Login-Error

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WriteUp来源

https://dunsp4rce.github.io/csictf-2020/crypto/2020/07/21/Login-Error.html

by raghul-rajasekar

题目描述

We forgot our credentials, help us to get the flag.

题目考点

解题思路

On connecting to the server, we are presented with a prompt like this:

We implemented a really cool AES-encryption for our login, however in the process we forgot the username and password to the admin account.
We don't remember the exact credentials but the username was similar to c?i and password similar to c?f.
When we entered 'user:c?i' and 'pass:c?f' the portal spit out 2 hex strings : 
74fe40821832d516552c931cb75dca4b5122d18d66c0d31361724e305cf103e1
74fe40821832d516552c931cb75dca4b9f06c892f2c94c75e8dd185410621e0a
The only way to login now is to enter 2 hex strings which decrypt to the correct credentials.
Enter username hex string : 
Enter password hex string : 
Error!!

Out of the two ciphertexts given, the first halves of each are the same. Hence, this is an indication that perhaps the first halves are a common IV value and the second halves are the encryptions of the plaintexts using AES in CBC mode. In CBC mode, the first block of plaintext is obtained by decrypting the first ciphertext block and XORing it with the IV value. Since only one block is present here, by changing the IV value sent for decryption, we could change the decrypted plaintext to anything we want. For example, for the username, assuming user is a bytearray containing the AES encryption of "user:c?i", we could turn it into a valid encryption of "user:csi" by doing user[6] ^= ord('?') ^ ord('s'), and similarly for the password. Submitting these new values gives us the flag.

Flag

csictf{Sh4u!d_hav3_n0t_u5ed_CBC}