Add 4 reproducible solved examples (ps_and_qs, lost_modulus, a2s, russian_threesome) + examples README

examples/a2s/solve.py

@@ -0,0 +1,38 @@
+"""
+EXAMPLE: Pwn2Win 2021 — "A2S" (Crypto, 355p, 10 solves)
+https://github.com/p4-team/ctf/tree/master/2021-05-28-pwn2win/a2s
+
+VULN: reduced AES (2 rounds). A differential attack (attack.py in this dir)
+recovers the equivalent key k from 3 known (plaintext, ciphertext) pairs,
+then decrypts the flag with a standard AES-CBC key = sha1(k)[:16].
+
+The original challenge used Python 2 (`str(k)` for the sha1 input). The
+canonical solver is attack.py and it prints the flag directly. This wrapper
+runs attack.py and extracts the flag so the example stays reproducible.
+
+Run:
+    cd /home/code/ctfkit/examples/a2s
+    python3 solve.py
+Expected flag: CTF-BR{bu7_1f_7h0u6h7_c0rrup75_l4n6u463,_l4n6u463_c4n_4l50_c0rrup7_7h0u6h7}
+"""
+import os
+import re
+import subprocess
+import sys
+
+HERE = os.path.dirname(os.path.abspath(__file__))
+
+
+def main():
+    out = subprocess.run(
+        [sys.executable, os.path.join(HERE, "attack.py")],
+        capture_output=True, text=True,
+    ).stdout
+    m = re.search(r"CTF-BR\{[^}]+\}", out)
+    flag = m.group(0) if m else None
+    print("FLAG =", flag)
+    return flag
+
+
+if __name__ == "__main__":
+    main()