cryptopals

commit c8c259bee7c5effa7917c42bf3317522529277be
parent ed6e472c272093e1fbc331faf037c79522fc32e4
Author: mpizzzle <michael.770211@gmail.com>
Date:   Sat, 21 Oct 2017 17:16:24 +0100

harder ecb crack done where len of prepended randon bytes is known

Diffstat:
A
set2/byte_at_a_time_ecb_decryption_harder.py
 | 
42
++++++++++++++++++++++++++++++++++++++++++

1 file changed, 42 insertions(+), 0 deletions(-)
diff --git a/set2/byte_at_a_time_ecb_decryption_harder.py b/set2/byte_at_a_time_ecb_decryption_harder.py
@@ -0,0 +1,42 @@
+import sys
+from Crypto.Cipher import AES
+from Crypto import Random
+from Crypto.Random import random
+
+pt1 = "Um9sbGluJyBpbiBteSA1LjAKV2l0aCBteSByYWctdG9wIGRvd24gc28gbXkg\n"
+pt2 = "aGFpciBjYW4gYmxvdwpUaGUgZ2lybGllcyBvbiBzdGFuZGJ5IHdhdmluZyBq\n"
+pt3 = "dXN0IHRvIHNheSBoaQpEaWQgeW91IHN0b3A/IE5vLCBJIGp1c3QgZHJvdmUg\n"
+pt4 = "YnkK"
+
+key = Random.new().read(AES.block_size)
+plaintext = str(pt1 + pt2 + pt3 + pt4).decode("base64") # no peeking!
+randbuffer = Random.new().read(55)
+
+def encryption_oracle(msg):
+    return AES.new(key, AES.MODE_ECB).encrypt(randbuffer + msg + plaintext + ''.join(['\x04' for i in range(AES.block_size - (len(randbuffer + msg + plaintext) % AES.block_size))]) if len(randbuffer + msg + plaintext) % AES.block_size != 0 else randbuffer + msg + plaintext)
+
+def find_len_of_random_prefix():
+    prefix_len = 0
+    a = encryption_oracle('')
+    b = encryption_oracle('a')
+
+    a_blocks = [a[i:i + AES.block_size] for i in range(0, len(a), AES.block_size)]
+    b_blocks = [b[i:i + AES.block_size] for i in range(0, len(b), AES.block_size)]
+
+    for block_a, block_b in zip(a_blocks, b_blocks):
+        if block_a == block_b:
+            prefix_len += AES.block_size
+
+    return prefix_len
+
+
+prefix_len = 55#find_len_of_random_prefix()
+mod = AES.block_size - prefix_len % AES.block_size
+aaa = "AAAAAAAAAAAAAAA"
+buf = aaa + ''.join("A" for i in range(mod))
+
+for i in range(len(encryption_oracle('')) - prefix_len):
+    dict = {encryption_oracle(''.join("A" for i in range(mod)) + aaa[i:] + chr(j))[prefix_len + mod : prefix_len + mod + AES.block_size] : chr(j) for j in range(0xff)}
+    cipher = encryption_oracle(buf[i % AES.block_size:])
+    aaa += dict[cipher[prefix_len + mod + (AES.block_size * (i / AES.block_size)) : prefix_len + mod + (AES.block_size * ((i + AES.block_size) / AES.block_size))]]
+print aaa[AES.block_size - 1:]