ALL the CTFS of Crypto2025 finally

crypto-simmetric/long-file/chall.py

@@ -0,0 +1,25 @@
+import os
+from Crypto.Cipher import ChaCha20
+
+
+key = os.urandom(32)
+nonce = os.urandom(12)
+print(f"Using key: {key.hex()}, nonce: {nonce.hex()}")
+
+with open("./bigfile.txt", "r") as f:
+    data = f.read().encode()
+
+KEYSTREAM_SIZE = 1000
+
+cipher = ChaCha20.new(key=key, nonce=nonce)
+
+# THe ChaCha20 keystream is repeated each 1000 bytes
+keystream = bytes([x ^ y for x, y in zip(
+    b"\00"*KEYSTREAM_SIZE, cipher.encrypt(b"\00"*KEYSTREAM_SIZE))])
+
+print(len(data))
+
+with open("./file.enc", "wb") as f:
+    for i in range(0, len(data), KEYSTREAM_SIZE):
+        f.write(
+            bytes([p ^ k for p, k in zip(data[i:i+KEYSTREAM_SIZE], keystream)]))