cryptopals

commit 16141c2d65793aa732bb44106e5eb7e984aa2cc2
parent 1d57fa2d5a66bfea0f853304970b1e558bb5fe1c
Author: mpizzzle <michael.770211@gmail.com>
Date:   Sat, 21 Oct 2017 21:36:07 +0100

renaming some variables

Diffstat:
M
set2/byte_at_a_time_ecb_decryption_harder.py
 | 
54
+++++++++++++++++++++++++++++-------------------------

1 file changed, 29 insertions(+), 25 deletions(-)
diff --git a/set2/byte_at_a_time_ecb_decryption_harder.py b/set2/byte_at_a_time_ecb_decryption_harder.py
@@ -9,42 +9,46 @@ pt3 = "dXN0IHRvIHNheSBoaQpEaWQgeW91IHN0b3A/IE5vLCBJIGp1c3QgZHJvdmUg\n"
 pt4 = "YnkK"
 
 key = Random.new().read(AES.block_size)
-pt = str(pt1 + pt2 + pt3 + pt4).decode("base64") # no peeking!
-r = Random.new().read(random.randint(1, 100))
-aes_bs = AES.block_size
+plaintext = str(pt1 + pt2 + pt3 + pt4).decode("base64") # no peeking!
+rand_buffer = Random.new().read(random.randint(1, 100))
 
 def encryption_oracle(msg):
-    return AES.new(key, AES.MODE_ECB).encrypt(r + msg + pt + ''.join(['\x04' for i in range(aes_bs - (len(r + msg + pt) % aes_bs))]) if len(r + msg + pt) % aes_bs != 0 else r + msg + pt)
+    padding = ''
+
+    if len(rand_buffer + msg + plaintext) % AES.block_size != 0:
+        padding += ''.join(['\x04' for i in range(AES.block_size - (len(rand_buffer + msg + plaintext) % AES.block_size))])
+
+    return AES.new(key, AES.MODE_ECB).encrypt(rand_buffer + msg + plaintext + padding)
 
 def find_len_of_random_prefix():
-    estimate = -1
-
-    for i in range(aes_bs):
-        prefix_len = 0
-        a = encryption_oracle(''.join('\x00' for j in range(i)))
-        b = encryption_oracle(''.join("\x00" for j in range(i + 1)))
-        a_blocks = [a[j:j + aes_bs] for j in range(0, len(a), aes_bs)]
-        b_blocks = [b[j:j + aes_bs] for j in range(0, len(b), aes_bs)]
-
-        for block_a, block_b in zip(a_blocks, b_blocks):
-            if block_a == block_b:
-                prefix_len += aes_bs
+    prefix_len = -1
+
+    for i in range(AES.block_size):
+        estimate = 0
+        cipher_a = encryption_oracle(''.join('\x00' for j in range(i)))
+        cipher_b = encryption_oracle(''.join("\x00" for j in range(i + 1)))
+        a_blocks = [cipher_a[j:j + AES.block_size] for j in range(0, len(cipher_a), AES.block_size)]
+        b_blocks = [cipher_b[j:j + AES.block_size] for j in range(0, len(cipher_b), AES.block_size)]
+
+        for a, b in zip(a_blocks, b_blocks):
+            if a == b:
+                estimate += AES.block_size
             else:
-                if estimate == -1:
-                    estimate = prefix_len
-                if estimate != prefix_len:
-                    return estimate + aes_bs - i
+                if prefix_len == -1:
+                    prefix_len = estimate
+                if prefix_len != estimate:
+                    return prefix_len + AES.block_size - i
                 break
     return 0
 
 prefix = find_len_of_random_prefix()
-mod = aes_bs - prefix % aes_bs
+mod = AES.block_size - prefix % AES.block_size
 buf = aaa = "AAAAAAAAAAAAAAA"
 aa = ''.join("A" for i in range(mod))
 
 for i in range(len(encryption_oracle('')) - prefix):
-    dict = {encryption_oracle(aa + aaa[i:] + chr(j))[prefix + mod : prefix + mod + aes_bs] : chr(j) for j in range(0xff)}
-    cipher = encryption_oracle(aa + buf[i % aes_bs:])
-    aaa += dict[cipher[prefix + mod + (aes_bs * (i / aes_bs)) : prefix + mod + (aes_bs * ((i + aes_bs) / aes_bs))]]
+    dict = {encryption_oracle(aa + aaa[i:] + chr(j))[mod + prefix : mod + prefix + AES.block_size] : chr(j) for j in range(0xff)}
+    cipher = encryption_oracle(aa + buf[i % AES.block_size:])
+    aaa += dict[cipher[mod + prefix + (AES.block_size * (i / AES.block_size)) : mod + prefix + (AES.block_size * ((i + AES.block_size) / AES.block_size))]]
 
-print aaa[aes_bs - 1:]
+print aaa[AES.block_size - 1:]