The main answer is that OATH HOTP only uses a hashed counter as the OTP whereas YubiKey OTPs contains the time differences compared to the last OTP. Thus, the YubiKey mitigates some attacks where several OTP's are harvested and then replayed at a different time. The attack that OATH HOTP is vulnerable against but YubiKey is not is a passive phishing attacker.
Link to OATH HOTP specification: http://www.ietf.org/rfc/rfc4226.txt
Our current chip/RAM/ROM size of our CPU is likely too limited to fit the OATH HOTP algorithm, but please contact Yubico to discuss this if you are interested.
/SimonStatistics: Posted by Simon — Fri Dec 05, 2008 3:30 pm
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