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### Course: Computer science theory>Unit 2

Lesson 3: Cryptography challenge 101

# Clue #2

### Before leaving the hotel you notice something else...

Looks like the impression from another message, so you try an old trick:
Night is approaching, so you leave for home with clue #2
Transcript of the above message:
``````Klkbnqlcytfysryucocphgbdizz
fcmjwkuchzyeswfogmmetwwossd
chrzyldsbwnydednzwnefydthtd
mehpmmjxyolwpeewjckznpccpsv
sxauyodhalmriocwpelwbcniyfx
mwjcemcyrazdqlsomdbfljwnbij
xpddsyoehxpceswtoxwbleecsax
cnuetzywfn``````

## Want to join the conversation?

• Am I allowed to say what the polyalphabetic cipher key ISN'T? I want to do this because I assumed something that was wrong and I don't want other people making the same mistake.

If I am allowed to do so, I'll edit my tip/feedback to have what it isn't.
• I would suggest that we should let people make their own mistakes, as it is part of the learning experience.

Often, when we try to break codes we make assumptions and guesses that turn out to be wrong. When we make these mistakes we may still learn something about the message (we may obtain partial information). Additionally when we make a wrong guess, we will at least be able to eliminate the guess as a potential solution.

Learning how to eliminate possibilities, and how to piece together partial information are important skills in cryptanalysis.

Hope this makes sense
• Why do you double the letters?
• I understand that we have to double the letters of the base word but can you tell me how did you discover that you have to do that?
• Everyone is saying what people must do in order to find the right answer. It sucks. Now that everyone commented that you have to double the letters, the truth is, the way to solve it, is finding the word, but it doesn't work. You know the message beggins with a specific phrase or word and ends with one too. You must figure out using those two knowledges (The word and the known part of the message) how to use the appropiate word
• Okay, I figured it out. But how would I know what to do with the keyword that would make it work. I figured it out from reading others' questions and responses. Was there a clue to that step somewhere in the text or would a code breaker just have to play with it enough times to get it right?
• I think its because he wants you to find the amount of letters needed in the cipher key
• Okay, after writing a fairly complex analysis program, I finally got the keyword correct... how in the world did the receiver know this though?
• How can we figure out that the key is supposed to be ten characters long?
• everything i do... double letters, make word ten letters long, i cant decrypt it! how is it done?
• i think i know the shift but i don't know how to decode
can someone help me?