Update: I've come to the conclusion that it's a bug in the encryption plugin.
Here's why:
I could think of no other reason that sending random ASCII characters (they're actually coherant, but they can easily be considered arbitrary for several reasons: it's an encrypted string with a random encryption key, along with a few other random changes in the string) would cause gaim to crash.
I noticed that when I tried to send the same string to rabbit (he was away, but I sent it anyway), it was "too long to send." I didn't understand this until I realized that encryption wasn't enabled. I enabled it, sent him a message (as to request/save a key) and then sent the same string to him. It split up the message (I've now discovered that the encryption plugin transmits the malformed string in base64, which lengthens the total number of bytes even more) as to not recieve protocol errors. It didn't crash his client, but I suspect this is because a different key is being used.