fix(otp): compare OTP codes in constant time#24
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Replace the short-circuiting === comparison in verifyTOTP with a constant-time byte-wise compare so the comparison time no longer leaks the matching-prefix length of the secret OTP. Also iterate over all window candidates without an early return to avoid leaking which window matched. Generated with [Linear](https://linear.app/better-auth/issue/OSS-438/non-constant-time-comparison-of-totphotp-code-in-verifytotp#agent-session-5c8cee8c) Co-authored-by: linear-code[bot] <222613912+linear-code[bot]@users.noreply.github.com>
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verifyTOTPcompared the attacker-suppliedotpagainst the server-computed code with===, which short-circuits on the first differing byte. SincegeneratedOTPis secret material derived from the HMAC over the shared secret, the comparison time leaks the matching-prefix length — a timing side channel for secret comparison in a low-level crypto primitive.Changes
constantTimeEqualhelper that XOR-accumulates over all bytes and folds in the length difference, never early-returning on a length or byte mismatch.verifyTOTPand iterate over all window candidates without returning on the first match, so neither the prefix length nor which window matched is leaked.Tests
===(observed ratio ~260x) and passes for the constant-time compare.All existing OTP tests pass unchanged.