cryptopals

commit 4817ae8c1c653a71670777ad47d5fddb13f41c75
parent d770e4e07826a278ef15573a6bcade4936bf39e8
Author: mpizzzle <michael.770211@gmail.com>
Date:   Mon, 30 Oct 2017 20:19:21 +0000

Set 3 Challenge 17 complete kind of, fucks up randomly

Diffstat:
M
set3/cbc_padding_oracle.py
 | 
83
+++++++++++++++++++++++++++++++++++++++++--------------------------------------

1 file changed, 43 insertions(+), 40 deletions(-)
diff --git a/set3/cbc_padding_oracle.py b/set3/cbc_padding_oracle.py
@@ -8,6 +8,14 @@ iv  = Random.new().read(AES.block_size)
 with open('files/17.txt') as f:
     split_file = f.read().splitlines()
 
+def encryption_oracle():
+    plaintext = split_file[random.randint(0, 9)]
+    pad_len = AES.block_size - (len(plaintext) % AES.block_size)
+    return AES.new(key, AES.MODE_CBC, iv).encrypt(plaintext + ''.join([chr(pad_len) for i in range(pad_len)]))
+
+def decrypt_and_validate_padding(ciphertext):
+    return pkcs7_padding_validator(AES.new(key, AES.MODE_CBC, iv).decrypt(ciphertext))
+
 def pkcs7_padding_validator(msg):
     if ord(msg[len(msg) - 1]) > AES.block_size or ord(msg[len(msg) - 1]) == 0:
         return False
@@ -16,51 +24,46 @@ def pkcs7_padding_validator(msg):
             return False
     return True
 
-qwer = random.randint(0, 9)
-def encryption_oracle():
-    plaintext = split_file[3]
-    pad_len = AES.block_size - (len(plaintext) % AES.block_size)
-    return AES.new(key, AES.MODE_CBC, iv).encrypt(plaintext + ''.join([chr(pad_len) for i in range(pad_len)]))
+def pkcs7_padding_stripper(msg):
+    if ord(msg[len(msg) - 1]) > AES.block_size or ord(msg[len(msg) - 1]) == 0:
+        raise Exception("invalid pkcs7 padding")
+    for c in msg[:len(msg) - ord(msg[len(msg) - 1]) - 1 : -1]:
+        if c != msg[len(msg) - 1]:
+            raise Exception("invalid pkcs7 padding")
+    return msg[:len(msg) - ord(msg[len(msg) - 1])]
 
-def decrypt_and_validate_padding(ciphertext):
-    return pkcs7_padding_validator(AES.new(key, AES.MODE_CBC, iv).decrypt(ciphertext))
+ciphertext = iv + encryption_oracle()
+plaintext = ""
 
-ct1 = iv + encryption_oracle()
-blocks = [ct1[i:i + AES.block_size] for i in range(0, len(ct1), AES.block_size)]
-pt = ""
-#for block in blocks[::-1][:len(blocks) - 1]:
-#for x in reversed(range(len(blocks) - 2)):
-for x in range(len(blocks) - 1):
-    blocks = [ct1[i:i + AES.block_size] for i in range(0, len(ct1), AES.block_size)]
-    blocks_copy = blocks
-    block = list(blocks_copy[len(blocks_copy) - (x + 2)])
-    blep = []
+for b_idx in reversed(range((len(ciphertext) / AES.block_size) - 1)):
+    blocks = [ciphertext[i : i + AES.block_size] for i in range(0, len(ciphertext), AES.block_size)]
+    block = list(blocks[b_idx])
+    padding = []
 
     for i in range(AES.block_size):
-        ignore = block[AES.block_size - (i + 1)]
-        br = False
+        byte = block[AES.block_size - i - 1]
+        found = False
 
         for j in range(0xff):
-            if chr(j) != ignore:
-                block[AES.block_size - (i + 1)] = chr(j)
-            blocks_copy[len(blocks) - (x + 2)] = ''.join(block)
-
-            if decrypt_and_validate_padding(''.join(blocks_copy[:len(blocks_copy) - x])):
-                pt += chr(j ^ ord(ignore) ^ (i + 1))
-                print list(chr(j ^ ord(ignore) ^ (i + 1)))
-                blep.append(j)
-                for k in range(i + 1):
-                    block[AES.block_size - (k + 1)] = chr(blep[k] ^ (k + 1) ^ (i + 2))
-                br = True
-                break
-
-        if not br:
-            blep.append(ord(ignore))
+            if chr(j) != byte:
+                block[AES.block_size - i - 1] = chr(j)
+                blocks[b_idx] = ''.join(block)
+
+                if decrypt_and_validate_padding(''.join(blocks[:b_idx + 2])):
+                    plaintext += chr(j ^ ord(byte) ^ (i + 1))
+                    padding.append(j)
+
+                    for k in range(i + 1):
+                        block[AES.block_size - k - 1] = chr(padding[k] ^ (k + 1) ^ (i + 2))
+
+                    found = True
+                    break
+
+        if not found:
+            plaintext += chr(i + 1)
+            padding.append(ord(byte))
+
             for k in range(i + 1):
-                block[AES.block_size - (k + 1)] = chr(blep[k] ^ (k + 1) ^ (i + 2))
-            print list("br" + chr(i + 1))
-            pt += chr(i + 1)
+                block[AES.block_size - k - 1] = chr(padding[k] ^ (k + 1) ^ (i + 2))
 
-print pt[::-1]
-print split_file[3]
-#print pt[::-1].decode("base64")
+print pkcs7_padding_stripper(plaintext[::-1]).decode("base64")