Assignment 2
Network Security
Karl-Johan Grahn
Roll no 03005202

3)

1. The error propagates: An error in the transmitted C1 corrupts P1 and P2, because C1 is not only decrypted to get P1 but is also XORed with the output of decrypting C2. No blocks beyond P2 are affected.

2. If there is an error in P1, this error is propagating through every ciphertext block. The corresponding bits of the received value is changed.

3. Bit errors in the incoming cipher block (bytes in this context) will cause bit error at the same bit positions in the first plain text block. This cipher block will then be fed to the shift register and cause bit errors in the plain text for as long as the erroneous bits stay in the shift register. Hence, for n-bit CFB the following n bytes will be garbled. After that, the system recovers, and all following bytes is decrypted correctly.