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I am currently stuck at an exercise for my Cryptography Class in University which is:

Alice wants to send a message $M = (M_1,M_2) =$ "Pay Bob \$100 from bank accout $12345$" to her bank (encrypted). Therefore she picks a random Initial Value ($IV$) coded $(IV, M_1, M_2)$ in 8-bit-ASCII, encrypts this and gets the Chiffre $C = (IV,C_1,C_2)$ which she sends to her bank.

Eve intercepts $C$ from Alice and wants to change it to $C'=(IV',C_1',C_2')$ with $M' =$ "Pay Eve \$500 from bank account $12345$"

  1. Give a valid $C'$ for $M'$ if Alice uses AES in Counter Mode
  2. Give a valid $C'$ for $M'$ if Alice uses AES in CBC-Mode

The $IV$ is transferred with the messsage so Eve is able to change it anyway.

Maybe one of you can explain how to do this I don't have any Idea.

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Think about what happens in CBC if you flip one bit of the IV. Then think about what happens in counter mode if you flip one bit of the ciphertext. –  mikeazo Jun 18 at 12:31
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