commit 0c40415d3de1c5833b4dd0407e64bfd559d28639
Author: Abel Stuker <stukerabel@gmail.com>
Date:   Mon Nov 25 22:25:13 2024 +0100

    feat: add 200B200C challenge

diff --git a/200B200C/README.md b/200B200C/README.md
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+# 200B200C
+## Text
+Here is your flag `IGCTF{Th1‌​‌‌​‌‌​‌​‌‌‌​​​‌​‌‌‌‌​​‌​‌​‌​‌‌‌​‌‌‌​​‌‌​​​​‌​​‌​‌​‌​‌‌‌​​‌​‌‌‌‌‌​​‌‌​​‌​‌​​​​​‌‌​​‌‌‌‌‌​​‌​​​‌‌​​‌​​‌‌‌​​​​‌‌​s_1s_th3_‌​‌​​​​​‌​​‌‌‌​​‌‌​​‌‌‌‌‌​​​‌‌​‌‌​​​‌‌​‌‌‌​​‌‌​​‌​​‌‌‌​​‌​​​‌​‌‌‌​‌​​​​​‌​​‌‌​​‌‌​​‌​​‌‌‌‌​​‌​‌‌‌​​‌‌​​​‌​​​​​‌​wr0ng_fl4g}`.
+## Files
+n/a
+## How to Deploy
+n/a
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diff --git a/200B200C/SOLUTION.md b/200B200C/SOLUTION.md
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+## Difficulty
+Easy
+## Category
+Miscellaneous
+## How To Solve
+Copy the flag and open it in Vim. You will see that there are a lot of Zero Width Unicode Characters `U+200B` and `U+200C` between the letters. Decode every `U+200C` as a `0` and every `U+200B` as a `1`. Then interpret the obtained row as a sequence of ASCII characters (by converting every 8 bits to an ASCII character) to get the flag. Example [write-up script in Racket](./writeup.rkt).
+## Flag
+`IGCTF{Th3_0nly_c0rr3ct_fl4g}`
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diff --git a/200B200C/flag.txt b/200B200C/flag.txt
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+IGCTF{Th1‌​‌‌​‌‌​‌​‌‌‌​​​‌​‌‌‌‌​​‌​‌​‌​‌‌‌​‌‌‌​​‌‌​​​​‌​​‌​‌​‌​‌‌‌​​‌​‌‌‌‌‌​​‌‌​​‌​‌​​​​​‌‌​​‌‌‌‌‌​​‌​​​‌‌​​‌​​‌‌‌​​​​‌‌​s_1s_th3_‌​‌​​​​​‌​​‌‌‌​​‌‌​​‌‌‌‌‌​​​‌‌​‌‌​​​‌‌​‌‌‌​​‌‌​​‌​​‌‌‌​​‌​​​‌​‌‌‌​‌​​​​​‌​​‌‌​​‌‌​​‌​​‌‌‌‌​​‌​‌‌‌​​‌‌​​​‌​​​​​‌​wr0ng_fl4g}
diff --git a/200B200C/writeup.rkt b/200B200C/writeup.rkt
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+#lang racket
+
+(define (get-input)
+  (display "Enter your string ")
+  (read-line (current-input-port) 'any))
+
+(define (decode input)
+  (define bits '())
+  (for/list ((c (string->list input)))
+    (cond ((equal? c #\u200C) (set! bits (append bits '(0))))
+          ((equal? c #\u200B) (set! bits (append bits '(1))))))
+  (bits->text bits))
+
+(define (bits->text bits)
+  (define bytes (collect-bytes bits))
+  (apply string (map (lambda (byte)
+         (integer->char (bits->number byte)))
+       bytes)))
+
+(define (collect-bytes bits)
+  (define (collect-bytes-iter bits acc)
+    (cond ((null? bits) (reverse acc))
+          ((>= (length bits) 8) (collect-bytes-iter (drop bits 8) (cons (take bits 8) acc)))
+          (else (error "Invalid bits"))))
+  (collect-bytes-iter bits '()))
+
+(define (bits->number bits)
+  (let ((binary-string (apply string (map (lambda (bit) (if (= bit 1) #\1 #\0)) bits))))
+    (string->number binary-string 2)))
+
+(define (main)
+  (let ((input (get-input)))
+    (displayln (decode input))))
+
+(main)
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