## Cryptology ePrint Archive: Report 2021/455

Guessing Bits: Improved Lattice Attacks on (EC)DSA

Chao Sun and Thomas Espitau and Mehdi Tibouchi and Masayuki Abe

Abstract: In the past 30 years, lattice reduction has proved to be one powerful tool of public-key cryptanalysis. Since the advent of the Hidden Number Problem, there has been an extensive study on attacks on (EC)DSA with nonce leakage. While lattice attacks require only a few signatures, it can't deal with small nonce bias compared with Bleichenbacher attack. Prior to this work, it is unknown how to utilize more signatures to improve lattice attacks on (EC)DSA. In this paper, we propose several approaches to improve lattice attacks. The key idea is that we can guess some bits of the secret key(or the nonces) and modify the standard lattice to increase the volume, thus making the lattice attack much easier. Besides, we observe that by filtering some specific signatures we are able to modify the lattice, so we can collect a large number of signatures and construct a lattice that is much easier to attack. With a combination of these techniques, we are able to improve lattice attacks on (EC)DSA. On the one hand, we are able to attack 160-bit modulus(and other modulus as well) (EC)DSA with 2-bit leakage within $2^{15}$ BKZ-30 operations with 90 signatures. On the other hand, with $2^{27}$ signatures available, we are able to attack 160-bit (EC)DSA with 2-bit leakage in just one BKZ-30 operation.

As a second contribution, we give an explanation for several questions unexplained in previous works. It was observed that SVP approaches(Kannan embedding) always outperform CVP approaches(nearest plane) and lattice attack is very sensitive to the Kannan Embedding factor, but these questions are not discussed in previous works. We give an explanation for completeness.

Last, we carry out some experiments on the TPM-Fail dataset. While the original attack utilizes around 40000 signatures, with a combination of our method, we are able to recover the secret with only 800 signatures available.

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