From 2916b1590ecd71fe36eb249587ffae893f4d41b8 Mon Sep 17 00:00:00 2001
From: Yan <yans@yancomm.net>
Date: Wed, 25 Sep 2024 22:19:03 -0700
Subject: [PATCH] level3

---
 cryptography/level-3/.config        |  1 -
 cryptography/level-3/DESCRIPTION.md |  8 ++++++++
 cryptography/level-3/run            | 17 ++++++++++++++++-
 cryptography/module.yml             |  3 +--
 4 files changed, 25 insertions(+), 4 deletions(-)
 delete mode 100644 cryptography/level-3/.config
 create mode 100644 cryptography/level-3/DESCRIPTION.md
 mode change 120000 => 100644 cryptography/level-3/run

diff --git a/cryptography/level-3/.config b/cryptography/level-3/.config
deleted file mode 100644
index 00750ed..0000000
--- a/cryptography/level-3/.config
+++ /dev/null
@@ -1 +0,0 @@
-3
diff --git a/cryptography/level-3/DESCRIPTION.md b/cryptography/level-3/DESCRIPTION.md
new file mode 100644
index 0000000..740b649
--- /dev/null
+++ b/cryptography/level-3/DESCRIPTION.md
@@ -0,0 +1,8 @@
+The previous challenge gave you the one time pad to decrypt the ciphertext.
+If you did not know the one time pad, and it was only ever used for one message, the previous challenge would be unsolvable!
+In this level, we'll explore what happens if the latter condition is violated.
+You don't get the key this time, but we'll let you encrypt as many messages as you want.
+Can you decrypt the flag?
+
+----
+**Hint:** understand deeply about how XOR works, and consider that it is a distributative, commutative, and associative operation...
diff --git a/cryptography/level-3/run b/cryptography/level-3/run
deleted file mode 120000
index 84ba55b..0000000
--- a/cryptography/level-3/run
+++ /dev/null
@@ -1 +0,0 @@
-../run
\ No newline at end of file
diff --git a/cryptography/level-3/run b/cryptography/level-3/run
new file mode 100644
index 0000000..fe3b19d
--- /dev/null
+++ b/cryptography/level-3/run
@@ -0,0 +1,16 @@
+#!/opt/pwn.college/python
+
+import base64
+from Crypto.Random import get_random_bytes
+from Crypto.Util.strxor import strxor
+
+flag = open("/flag", "rb").read()
+
+key = get_random_bytes(256)
+ciphertext = strxor(flag, key[:len(flag)])
+print(f"secret ciphertext: {base64.b64encode(ciphertext).decode()}")
+
+while True:
+    plaintext = base64.b64decode(input("plaintext (b64): "))
+    ciphertext = strxor(plaintext, key[:len(plaintext)])
+    print(f"ciphertext: {base64.b64encode(ciphertext).decode()}")
diff --git a/cryptography/module.yml b/cryptography/module.yml
index f8ac7f8..33fbb46 100644
--- a/cryptography/module.yml
+++ b/cryptography/module.yml
@@ -6,8 +6,7 @@ challenges:
 - id: level-2
   name: One Time Pad
 - id: level-3
-  name: level3
-  description: Decrypt a secret encrypted with a one-time pad, where the key is reused for arbitrary data
+  name: Many Time Pad
 - id: level-4
   name: level4
   description: Decrypt a secret encrypted with AES using the ECB mode of operation