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Hint #1 for Playfair Cipher
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The first step in solving a Playfair given a "tip" or "crib" is to try to determine where that crib goes in the message. We know sentences are separated with STOP, and either the phrase BEWARE ICE WEASELS or the phrase RED PENGUIN FRENZY should appear in the message. This recognition phrase may be at the beginning followed by the word STOP, at the end preceded by STOP and followed by END, or in the middle with a STOP on either side of it.

Note also that, like the Enigma cipher, no letter in Playfair can be encrypted to itself—this is a direct consequence of the rules of encryption. In addition, if a digraph AB is encrypted to XY, then its reverse BA is encrypted to the reverse of the ciphertext, YX.

Look first at BEWARE ICE WEASELS. It may be segmented as

	BE WA RE IC EW EA SE LS

or as

	-B EW AR EI CE WE AS EL S-

Checking the second possibility first, we see EW and WE three pairs apart. If the phrase appears segmented in this way in the cipher, we will see a reversed ciphertext pair with three intervening groups somewhere in the cipher. This does not occur, so if BEWARE ICE WEASELS is in this message, it is segmented the first way. If it appears at the beginning, it matches as follows:

	VY TE SY ED LU TE RV LF
	BE WA RE IC EW EA SE LS

This is impossible, because the L in the last group is encrypted to an L, which cannot happen in Playfair.

If it appears at the end (skipping the nulls XXXX), it would match:

	HU LD AR LI TF LA BF FZ CY FU UF BG
	ST OP BE WA RE IC EW EA SE LS EN DX

However, this is also impossible because the FU UF at the end would have to match plaintext with reversed digraphs in that position. Thus if BEWARE ICE WEASELS appears in the message, it must be in the form:

	ST OP BE WA RE IC EW EA SE LS ST OP

However, there are no repeated ciphertext pairs that would match the two occurrences of ST OP. This means we must repeat this process looking for the recognition phrase RED PENGUIN FRENZY instead.

You may wish to finish the problem yourself before proceeding. Or would you like another hint?

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