
A Kelsey–Kohno Attack Variant against SHA-2 Message Digests in HSS/LMS and XMSS
Falko Strenzke's ePrint 2026/2635 presents a multi-target second-preimage attack on message hashing in HSS/LMS and XMSS with SHA-256 and SHA-256/192. It herds chaining states from many collected signatures into one state, defeating the key and leaf-index prefix. With 2^64 signatures, SHA-256 cost falls from 2^256 to about 2^166 (2^156 at maximal message length). SHAKE256 parameter sets are unaffected.
other· Added 26 Oct 2026, published 25 Oct 2026







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