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crypto/cipher-block-chaining-cbc-mac-priv.md
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@ -24,11 +24,17 @@ With CBC-MAC, we can generate two signatures `t` and `t'` for the messages `m` a
To keep things simple, we are going to work on a single block for each message.
We can see below how signing both messages works \(NB: both signatures are completely independent from each other\):![](https://pentesterlab.com/cbc-mac/cbc-mac-1.png)
We can see below how signing both messages works \(NB: both signatures are completely independent from each other\):
If we try to concatenate those messages, the signature is no longer valid \(since `t` is now the IV for the second block where it was only NULL before\):![](https://pentesterlab.com/cbc-mac/cbc-mac-2.png)
![](https://pentesterlab.com/cbc-mac/cbc-mac-1.png)
However, if we XOR `m'` and `t`, the signature is now `t'`:![](https://pentesterlab.com/cbc-mac/cbc-mac-3.png)
If we try to concatenate those messages, the signature is no longer valid \(since `t` is now the IV for the second block where it was only NULL before\):
![](https://pentesterlab.com/cbc-mac/cbc-mac-2.png)
However, if we XOR `m'` and `t`, the signature is now `t'`:
![](https://pentesterlab.com/cbc-mac/cbc-mac-3.png)
## Implementation