sasl-otp: use constant-time hash comparison #259
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This PR fixes a timing side-channel vulnerability in the OTP SASL
authentication mechanism.
The OTP verification logic used memcmp() to compare cryptographic
hash values. Because memcmp() exits on the first mismatch, execution
time varied depending on the number of matching prefix bytes, leaking
information through observable timing differences.
The Dovecot codebase already provides mem_equals_timing_safe() and
uses it for other authentication mechanisms (e.g., APOP, CRAM-MD5,
SCRAM, password verification). This patch replaces memcmp() with
mem_equals_timing_safe() in the OTP mechanism to ensure constant-time
comparison.