In this paper, we present a practical attack on the signature scheme SFLASH proposed by Patarin, Goubin and Courtois in 2001 following a design they had introduced in 1998. The attack only needs the public key and requires about one second to forge a signature for any message, after a one-time computation of several minutes. It can be applied to both SFLASH v2 which was accepted by NESSIE, as well as to SFLASH v3 which is a higher security version.
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