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10

One of the advantages is purely on the human side of security. From RFC 6238's abstract: The HOTP algorithm specifies an event-based OTP algorithm, where the moving factor is an event counter. The present work bases the moving factor on a time value. A time-based variant of the OTP algorithm provides short-lived OTP values, which ...


8

The HOTP standard describes the resynchronization algorithm (section 7.4). Basically, the server remembers the last value $C$ of the counter for which a correct password was presented. When a new password is to be verified, the server tries $C+1$, $C+2$... until one matches, or $C+w$ is reached for some $w$ called the "window size". The intended scenario is ...


6

It looks to me that the original intent was to make sure that all bits of the hash digest have an equal chance to contribute to the truncated portion. But one of the properties of a secure hash function is to ensure that a single bit change results in a cascade that yields changing bits across the entire digest. If you don't trust this property in the hash ...


4

Why stop at 8 digits? 10 digits will be even more secure. Or 12. The output of the HOTP algorithm is 160 bits so you could go all the way to about 48 digits. Bottom line: 6 digits is secure enough for most applications and that is all that counts. Any more is inconvenient for the user and slightly more expensive when used in a hardware token (8 digit ...


4

The usual resynchronization method involves getting several consecutive codes from the token and then running the algorithm once with a very large look-ahead window until the set of consecutive codes are found. The number of consecutive codes needed depends on how far off the token is. With a typical token, two codes would suffice to handle a desynch of ...


3

There is no "fresh client" with HOTP. The whole counter business is based on the idea that there is a single client, who maintains his counter which is more-or-less synchronized with the server counter. The synchronization window is just a way to cope with small unsynchronization events which come from realistic situations (e.g. your 3-year-old played with ...


2

It is for user experience reasons, as you surmise, but the security is not compromised as much as you may think. Most implementations use 6 digit HOTP/TOTP schemes and design their implementation of the scheme to give them a security level they are comfortable with. For HOTP, the key parameter that allows 6 digits to be secure enough is the throttling ...


1

RFC 4226, section 7.5 defines two shared key generation schemes: deterministic and random. I would suggest that you use the deterministic scheme, which only requires the server to store a single "master key": "Deterministic Generation A possible strategy is to derive the shared secrets from a master secret. The master secret will be stored at ...


1

I agree that Gilles' interpretation in the comments is the only one that makes sense; the RFC clearly contains an editorial error, and should read either (emphasis indicates corrections): "If the value calculated by the authentication server matches the value calculated by the client, then the HOTP value is validated." or: "If the value received by ...


1

Yes, HOTP can include a PIN/Password also. If you check RFC 4226, it says Composite Shared Secrets It may be desirable to include additional authentication factors in the shared secret K. These additional factors can consist of any data known at the token but not easily obtained by others. Examples of such data include: * PIN or Password ...


1

It looks like unnecessary window dressing to me. As far as I can see, there is absolutely no reason to use this scheme instead of just choosing the first four bytes of the hash. It looks like unnecessary complexity -- or, as fgrieu put it, over-engineering. If the hash function is any good, then all this should be unnecessary. And if the hash function ...


1

As I understand, the user's token normally can't be reset (without destroying it). So, the assistance would consist in either giving a new token to the user (and declaring the old one invalid), or in stepping the server ahead until it matches again (i.e. running the algorithm once with a really large window size).



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