Expose `not_before` and `not_after` for each Certificate

[woodruffw]

These are easy enough to extract with X509_get0_notBefore and X509_get0_notAfter.

Each returns an ASN1_TIME *, which can be dumped to a BIO using ASN1_TIME_print.

[hugmyndakassi]

Hi, regarding that. I have tinkered a bit with it already.

It seems straightforward to go from ASN1_TIME to tm with ASN1_TIME_to_tm(X509_get0_notBefore(cert) (e.g. in the Certificate ctor). According to the documentation the converted time in tm is GMT (= UTC). Perfect.

Now what I am wondering is how you envision such an exposure of these points of data.

Exposing tm directly via const reference would be doable, of course. But I find tm always a bit unwieldy with its semantics of year-1900 ... and months and days start at 0 and so on. However, it would still be easier for a developer to go from tm to something else, so perhaps that's an argument in favor of exposing it anyway.

Would it be acceptable to introduce a struct which contains year, month, day (of month), hour, minute and second? E.g.

typedef struct
{
unsigned int year;
unsigned int month;
unsigned int day;
unsigned int hour;
unsigned int minute;
unsigned int second;
} cert_timestamp;

And is std::optional the right way to go for when this value doesn't exist? I know, formally it should always exist, but what to do if the OpenSSL call failed or if we have something signed with some awkward self-signed certificate?

[woodruffw]

I agree re: tm being unwieldy.

A custom struct seems reasonable, but before we commit to that completely: is there something in std::chrono we can reuse? chrono::time_point seems like it's roughly appropriate, although I'm not sure if there's a convenient way to construct it in our case.

Either way: I think we should raise an exception in the constructor if these time fields don't exist. AFAICT x.509 requires them, so we should as well.