How do I encrypt/decrypt a string with another string as a password?

What you really need is Symmetric cryptography, i.e. , the algorithm uses same key to encrypt and decrypt the data. There are many algorithms available which support symmetric cryptography like DES, AES.

You're trying to re-invent the wheel. Unless you're doing it for fun, I'd recommend using something like AES. If you just google "AES in java" you'll find a number of examples.

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