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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.

share|improve this question
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 '14 at 12:31

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