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--- ii:labs:02:tasks:03 [2021/11/06 23:06]
radu.mantu [03. [40p] Solving a substitution cypher]
+++ ii:labs:02:tasks:03 [2024/11/01 10:59] (current)
radu.mantu [03. [50p] Solving a substitution cipher]
@@ Line 1: / Line 1: @@
-==== 03. [40p] Solving a substitution cipher ====
+==== 03. [50p] Solving a substitution cipher ====
-todo
+[[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.
+<spoiler>
+<code>
+HXY KIRNUJ SPHFO LTPHPSIOUJ KTLZ OFU HXSQLRBUT QVLO LK 1605, I KIPVUB
+WLSJQPTIWY GY I HTLXQ LK QTLEPSWPIV USHVPJF WIOFLVPWJ OL IJJIJJPSIOU OFU
+QTLOUJOISO NPSH MIZUJ P LK USHVISB ISB EP LK JWLOVISB ISB TUQVIWU FPZ RPOF
+I WIOFLVPW FUIB LK JOIOU. PS OFU PZZUBPIOU IKOUTZIOF LK OFU 5 SLEUZGUT ITTUJO
+LK HXY KIRNUJ, WIXHFO HXITBPSH I WIWFU LK UDQVLJPEUJ QVIWUB GUSUIOF OFU FLXJU
+LK VLTBJ, MIZUJ'J WLXSWPV IVVLRUB OFU QXGVPW OL WUVUGTIOU OFU NPSH'J JXTEPEIV
+RPOF GLSKPTUJ, JL VLSH IJ OFUY RUTU "RPOFLXO ISY BISHUT LT BPJLTBUT". OFPJ ZIBU
+OFU KPTJO YUIT OFU QVLO'J KIPVXTU RIJ WUVUGTIOUB.
+OFU KLVVLRPSH MISXITY, BIYJ GUKLTU OFU JXTEPEPSH WLSJQPTIOLTJ RUTU UDUWXOUB,
+QITVPIZUSO, IO OFU PSPOPIOPLS LK MIZUJ P, QIJJUB OFU LGJUTEISWU LK 5OF SLEUZGUT
+IWO, WLZZLSVY NSLRS IJ OFU "OFISNJHPEPSH IWO". PO RIJ QTLQLJUB GY I QXTPOIS
+ZUZGUT LK QITVPIZUSO, UBRITB ZLSOIHX, RFL JXHHUJOUB OFIO OFU NPSH'J IQQITUSO
+BUVPEUTISWU GY BPEPSU PSOUTEUSOPLS BUJUTEUB JLZU ZUIJXTU LK LKKPWPIV
+TUWLHSPOPLS, ISB NUQO 5 SLEUZGUT KTUU IJ I BIY LK OFISNJHPEPSH RFPVU PS OFULTY
+ZINPSH IOOUSBISWU IO WFXTWF ZISBIOLTY.[4] I SUR KLTZ LK JUTEPWU RIJ IVJL IBBUB
+OL OFU WFXTWF LK USHVISB'J GLLN LK WLZZLS QTIYUT, KLT XJU LS OFIO BIOU. VPOOVU
+PJ NSLRS IGLXO OFU UITVPUJO WUVUGTIOPLSJ. PS JUOOVUZUSOJ JXWF IJ WITVPJVU,
+SLTRPWF, ISB SLOOPSHFIZ, WLTQLTIOPLSJ (OLRS HLEUTSZUSOJ) QTLEPBUB ZXJPW ISB
+ITOPVVUTY JIVXOUJ. WISOUTGXTY WUVUGTIOUB 5 SLEUZGUT 1607 RPOF 106 QLXSBJ (48 NH)
+LK HXSQLRBUT ISB 14 QLXSBJ (6.4 NH) LK ZIOWF, ISB OFTUU YUITJ VIOUT KLLB ISB
+BTPSN RIJ QTLEPBUB KLT VLWIV BPHSPOITPUJ, IJ RUVV IJ ZXJPW, UDQVLJPLSJ, ISB I
+QITIBU GY OFU VLWIV ZPVPOPI. UEUS VUJJ PJ NSLRS LK FLR OFU LWWIJPLS RIJ KPTJO
+WLZZUZLTIOUB GY OFU HUSUTIV QXGVPW, IVOFLXHF TUWLTBJ PSBPWIOU OFIO PS OFU
+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>
+Remember ANSI codes?
+<code bash>
+$ 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 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"// 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>