I've read several texts which say that if the entire plaintext is a multiple of the block-size padding is not required (and not using padding would not mean a loss of security). I generally disagree on this. In some special cases, for example in CTR of OFB mode padding could even unveil more information about the block-cipher output than not using it. But many currently used paddings are randomized.

When using a randomized padding with modern block ciphers the smallest change in the block-cipher input will change the entire block-cipher output. Therefore the randomness of the padding contributes to the encryptions security.

My first question is whether the above statement is correct or not and why. And second: Are modes like CBC/PCBC more secure than modes like CTR/OFB? Or does it depend on the used padding?

  • $\begingroup$ There are two kinds of padding: 1) Padding required by the mode of operation (for CBC but not CTR) 2) Padding that aims to make traffic analysis harder. These kinds of padding should not be confused. $\endgroup$ – CodesInChaos May 1 '14 at 15:34

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