Hot answers tagged cbc-mac
9
I'll assumme All ciphered blocks means the same as ciphertext for CBC-Encryption with implicit zero IV, while CBC-MAC is the last block of that.
All ciphered blocks is unsafe as a message authenticator for messages longer than one block, for it succumbs to a trivial attack (here with two blocks):
Eve intercepts message $M=M_0||M_1$ and its authenticator ...
4
1) The adversary queries the oracle (with some randomly chosen message $m$) and gets as a result a message $m=m_1|m_2|...$ and its tag $t=(t_0,F_{k_2}(t_r))$. She then draws $\rho$ uniformly at random in $\{0,1\}^n$ and outputs the message $m=\rho\oplus m_1|m_2|...$ and its (valid) tag $t=(\rho\oplus t_0,F_{k_2}(t_r))$.
2) The adversary queries the oracle ...
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