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Leli avatar Crypto

OFB mode discussion

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Leli

In OFB mode, I understand that a bit flip in $c_i$ for $i > 0$ only causes a bit flip in message block $m_i$. However, how is it possible for a bit flip in $c_0$ (i.e, in IV) will result in all the plaintext blocks being recovered incorrectly.

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kelalaka
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kelalaka avatar Crypto
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kelalaka

OFB mode produces a stream by feedbacking the output of the previous encryption as the input of the next block;

$$O_j = E_K(I_j)$$ $$I_j = O_{j-1}$$ $$I_0 = IV$$

The output stream $O_j$ is used to x-or to produce the ciphertext/plaintext depending on the encryption/decryption mode.

  • $C_j = P_j \oplus O_j$ for encryption
  • $P_j = C_j \oplus O_j$ for decryption.

Now if you modify the IV, then the output of the $O_1$ will change to something else. Then the next stream input $I_2$ will use this changed output as input so that $O_2$ is changed, too. The rest follows and you will get different output streams this will result in different decrypted plaintexts.

One can follow from the red bit flipping and the affected red lines from the below figure;

enter image description here

And the green is used for the effects of the ciphertext modification.

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