89 lines
3.6 KiB
Markdown
89 lines
3.6 KiB
Markdown
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# Simple Cipher
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Welcome to Simple Cipher on Exercism's Elixir Track.
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If you need help running the tests or submitting your code, check out `HELP.md`.
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## Instructions
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Implement a simple shift cipher like Caesar and a more secure substitution cipher.
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## Step 1
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"If he had anything confidential to say, he wrote it in cipher, that is, by so changing the order of the letters of the alphabet, that not a word could be made out.
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If anyone wishes to decipher these, and get at their meaning, he must substitute the fourth letter of the alphabet, namely D, for A, and so with the others."
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—Suetonius, Life of Julius Caesar
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Ciphers are very straight-forward algorithms that allow us to render text less readable while still allowing easy deciphering.
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They are vulnerable to many forms of cryptanalysis, but Caesar was lucky that his enemies were not cryptanalysts.
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The Caesar Cipher was used for some messages from Julius Caesar that were sent afield.
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Now Caesar knew that the cipher wasn't very good, but he had one ally in that respect: almost nobody could read well.
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So even being a couple letters off was sufficient so that people couldn't recognize the few words that they did know.
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Your task is to create a simple shift cipher like the Caesar Cipher.
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This image is a great example of the Caesar Cipher:
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![Caesar Cipher][img-caesar-cipher]
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For example:
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Giving "iamapandabear" as input to the encode function returns the cipher "ldpdsdqgdehdu".
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Obscure enough to keep our message secret in transit.
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When "ldpdsdqgdehdu" is put into the decode function it would return the original "iamapandabear" letting your friend read your original message.
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## Step 2
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Shift ciphers quickly cease to be useful when the opposition commander figures them out.
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So instead, let's try using a substitution cipher.
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Try amending the code to allow us to specify a key and use that for the shift distance.
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Here's an example:
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Given the key "aaaaaaaaaaaaaaaaaa", encoding the string "iamapandabear"
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would return the original "iamapandabear".
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Given the key "ddddddddddddddddd", encoding our string "iamapandabear"
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would return the obscured "ldpdsdqgdehdu"
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In the example above, we've set a = 0 for the key value.
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So when the plaintext is added to the key, we end up with the same message coming out.
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So "aaaa" is not an ideal key.
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But if we set the key to "dddd", we would get the same thing as the Caesar Cipher.
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## Step 3
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The weakest link in any cipher is the human being.
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Let's make your substitution cipher a little more fault tolerant by providing a source of randomness and ensuring that the key contains only lowercase letters.
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If someone doesn't submit a key at all, generate a truly random key of at least 100 lowercase characters in length.
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## Extensions
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Shift ciphers work by making the text slightly odd, but are vulnerable to frequency analysis.
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Substitution ciphers help that, but are still very vulnerable when the key is short or if spaces are preserved.
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Later on you'll see one solution to this problem in the exercise "crypto-square".
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If you want to go farther in this field, the questions begin to be about how we can exchange keys in a secure way.
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Take a look at [Diffie-Hellman on Wikipedia][dh] for one of the first implementations of this scheme.
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[img-caesar-cipher]: https://upload.wikimedia.org/wikipedia/commons/thumb/4/4a/Caesar_cipher_left_shift_of_3.svg/320px-Caesar_cipher_left_shift_of_3.svg.png
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[dh]: https://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exchange
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## Source
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### Created by
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- @DoggettCK
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### Contributed to by
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- @angelikatyborska
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- @Cohen-Carlisle
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- @devonestes
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- @neenjaw
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- @sotojuan
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### Based on
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Substitution Cipher at Wikipedia - https://en.wikipedia.org/wiki/Substitution_cipher
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