One time password (Event or Time Synchronous …)

Search queries as tracted by this blog statistic system have shown us that quite a few people have ended here in search of information on which of this 2 one time password system was the best if any. We believe than when applicable , time synchronous one time password delivers more security than event synchronous. Following are some explanations on where the added value is coming from.

How to produce a one time password :

Each one time password vendor appears to have a different recipe for one time password generation , however the main principles behind those recipe is the same for all the systems we have evaluated. A one time password “token” contains a (long enough) key that is shared with the validation server. At time a new password is required , the value of an always increasing counter is read , and the value of this counter is used as input for a key dependant transformation that output an encoded password , which can be validated by recalculating a range of possible values (knowing where counter is supposed to start ) and trying to find a positive match for the received password in the calculated range…

If the transformation is well choosen , the sequence of passwords one can so generate appears random even for an adversary able to guess the value of the counter (the key still being a secret…). For an example of a known to work recipe , look to the OATH HOTP algorithm as described in rfc 4226.

Event Synchronous, Time Synchronous …

An event synchronous one time password token stores a counter , and increase it prior to generate a new password. A time synchronous token reads the necessary always increasing counter from a local clock that is sufficiently synchronized with validation server time.

The difference of value in between the 2 methods come mainly from the way validation happen. Regardless of the technology used , it is not possible to be perfectly synchronized in between the token and the validation side , hence for each validation a certain “window” (range of possible counter value is considered). In the case of an event synchronous system , the window is frozen as long no new valid password has been presented. Adversaries can patiently present new random password value , gradually increasing their chances to find a positive match. To prevent this to happen , the account will have to be locked down , this resulting in easy denial of service attack on the system. In contrast in a time synchronous system , the “window” is always moving forward making the conduction of brute force password guessing an extremely difficult target. Another advantage of time synchronous password is that they expire after a few minutes , some consider that this can make phishing less likely to succeed.

What do we support

KerPass delivers “time synchronous” one time password on mobile phone.