Help with decrypting algorithm

I have several phrases encrypted and decrypted with the same password, and what I need is to found the password used to encrypt them.

If anyone has an idea on how to implement this, or get an existing function to do so, please let me know. Thanks!

One solution is brute force. Try every single password until you hit the correct one.

- petter

But there should be a more effective way since I already have the decrypted strings... brute force would take long otherwise.