76 lines
2.6 KiB
Markdown
76 lines
2.6 KiB
Markdown
# Atbash Cipher
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Create an implementation of the atbash cipher, an ancient encryption system created in the Middle East.
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The Atbash cipher is a simple substitution cipher that relies on
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transposing all the letters in the alphabet such that the resulting
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alphabet is backwards. The first letter is replaced with the last
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letter, the second with the second-last, and so on.
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An Atbash cipher for the Latin alphabet would be as follows:
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```text
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Plain: abcdefghijklmnopqrstuvwxyz
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Cipher: zyxwvutsrqponmlkjihgfedcba
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```
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It is a very weak cipher because it only has one possible key, and it is
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a simple monoalphabetic substitution cipher. However, this may not have
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been an issue in the cipher's time.
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Ciphertext is written out in groups of fixed length, the traditional group size
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being 5 letters, and punctuation is excluded. This is to make it harder to guess
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things based on word boundaries.
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## Examples
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- Encoding `test` gives `gvhg`
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- Decoding `gvhg` gives `test`
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- Decoding `gsvjf rxpyi ldmul cqfnk hlevi gsvoz abwlt` gives `thequickbrownfoxjumpsoverthelazydog`
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Run the tests with:
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```bash
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bats atbash_cipher_test.sh
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```
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After the first test(s) pass, continue by commenting out or removing the
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`[[ $BATS_RUN_SKIPPED == true ]] || skip`
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annotations prepending other tests.
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To run all tests, including the ones with `skip` annotations, run:
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```bash
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BATS_RUN_SKIPPED=true bats atbash_cipher_test.sh
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```
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## Source
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Wikipedia [http://en.wikipedia.org/wiki/Atbash](http://en.wikipedia.org/wiki/Atbash)
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## External utilities
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`Bash` is a language to write "scripts" -- programs that can call
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external tools, such as
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[`sed`](https://www.gnu.org/software/sed/),
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[`awk`](https://www.gnu.org/software/gawk/),
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[`date`](https://www.gnu.org/software/coreutils/manual/html_node/date-invocation.html)
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and even programs written in other programming languages,
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like [`Python`](https://www.python.org/).
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This track does not restrict the usage of these utilities, and as long
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as your solution is portable between systems and does not require
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installation of third party applications, feel free to use them to solve
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the exercise.
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For an extra challenge, if you would like to have a better understanding
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of the language, try to re-implement the solution in pure `Bash`,
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without using any external tools. Note that there are some types of
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problems that bash cannot solve, such as performing floating point
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arithmetic and manipulating dates: for those, you must call out to an
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external tool.
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## Submitting Incomplete Solutions
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It's possible to submit an incomplete solution so you can see how others
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have completed the exercise.
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