Score:1

CBC MAC forgery

ru flag

$\text{CBC-MAC}$ is defined as follows:

  • $\text{CBC-MAC}_k() = IV $

  • $\text{CBC-MAC}_k(m_1, . . . , m_n) = E_k(m_n \oplus \text{CBC-MAC}_k(m_1, . . . , m_{n−1}))$

My lecture notes say that this is secure for fixed-length messages but forgeries are possible with arbitrary-length messages. I'm trying to come up with a proof for this.

DannyNiu avatar
vu flag
Query for the MAC of 2 different messages, then what can you do by XOR'ng 2 separate message blocks to the CBC-MAC of the queried messages?
kelalaka avatar
in flag
We had some Q/A for this. Well, did you look at Wiki page? It all explained there..
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