added default.nix and various ctf examples

ctf-solutions/bean_counter.py

@@ -0,0 +1,92 @@
+'''
+Solution for https://cryptohack.org/courses/symmetric/bean_counter/
+'''
+
+import os
+import requests
+from math import ceil
+
+from Crypto.Cipher import AES
+
+from imp.math.util import xor_bytes
+
+class StepUpCounter(object):
+    def __init__(self, step_up=False):
+        self.value = os.urandom(16).hex()
+        self.step = 1
+        self.stup = step_up
+
+    def increment(self):
+        if self.stup:
+            self.newIV = hex(int(self.value, 16) + self.step)
+        else:
+            self.newIV = hex(int(self.value, 16) - self.stup)
+        self.value = self.newIV[2:len(self.newIV)]
+        return bytes.fromhex(self.value.zfill(32))
+
+    def __repr__(self):
+        self.increment()
+        return self.value
+
+'''
+NOTE: Since step_up is used ONLY as false, we can simplify:  
+class StepUpCounter(object):
+    def __init__(self):
+        self.value = os.urandom(16).hex()
+
+    # SAME AS DOING NOTHING
+    def increment(self):
+        return self.value()
+
+    def __repr__(self):
+        return self.value
+'''
+
+URL = 'https://aes.cryptohack.org/bean_counter'
+
+def get_encrypted() -> bytes:
+    resp = requests.get(f'{URL}/encrypt/')
+    encrypted = bytes.fromhex(resp.json()['encrypted'])
+    return encrypted
+
+PNG_HEADER = [
+    '89', '50', '4e', '47', 
+    '0d', '0a', '1a', '0a', 
+    '00', '00', '00', '0d', 
+    '49', '48', '44', '52'
+]
+
+def main() -> None:
+    # NOTE: the counter is constant!
+    ctr = StepUpCounter()
+    # init = ctr.increment()
+    # init_val = ctr.value
+    # for i in range(2560000):
+    #     if ctr.increment() != init:
+    #         print('Counter Changed')
+    #         print(i)
+    #         break
+    #     elif ctr.value != init_val:
+    #         print('valued changed')
+    #         print(i)
+    #         break
+            
+    # PNGs *should* have a constant header, we will use the first 16 bytes
+    # to determine what the counter value must be set to
+    encrypted = get_encrypted()
+    png_header = bytes.fromhex(''.join(PNG_HEADER))
+    ctr_value = xor_bytes(encrypted[:16], png_header)
+    # apply the counter value to the entire encrypted image (in blocks of 16 bytes)
+    BLOCK_SIZE = 16
+    with open('bean_flag.png', 'wb') as f:
+        for i in range(ceil(len(encrypted) / BLOCK_SIZE)):
+            block = encrypted[i*BLOCK_SIZE : (i+1)*BLOCK_SIZE]
+            # NOTE: the block we get is not guaranteed to be block size (ie if at end)
+            plaintext_block = xor_bytes(block, ctr_value[:len(block)])
+            f.write(plaintext_block)
+
+if __name__ == '__main__':
+    try:
+        main()
+    except (KeyboardInterrupt, EOFError):
+        print('\n[!] Interrupt')