**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**Date: **received 24 Apr 2019, withdrawn 29 Apr 2019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**Available format(s): **(-- withdrawn --)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)

**Short URL: **ia.cr/2019/424

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