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Use a more "constant-time" approach for validating one-time passwords #70

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Merged
jchambers merged 1 commit into from
Aug 17, 2026
Merged

Use a more "constant-time" approach for validating one-time passwords #70

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4 changes: 3 additions & 1 deletion src/main/java/com/eatthepath/otp/HmacOneTimePasswordGenerator.java
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Original file line number Diff line number Diff line change
Expand Up @@ -275,7 +275,9 @@ public boolean validateOneTimePassword(final SecretKey key, final long counter,
* @see <a href="https://datatracker.ietf.org/doc/html/rfc4226#section-7">HOTP: An HMAC-Based One-Time Password Algorithm (RFC 4226) - Security Requirements</a>
*/
public boolean validateOneTimePassword(final SecretKey key, final long counter, final int oneTimePassword) throws InvalidKeyException {
return generateOneTimePassword(key, counter) == oneTimePassword;
// This looks a little goofy, but the idea is to ward off any CPU-specific optimizations that might make
// direct integer comparison a not-constant-time operation
return (generateOneTimePassword(key, counter) ^ oneTimePassword) == 0;
}

/**
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