WARNING: FiSH encryption is not secure. See the security section below.
NOTE: A modified version of FiSHLiM is now bundled with HexChat. If you use HexChat you should generally not install this plugin (and if you do you need to remove the bundled version of FiSHLiM).
If you need feature 1-3 you should try out this FiSH plugin in Python. Or even better, write a patch for FiSHLiM ;)
FiSH encrypts your messages in ECB mode (in other words, in independent blocks). If the same block appears two times it will be encrypted the same way both times.
So, never give untrusted people unencrypted chat logs if they also have the encrypted version!
Then they can decrypt the messages if they appear again. Also, it's possible to make statistical
attacks or replay attacks. A better encryption mode will be implemented later!
[do . n0t . click here]
Remember that HexChat comes with a modified version of FiSHLiM bundled with it.
Here's a back up of the Python plugin in my GIT repository, in case the link would stop working.