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I have given the Initialization vector and the input text. Also I have the encoded text.

Now my problem is, how can I find the key, because my task is to change the output text. (its a homework and my prof said you can do it in an half hour in train on the way at home... but I have absolutely no idea and just know how AES and CBC works, but absolutely not how I have to start by this task.

The task: first 16 byte are the initialization vector and the second 16bytes are my ciphertext. And I have my text that is "Send 5$ to Carl". Now I THINK that I need first the key cause my task is to change the message "Send 5$ to Carl" to "Send 50$ to Bob.

Artjom B.
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    You probably need to describe what you have, what you need and what you already accomplished with some code. You can't find an AES key just like that. Brute forcing an AES key will take *many* years. You probably didn't understand something. – Artjom B. Jun 07 '15 at 13:13
  • the task: first 16 byte are the initialization vector and the second 16bytes are my chiffretext. And I have my text that ist "Send 5$ to Carl". Now I THINK that i need first the key cause my task is to change the message "Send 5$ to Carl" to "Send 50$ to Bob. – max Jun 07 '15 at 13:24
  • No, you don't need the key. Hint: look at what CBC mode does during decryption. Another hint: how many blocks do you have? – Artjom B. Jun 07 '15 at 13:40

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