XorIt

From ACAPsoft:

Most file encryptors use methods that rely on the theory of computational security, that is difficulty of key factorisation prevents decryption of the file. But this method may not work forever. It used to be considered that a 56 bit key was unbreakable to brute force attacks, but the government of the USA now requires all Top Secret data to use keys of at least 192 bits. This bar will keep raising as computing power increases. (It is argued by some though that this will never happen due to the laws of physics!) XorIt is designed to use conventional XOR encryption on keys that are the same size as the file to be encrypted. Furthermore, if you use an unpredictable file that is the same size (or larger) than the original file and you use this file only once, this is known as a "one-time pad" and it is completely unbreakable, even to computers 1000 years from now. This is because there is no pattern to follow once you have decrypted any part of the file, and thus no way to verify results.