A decent MUA would understand that there may be more than one cert with the same SKI. AKI/SKI is just a hint. As such, it's nice (maybe leading to better performance) if there's one and only one cert with a particular SKI. But it's not required. In fact, if you use one of the key hash methods for calculating the AKI/SKI and there are several certificates with the same subject public key (as with multiple cross-certificates for a single CA), these certificates will probably all have the same SKI. Note also that AKI/SKI chaining SHOULD NOT be checked during path validation. To be more explicit, it's NOT true that the SKI of a CA certificate must match the AKI of a certificate signed by that CA. Why? Because the CA certificate may have been issued by a CA that uses a different AKI/SKI computation technique than the CA who's the subject of the CA certificate. Thanks, Steve Peter Gutmann wrote: > > "Al Arsenault" <aarsenau@xxxxxxx> writes: > > >Why does an AKI/SKI have to be globally unique? > > > >It strikes me that the AKI/SKI extensions are simply "search aids". That is, > >they merely help the application (RP) determine which of a set of possible > >certificates to use is the correct one. While it would be kind of cool if an > >SKI/AKI would be globally unique (it would make searching a repository a > >little easier/quicker IN SOME CASES), it doesn't strike me as a huge deal. > > You send me a CMS signed message (say) with the key identified by sKID = 0. > My MUA finds a cert with sKID = 0 and tries to verify the signature. The > verification fails, and I get a warning saying that the forces of darkness > are tampering with my communications, I could be under attack, and the world > is about to end. > > You send me a CMS signed message (say) with the key identified by sKID = > aildhfsdfsjklhgdfjghkf. My MUA can't find a cert with that sKID and > displays an informational message saying that it couldn't find a signing > key, and perhaps I should contact the sender for more information. > > There's a big difference in terms of usability. > > Peter.
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