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 $12345$"
- Give a valid $C'$ for $M'$ if Alice uses AES in Counter Mode
- 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.