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I read: "$Z_0$ serves as an initial vector for CBC-like encryption, R is used to post-whitening each ciphertext block $C_i$ by $i ∗ R$".

but I don't understand what is the "post-whitening". Can you help me to solve this problem?

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Wikipedia states that key whitening increases security:

In cryptography, key whitening is a technique intended to increase the security of an iterated block cipher. It consists of steps that combine the data with portions of the key.

The most common form of key whitening is xor-encrypt-xor -- using a simple XOR before the first round and after the last round of encryption.

The first block cipher to use a form of key whitening is DES-X, which simply uses two extra 64-bit keys for whitening, beyond the normal 56-bit key of DES. This is intended to increase the complexity of a brute force attack, increasing the effective size of the key without major changes in the algorithm. DES-X's inventor, Ron Rivest, named the technique whitening. In cryptography, key whitening is a technique intended to increase the security of an iterated block cipher. It consists of steps that combine the data with portions of the key (most commonly using a simple XOR) before the first round and after the last round of encryption.

The "post-whitening" consists into applying the whitening after the encryption function: encrypt-xor.

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