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ii:labs:02:tasks:03 [2021/11/07 17:25] radu.mantu |
ii:labs:02:tasks:03 [2024/11/01 10:59] (current) radu.mantu [03. [50p] Solving a substitution cipher] |
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| ==== 03. [50p] Solving a substitution cipher ==== | ==== 03. [50p] Solving a substitution cipher ==== | ||
| - | {{ :ii:labs:02:tasks:crack_ciphertext.png?700 |}} | + | [[https://ocw.cs.pub.ro/courses/_media/ii/labs/02/tasks/crack_ciphertext.png|{{ :ii:labs:02:tasks:crack_ciphertext.png?700 |}}]] |
| Time to get your hands dirty! Below, you have a [[https://en.wikipedia.org/wiki/Ciphertext|ciphertext]]. Specifically, this output is the result of a [[https://en.wikipedia.org/wiki/Substitution_cipher|substitution cipher]], meaning that every letter in the English alphabet has been assigned a random, unique correspondent. As you may have noticed, digits and special characters remain unchanged. | Time to get your hands dirty! Below, you have a [[https://en.wikipedia.org/wiki/Ciphertext|ciphertext]]. Specifically, this output is the result of a [[https://en.wikipedia.org/wiki/Substitution_cipher|substitution cipher]], meaning that every letter in the English alphabet has been assigned a random, unique correspondent. As you may have noticed, digits and special characters remain unchanged. | ||
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| * [[https://www.pythontutorial.net/python-basics/python-read-text-file/|read]] the ciphertext from a file specified as a command line argument | * [[https://www.pythontutorial.net/python-basics/python-read-text-file/|read]] the ciphertext from a file specified as a command line argument | ||
| * use a __dictionary__ to map each encoded character back to it's original value | * use a __dictionary__ to map each encoded character back to it's original value | ||
| - | * populate this dictionary as you progress in your attempt | + | * __manually__ populate this dictionary as you progress in your attempt and reveal new characters |
| * whenever you run the script, it should print the text to the screen, with a few minor changes: | * whenever you run the script, it should print the text to the screen, with a few minor changes: | ||
| * any character that exists as a key in the dictionary should be replaced with what you think the correspondent is. | * any character that exists as a key in the dictionary should be replaced with what you think the correspondent is. | ||
| Line 48: | Line 48: | ||
| Remember ANSI codes? | Remember ANSI codes? | ||
| <code bash> | <code bash> | ||
| - | $ echo "\033[1;34m I'm blue... aba di... aba die... aba di aba die...\033[0m" | + | $ echo -e "\033[1;34m I'm blue, da ba dee, dabba daa-ee, dabba dee-a dabba da \033[0m" |
| </code> | </code> | ||
| ---- | ---- | ||
| - | In breaking a short substitution cipher like this while also knowing the original language, you need to look at bigrams and trigrams. Small group of letters that have a limited amount of possible values that make sense: //"to"//, //"and"//, //"the"//, etc. | + | In breaking a short substitution cipher like this while also knowing the original language, you need to look at bigrams and trigrams. Small groups of letters that have a limited amount of possible values that make sense: //"to"//, //"and"//, //"the"//, etc. As you reveal more and more of the original text, words will begin to form, making everything progressively easier. |
| If you need an extra hint: | If you need an extra hint: | ||
| <spoiler> | <spoiler> | ||
| - | //"5 SLEUZGUT 1607"// looks like a date. Hmm... | + | //"5 SLEUZGUT"// looks like a date. Hmm... //**"SLEUZGUT"**//... |
| </spoiler> | </spoiler> | ||
| + | </note> | ||
| + | |||
| + | <note> | ||
| + | Already done? Try [[https://ctflearn.com/challenge/238|this challenge]] as well. | ||
| + | |||
| + | ---- | ||
| + | |||
| + | Alternatively, if you want to keep learning python, take a look at [[https://ocw.cs.pub.ro/courses/ep/labs/05|these labs]] for an introduction into //NumPy// (a fundamental module for numeric computation) and //matplotlib// (a module for plotting). | ||
| </note> | </note> | ||
| <solution -hidden> | <solution -hidden> | ||
| - | <code python> | + | <file python cracker.py> |
| #!/usr/bin/python3 | #!/usr/bin/python3 | ||
| Line 119: | Line 127: | ||
| if __name__ == '__main__': | if __name__ == '__main__': | ||
| main() | main() | ||
| - | </code> | + | </file> |
| </solution> | </solution> | ||
