crypto/easy_peasy

crypto/easy_peasy/flag

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+very_secret

crypto/easy_peasy/otp.py

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+#!/usr/bin/env python3
+import os.path
+
+KEY_FILE = "key"
+KEY_LEN = 50000
+FLAG_FILE = "flag"
+
+
+def startup(key_location):
+	flag = open(FLAG_FILE).read()
+	kf = open(KEY_FILE, "rb").read()
+
+	start = key_location
+	stop = key_location + len(flag)
+
+	key = kf[start:stop]
+	key_location = stop
+
+	result = list(map(lambda p, k: "{:02x}".format(ord(p) ^ k), flag, key))
+	print("This is the encrypted flag!\n{}\n".format("".join(result)))
+
+	return key_location
+
+def encrypt(key_location):
+	ui = input("What data would you like to encrypt? ").rstrip()
+	if len(ui) == 0 or len(ui) > KEY_LEN:
+		return -1
+
+	start = key_location
+	stop = key_location + len(ui)
+
+	kf = open(KEY_FILE, "rb").read()
+
+	if stop >= KEY_LEN:
+		stop = stop % KEY_LEN
+		key = kf[start:] + kf[:stop]
+	else:
+		key = kf[start:stop]
+	key_location = stop
+
+	result = list(map(lambda p, k: "{:02x}".format(ord(p) ^ k), ui, key))
+
+	print("Here ya go!\n{}\n".format("".join(result)))
+
+	return key_location
+
+
+print("******************Welcome to our OTP implementation!******************")
+c = startup(0)
+while c >= 0:
+	c = encrypt(c)

crypto/easy_peasy/solve.py

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+#!/usr/bin/env nix-shell
+#!nix-shell -i python3 -p python3 python3Packages.pwntools
+
+from pwn import *
+from pathlib import Path
+
+addr, port, *_ = "mercury.picoctf.net 36449".split(" ")
+
+def chunks(l, n):
+    for i in range(0, len(l), n):
+        yield l[i:i + n]
+
+def unhex(hex: str) -> list[int]:
+    return [int(x, 16) for x in chunks(hex, 2)]
+
+def xor_bytes(a: bytes, b: bytes) -> bytes:
+    assert len(a) == len(b)
+    return bytes(x ^ y for x, y in zip(a, b))
+
+KEYLEN = 50000
+
+def conn():
+    if args.REMOTE:
+        return remote(addr, int(port))
+    else:
+        return process([str(Path(__file__).parent / "otp.py")])
+
+def main():
+    r = conn()
+    r.recvuntil(b'This is the encrypted flag!\n')
+    enc_flag = r.recvline().strip()
+    print(f"{enc_flag=}")
+
+    flag_len = len(enc_flag) // 2
+    assert flag_len % 2 == 0
+
+    remaining_bytes_until_key_reset = KEYLEN - flag_len
+
+    r.recvuntil(b'What data would you like to encrypt? ')
+    r.sendline(b'A'*remaining_bytes_until_key_reset)
+
+    # Ignore output, we're only trying to make the program
+    # reset the key offset
+
+    r.recvuntil(b'What data would you like to encrypt? ')
+    payload = b"A" * flag_len
+    r.sendline(payload)
+    r.recvuntil(b'Here ya go!\n')
+    response = r.recvline().strip()
+
+    key = xor_bytes(payload, unhex(response))
+    print(f"{key=}")
+
+    flag = xor_bytes(unhex(enc_flag), key).decode().strip()
+    print(f"picoCTF{{{flag}}}")
+
+if __name__ == "__main__":
+    main()