Re: DER encoding of KeyUsage BIT STRING

[Peter Gutmann <pgut001@xxxxxxxxxxxxxxxxx>]

John Thielens <johnt@xxxxxxxxxxxx> writes:

>Ordinarily, I wouldn't even notice.  But in this case, the certificate goes
>through a toolkit that parses the certificate into an internal canonical form
>and then regenerates its own DER encoded certificate, thereby changing the
>second encoding into the first.

Is there really a toolkit out there which does that?  Wouldn't that break about
90% of the certificates in existence?

(I'm not just being facetious with that question, I can't imagine how anyone
 could field a toolkit which recodes certs into correct DER without finding
 that almost everything they try fails to work after the recoding.  Which
 toolkit is this?  I should probably put a warning about this in the style
 guide).

>1) the "unusual" CA is at fault for generating an improper DER encoding with
>trailing 0's explicit in a BIT STRING.

Yes, look up the rules for named bit strings in X.680/690 (I don't have a copy
handy at the moment so I can't quote chapter and verse).  OTOH the CA isn't
that unusual, a lot of CAs and implementations do this (thus my surprise that
the toolkit managed to get past any field testing with that behaviour).

Peter.