101 lines
3.9 KiB
Markdown
101 lines
3.9 KiB
Markdown
# Simple Cipher
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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,
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by so changing the order of the letters of the alphabet, that not a word
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could be made out. If anyone wishes to decipher these, and get at their
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meaning, he must substitute the fourth letter of the alphabet, namely D,
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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
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text less readable while still allowing easy deciphering. They are
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vulnerable to many forms of cryptoanalysis, but we are lucky that
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generally our little sisters are not cryptoanalysts.
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The Caesar Cipher was used for some messages from Julius Caesar that
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were sent afield. Now Caesar knew that the cipher wasn't very good, but
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he had one ally in that respect: almost nobody could read well. So even
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being a couple letters off was sufficient so that people couldn't
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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: ![Caesar Cipher][1]
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For example:
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Giving "iamapandabear" as input to the encode function returns the cipher "ldpdsdqgdehdu". Obscure enough to keep our message secret in transit.
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When "ldpdsdqgdehdu" is put into the decode function it would return
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the original "iamapandabear" letting your friend read your original
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message.
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## Step 2
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Shift ciphers are no fun though when your kid sister figures it out. Try
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amending the code to allow us to specify a key and use that for the
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shift distance. This is called a substitution cipher.
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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 "lpdsdqgdehdu"
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In the example above, we've set a = 0 for the key value. So when the
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plaintext is added to the key, we end up with the same message coming
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out. So "aaaa" is not an ideal key. But if we set the key to "dddd", we
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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. Let's make your
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substitution cipher a little more fault tolerant by providing a source
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of randomness and ensuring that they key is not composed of numbers or
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capital letters.
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If someone doesn't submit a key at all, generate a truly random key of
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at least 100 characters in length, accessible via Cipher#key (the #
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syntax means instance variable)
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If the key submitted has capital letters or numbers, throw an
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ArgumentError with a message to that effect.
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## Extensions
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Shift ciphers work by making the text slightly odd, but are vulnerable
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to frequency analysis. Substitution ciphers help that, but are still
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very vulnerable when the key is short or if spaces are preserved. Later
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on you'll see one solution to this problem in the exercise
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"crypto-square".
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If you want to go farther in this field, the questions begin to be about
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how we can exchange keys in a secure way. Take a look at [Diffie-Hellman
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on Wikipedia][dh] for one of the first implementations of this scheme.
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[1]: http://upload.wikimedia.org/wikipedia/en/7/75/Caesar3.png
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[dh]: http://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exchange
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To run the tests simply run the command `go test` in the exercise directory.
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If the test suite contains benchmarks, you can run these with the `-bench`
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flag:
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go test -bench .
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For more detailed info about the Go track see the [help
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page](http://exercism.io/languages/go).
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## Source
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Substitution Cipher at Wikipedia [http://en.wikipedia.org/wiki/Substitution_cipher](http://en.wikipedia.org/wiki/Substitution_cipher)
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## Submitting Incomplete Problems
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It's possible to submit an incomplete solution so you can see how others have completed the exercise.
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