If an Initialization Vector (IV) is reused (under the same encryption key) in Cipher Feedback Mode (CFB), what is the effect upon security?

I know there is some negative effect upon security, but I can't figure out if it leaks information about just the blocks with the same IV, or if it destroys security for all the data encrypted under that key.


1 Answer 1


It just leaks information about the blocks with the same IV.

Specifically, if the two messages encrypted with the same IV started with blocks $M_0$ and $M'_0$, then the attacker learns the value $M_0 \oplus M'_0$ (and if those where the same, then the attacker also learns the corresponding xor with the second block, etc)

However, the attacker learns nothing about the key, or about any message that wasn't encrypted with a shared IV.


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