# Tag Info

4

One of the basic security requirements of a block cipher mode of operation is that it is indistinguishable under chosen plaintext attack (IND-CPA). Essentially, this means that, if an attacker chooses two messages $m_A$ and $m_B$ and the defender randomly returns either $\text{Encrypt}(K, m_A)$ or $\text{Encrypt}(K, m_B)$ (with $K$ kept secret from the ...

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You do not get semantic security; a chosen plaintext attack can (with high probability) distinguish this mode from random. Consider the case where you are encrypting a two block message $(B, B \oplus 1)$ (for an arbitrary value B). Then, if IV (which I assume is selected randomly) happens to have an lsbit of 0 ($p = 0.5$), then the two ciphertext blocks ...

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In essence it seems to be a sort of mixture of CFB and CTR. I see a possible issue where encrypting sequential values will show up as repeating patterns in the ciphertext. Consider the following 4 bit example. $$PT_0 = 1010$$ $$PT_1 = 1011$$ $$PT_2 = 1100$$ $$PT_3 = 1101$$ $$IV = 1110$$ Assuming $PB$ is the plaintext block directly before encryption. ...

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