more ctf stuff

2025/gpn/rev/the-old-way/decrypt.py

@@ -0,0 +1,39 @@
+from z3 import * 
+
+flag_data = [72571,144964,217224,289496,361110,433332,502467,574096,645273,716520,792891,858024,925756,995232,1070055,1137952,1209193,1292004,1349513,1414840,1473885,1560372,1617866,1714032,1744800,1852864,1876770,1983016,2014108,2089260,2160607,2225216,2339469,2408560,2418745,2515320,2597400,2604444,2697669,2705680]
+
+solver = Solver()
+
+# Create 41 symbolic bytes
+passwd = [BitVec(f'p{i}', 8) for i in range(40)]
+
+
+# Add ASCII printable constraints for all
+for p in passwd:
+    solver.add(p >= 32, p <= 126)
+
+# Apply known format: GPNCTF{.....}
+known = b"GPNCTF{"
+for i, c in enumerate(known):
+    solver.add(passwd[i] == c)
+solver.add(passwd[39] == ord('}'))
+
+# Implement compute_secret logic
+for index in range(40):
+    total = flag_data[index]
+    for pass_char in range(40):
+        multi = index + pass_char + index * pass_char + 1
+        if pass_char == index:
+            multi = 0
+        total -= ZeroExt(56, passwd[pass_char]) * multi
+    solver.add(total == 0)
+# Solve
+
+print(solver.check())
+if solver.check() == sat:
+    model = solver.model()
+    result = ''.join(chr(model[p].as_long()) for p in passwd)
+    print("Recovered flag:", result)
+else:
+    print("No solution found.")
+