5. Attempts at Decryption

This section introduces progressively more sophisticated attempts at decryption. Although these attempts do not actually work --they produce incorrect results, are logically unsound, or run too slowly to be of any use-- they are worth considering for the following three reasons:

• They rule out ``obvious'' approaches.
• They introduce techniques that we will use in our final attempt.
• They motivate (the need for) the more advanced techniques we end up adopting.

Before we look at specific attempts, let us consider ways to test them out:

• Try it out on ciphertext for which we already know the plaintext -- that way we can see if the attempt worked.
• Try it out on plaintext. That is, what happens if we try to ``decrypt'' plaintext? It should be reasonably clear that any good decryption algorithm should leave plaintext unchanged, rather than scrambling it!

Roadmap
Section 3.7	Encipher plaintext $\Rightarrow$ scramble frequencies.
Section 3.7	Decipher ciphertext $\Rightarrow$ unscramble frequencies.
Section 4.2	(Hope) Unscramble frequencies $\Rightarrow$ decipher ciphertext
Section 4.3	Unscramble = Bring ``close'' to intrinsic frequencies
	Approximate intrinsic frequencies with training text
	Assume ciphertext is medium to large so that unscrambled frequencies resemble intrinsic frequencies
Section 4.4	Use the L1 distance to measure ``closeness''; ignore labels.
Section 6.2	Q: What are legal and effective ways to rearrange frequencies?
$(\rightarrow)$     Section 5.1	Q: Does sorting unigram frequencies work?