In the LRW mode discussion on page 34/161 of the LibTomCrypt manual, available here for example


there is the following comment:

"The tweak value is NOT a nonce or IV value it must be random and secret"

Nonetheless, in the original LRW article


this observation is made:

"We wish to carefully distinguish between the function of the key, which is to provide uncertainty to the adversary, and the role of the tweak, which is to provide variability.The tweak is not intended to provide additional uncertainty to an adversary. Keeping the tweak secret need not provide any greater cryptographic strength."

They also say:

"For this operator, we call the new (second) input a “tweak” rather than a “nonce”or “initialization vector,” but the intent is similar. "

And in the proposed TBC mode, the tweak plays an analogous role to the IV in CBC mode.

Would anyone be able to clarify the meaning of the author in the LibTomCrypt document? I am not expecting this to be an error, but nonetheless I cannot explain this seeming contradiction...

  • 2
    $\begingroup$ I expect it to be an error. The wording seems strange, and I suspect it is an editing error of some sort. (A tweakable block cipher should look like an ideal cipher keyed by the tweak, and an ideal cipher retains its properties even under known keys.) $\endgroup$
    – K.G.
    Mar 27 at 20:49

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