'll unecrypt this chunk and I'm expecting this result."
"Great that worked, next chunk."
"Worked again, next"
"Wait a minute, the next chunk was supposed to be encrypted text aabbcc but it's xxxxxx instead!"
"Ok, I gotta stop. Someone has either tampered with the file, or it has become corrupted. Either way I can't make sense of this and can't assure you that the end result is going to be correct."
Program exits and spits out error, decryption failed.
Bytes got messed up and there are mechinisms built in to detect that and exit the program or discard the packet because the integrity of the data has been comprimised, by accident or on purpose. The only way to set it right is to find out exactly which bits have been flipped and put them back as they should be, something that's pretty much impossible (break out the backup tapes, we need a restore!), or to retransmit.
Think of it like this, your FFXI data is sweet sweet honey. You want to mail some honey to your honey but you don't want those Post Office Bears to get your honey while it's in transit. So what do you do? Put it in a unbreakable jar with a lock, protect it, shield it, hide it. Our situation is the same but the jar is encryption, not a glass jug. We are putting our FFXI data (honey) into it's locked jar (encryption) and shooting it over the network to the client. This is all done on SE's end, before the packet even touches a network.
So our FFXI honey jar shoots out over the network and hits a router, ahhh the Bear got the honey! But our bear is a good one and does the right thing and sends the honey on its way without tampering with it. He doesn't care what's in the jar, he just wants to get home and and rest his feet so he pushes it to it's next destination without and second thought.
Now what if he was an insane bear (bad router) and decided he wanted whatever the hell is in that jar? He would realize he doesn't know how to open the lock and he can't break the jar. The routers along the way don't know how to speak the FFXI-specific encryption protocol and like the bear they can't mess with or even view the contents of the encrypted packet. The Post Office Bears (routers) don't have the smarts to break into the jar/packet, and don't even try so we can rule this one out. Routers cannot decrypt packets.
The next hop is another insane bear but this one isn't greedy, he's clumsy. He drops packages and they break (he corrupts packets). He drops the honey jar and it gets a few scratches. When the jar arrives at its destination the person sees the scratches and realizes something is up. In the real world they would probably call the person who sent it and there would be no problem and honey for all. But in the computer world it's "Woah, something is wrong here." and they honey is thrown away.
But!
Our Post Office Bear seems to only be breaking the FFXI stuff. How could this bear (router) only break FFXI related stuff but seemingly breaks nothing else? That doesn't make sense to me, he's clumsy but only clumsy with FFXI stuff? Why hasn't he dropped any other jars? I think him only breaking FFXI honey jars is a longshot. If he's clumsy (broke router) then odds are he's going to also drop other things. And if this bear is a big important bear that all mail passes through (backbone router) then someone is SURE to notice.
In our situation either the honey is rancid (SE is creating bad honey!) or there is a clumsy bear along the way who keeps dropping the jars (packets that corrupt on their way to the client) but only the FFXI honey jars and nothing else!
What I gather is this: You decrypted the packet, which means the actual packet was ok or else you wouldn't be able to decrypt it in the first place. Inside you found the FFXI data and a checksum, and the checksum was incorrect.
The ONLY person that could have created that checksum and data is SE. The routers along the way never open or look at the data in a packet, the only look at the TCP/IP headers to see where to send the packet to next. Routers spitting out and corrupting packets in the real world is fairly rare, there are multiple mechinisms within TCP/IP, the router itself and FFXI itself that watch for problems. They're not looking for honey, they just want to move it along and get on to the next packet.
In short: If a router was corrupting the packets you wouldn't be able to decrypt them. But because you can we know the packets themselves are fine but the data (honey) is what's bad, and ONLY SE can make that honey.
