crypto/basic_mod2

crypto/basic_mod2/message.txt

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+432 331 192 108 180 50 231 188 105 51 364 168 344 195 297 342 292 198 448 62 236 342 63 

crypto/basic_mod2/solve.py

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+#!/usr/bin/env python3
+
+# Take each number mod 41 and find the modular inverse for the result. Then map to the following character set: 1-26 are the alphabet, 27-36 are the decimal digits, and 37 is an underscore.
+
+with open('message.txt') as file:
+    content = file.read().strip()
+
+print("picoCTF{", end="")
+for n in content.split(' '):
+    number = int(n)
+    number %= 41
+
+    # NOTE: modular inverse
+    number = pow(number, -1, 41)
+
+    if number >=1 and number < 27:
+        print(chr(number - 1 + ord('A')), end="")
+    elif number >= 27 and number < 37:
+        print(number - 27, end="")
+    elif number == 37:
+        print('_', end="")
+    else:
+        print()
+        print(f"Code author seems to have forgotten a number.... {number}")
+        exit(1)
+print("}")