sm2: fix SM2PKE biased k generation [SECURITY]
#1600
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This fixes a critical security vulnerability originally reported as GHSA-w3g8-fp6j-wvqw by @XlabAITeam. Previously the module defined its own `next_k` function which invoked the `RandomBits::try_random_bits` API to generate a `k` value via rejection sampling, however it was passing previous calculation of the number of bytes to generate to an API expecting a number of bits. This bits/bytes confusion lead to highly biased values for `k` consisting of mostly zeros. Such a bias can be exploited for full plaintext recovery, and potentially key recovery. All ciphertexts ever encrypted using this API are vulnerable. This commit replaces the `next_k` function with the recently added `Generate` trait (#1586), using `NonZeroScalar::try_generate_from_rng` which is backed by the rejection sampling implemenatation from `crypto-bigint`.
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This fixes a critical security vulnerability originally reported as GHSA-w3g8-fp6j-wvqw by @XlabAITeam.
Previously the module defined its own
next_kfunction which invoked theRandomBits::try_random_bitsAPI to generate akvalue via rejection sampling, however it was passing previous calculation of the number of bytes to generate to an API expecting a number of bits. This bits/bytes confusion lead to highly biased values forkconsisting of mostly zeros.Such a bias can be exploited for full plaintext recovery, and potentially key recovery. All ciphertexts ever encrypted using this API are vulnerable.
This commit replaces the
next_kfunction with the recently addedGeneratetrait (#1586), usingNonZeroScalar::try_generate_from_rngto generatekwhich is backed by the rejection sampling implementation fromcrypto-bigint.