Cryptology ePrint Archive: Report 2019/424

Preimage Security of KNOT-Hash

Raghvendra Rohit

Abstract: KNOT is a Round 1 submission of the ongoing NIST lightweight cryptography project. In this short note, we show that the preimage security of KNOT-Hash instances with squeezing rate half the state size is lower than the claimed security. Our attack exploits the non-randomness properties of the KNOT Sbox which reduce the preimage complexities.

In particular, if $2n$ is the squeezing rate then the preimage security is approximately $(\text{log\textsubscript{2}}(\frac{3}{4}))^{-n} \times 2^{\frac{3n}{4}} \times (\text{log\textsubscript{2}}(3))^{\frac{n}{2}}$. For $n = 64$, 96 and 128, the former bound translates to $2^{125.28}$, $2^{187.92}$ and $2^{250.57}$, respectively.

Category / Keywords: secret-key cryptography / KNOT, NIST lightweight cryptography project, Preimage

Date: received 24 Apr 2019, withdrawn 29 Apr 2019

Contact author: rsrohit at uwaterloo ca

Available format(s): (-- withdrawn --)

Note: Thanks to the KNOT designers for pointing out the inconsistencies in Step 1 and Step 2 of the attack (Section 3). The time complexities indeed exceed the claimed security level and hence the mentioned attack does not work in the current scenario.

Version: 20190429:223635 (All versions of this report)

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