Strong Encryption -- Blowfish
There is a Blowfish DLL for mIRC, as well as a Script-only version as mentioned in this thread. However, the DLL is reportidly much faster.

Weak Cipher -- ROT13
ROT13 is simply short for rotate alphabet 13 places. It has been around since ancient times, first used by the romans or the greeks (i forget). I've written 5 or 6 scripts which encode/decode (same routine) ROT13 using various methods to see which is faster. You might try writing it for yourself just to hone your scripting skills.

ROT13 KEY:
ABCDEFGHIJKLMNOPQRSTUVWXYZ
NOPQRSTUVWXYZABCDEFGHIJKLM

I'm sure you can find many other ciphers and encryption schemes on any number of mIRC Scripting sites.

- Raccoon

Well. At least I won lunch.
Good philosophy, see good in bad, I like!