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Codebook (ECB), Cipher Block Chaining (CBC), Cipher Feedback (CFB), and Counter (CTR). Which of these four modes can perform most of the decryption work in parallel? Why?

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    $\begingroup$ Hint: Look at the decryption formulas and check whether there are dependencies between the decrypted plaintext blocks. $\endgroup$ – SEJPM Feb 17 '18 at 15:08
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    $\begingroup$ All of them support parallel decryption. But CBC and CFB cannot parallelize encryption. $\endgroup$ – CodesInChaos Feb 17 '18 at 15:55
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    $\begingroup$ The only disadvantage of CBC and CFB is that decryption requires access to the previous block of ciphertext. This could be an issue if access to the previous block is expensive (e.g. in a previous sector on disk encryption). Then again, using CBC or CFB for disk encryption is a bad idea for more than just that reason. $\endgroup$ – Maarten Bodewes Feb 17 '18 at 22:58
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I shall summarize the comments as an answer:

The TL;DR is:

All of them support parallel decryption. But CBC and CFB cannot parallelize encryption.

(by CodesInChaos)

The strategy to seeing this is:

Look at the decryption formulas and check whether there are dependencies between the decrypted plaintext blocks.

(by SEJPM)

And the caveat to this, where the above TL;DR doesn't hold is:

The only disadvantage of CBC and CFB is that decryption requires access to the previous block of ciphertext. This could be an issue if access to the previous block is expensive (e.g. in a previous sector on disk encryption). Then again, using CBC or CFB for disk encryption is a bad idea for more than just that reason.

(by Maarten Bodewes)

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