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In my course there is a remark saying that in CFB encryption mode, the plaintext must not a multiple of the block length.

Why?

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CFB mode has no such length restrictions; it can be used to encrypt an arbitrary length plaintext.

Perhaps you misunderstood what was said.

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  • $\begingroup$ That's what I thought. But it is clearly written black on white. Thanks for the confirmation :) $\endgroup$ – tomak Jan 23 '18 at 15:42
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For CFB mode the input does not have to be a multiple of the block size; i.e. no padding or ciphertext stealing is required as with the ECB or CBC mode of operation.

Maybe that has been badly translated into "must not" as in "is not required to".

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