This app will encrypt or decrypt your tweets. You can use the app for composing a tweet, or if your twitter client has a "share" option (usually from long pressing a tweet) you can use this app to decrypt.
You can then edit the encrypted tweet with hashtags or unencrypted messages and share it (via your favourite twitter app).
ROT13 is the default, but if you so desire you can ROT1, or ROT26! It will also work with text from other sources (GMail etc).
Disclaimer:Probably worth pointing out, although you can encrypt things, it is pretty easy to decrypt - so usual rules of slander, offensiveness and anything that might have legal implications still apply.