frost-core: refresh shares with smaller min_signers will reduce group security

ZF FROST is a Rust implementation of FROST (Flexible Round-Optimised Schnorr Threshold signatures). In versions 2.0.0 through 2.1.0, refresh shares with smaller min_signers will reduce security of group. The inability to change min_signers (i.e. the threshold) with the refresh share functionality (frost_core::keys::refresh module) was not made clear to users. Using a smaller value would not decrease the threshold, and attempts to sign using a smaller threshold would fail. Additionally, after refreshing the shares with a smaller threshold, it would still be possible to sign with the original threshold, potentially causing a security loss to the participant's shares. This issue is fixed in version 2.2.0.

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缺少必要的密码学步骤

FROST 安全漏洞

FROST是Zcash Foundation开源的一个Rust库。 FROST 2.0.0至2.1.0版本存在安全漏洞，该漏洞源于使用较小min_signers刷新共享会降低组安全性。

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