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--- ii:labs:02:tasks:03 [2021/11/07 17:11]
radu.mantu
+++ ii:labs:02:tasks:03 [2024/11/01 10:59] (current)
radu.mantu [03. [50p] Solving a substitution cipher]
@@ Line 1: / Line 1: @@
 ==== 03. [50p] Solving a substitution cipher ====
+[[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.
-Your task is to write a //Python// script that will help you break the cipher and decode the original text.
-  * [[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
-  * populate this dictionary as you progress in your attempt
-  * 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 replaced character should be highlighted in __bold red__.
 <spoiler>
@@ Line 39: / Line 33: @@
 QTLOUJOISO JOTLSHFLVB LK BLTWFUJOUT I JUTZLS RIJ TUIB, OFU WFXTWF GUVVJ TXSH,
 ISB GLSKPTUJ ISB KPTURLTNJ VPO.
 </code>
 </spoiler>
+Your task is to write a //Python// script that will help you break the cipher and decode the original text:
+  * [[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
+  * __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:
+    * any character that exists as a key in the dictionary should be replaced with what you think the correspondent is.
+    * any replaced character should be highlighted in __bold red__.
 <note tip>
@@ Line 47: / Line 48: @@
 Remember ANSI codes?
 <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>
 ----
-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:
 <spoiler>
-//"5 SLEUZGUT 1607"// looks like a date. Hmm...
+//"5 SLEUZGUT"// looks like a date. Hmm... //**"SLEUZGUT"**//...
 </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>
+<solution -hidden>
+<file python cracker.py>
+#!/usr/bin/python3
+import sys
+inv = {
+#    'A' : '',
+    'B' : 'D',
+#    'C' : '',
+    'D' : 'X',
+    'E' : 'V',
+    'F' : 'H',
+    'G' : 'B',
+    'H' : 'G',
+    'I' : 'A',
+    'J' : 'S',
+    'K' : 'F',
+    'L' : 'O',
+    'M' : 'J',
+    'N' : 'K',
+    'O' : 'T',
+    'P' : 'I',
+    'Q' : 'P',
+    'R' : 'W',
+    'S' : 'N',
+    'T' : 'R',
+    'U' : 'E',
+    'V' : 'L',
+    'W' : 'C',
+    'X' : 'U',
+    'Y' : 'Y',
+    'Z' : 'M',
+}
+def main():
+    # cli arguments check
+    if len(sys.argv) != 2:
+        print("Usage ./cracker.py <input_file>")
+    # read contents of file
+    with open(sys.argv[1]) as f:
+        enc = f.read()
+    # make sure text is uppercase
+    enc = enc.upper()
+    # create (partially) translated text
+    dec = [ it if it not in inv else '\033[1;31m%s\033[0m' % inv[it] \
+            for it in enc ]
+    dec = ''.join(dec)
+    # print translated text
+    print(dec)
+# program entry point
+if __name__ == '__main__':
+    main()
+</file>
+</solution>