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fix(crypto): clarify gf_mul comment in split_secret
It is the random coefficient (coeffs[i]), not the secret byte directly,
that passes through gf_mul. The secret byte only goes through gf_add
(XOR, inherently branchless). Security intent unchanged.
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crates/crypto/src/shamir.rs
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crates/crypto/src/shamir.rs
···
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// Polynomial: f(x) = secret[i] + coeffs[i]·x in GF(2^8).
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// f(0) = secret[i]. Shares are f(1), f(2), f(3).
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//
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// Secret bytes are in the first argument of gf_mul. The polynomial
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// reduction inside gf_mul is branchless (mask-based), so bit patterns
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// of the secret are not observable through branch timing. The `if b & 1`
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// branch in gf_mul is on the public share index.
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// Secret-bearing coefficient bytes are in the first argument of gf_mul.
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// The polynomial reduction inside gf_mul is branchless (mask-based), so
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// bit patterns of the coefficients are not observable through branch
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// timing. The `if b & 1` branch in gf_mul is on the public share index.
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for i in 0..32 {
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let s = secret[i];
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let a = coeffs[i];