SmimeUtil is missing the API to return the "Signed by <from-address>", which email clients generally display

[bbottema]

If you open a signed message in for example Outlook or Thunderbird, they will show a field right under "To:", named "Signed by:". After a lot of research, it turns out it is the subject of a certificate, the person's address to which the certificate was issued to.

I'll provide a pull-request that pulls out this subject from the first available certificate in the message (partially based on this SO answer):

X509Certificate cert = ...;

X500Name x500name = new JcaX509CertificateHolder(cert).getSubject();
RDN cn = x500name.getRDNs(BCStyle.CN)[0];

return IETFUtils.valueToString(cn.getFirst().getValue());

[bbottema]

I provided the pull request.

The only thing I'm not sure about is whether the first certificate will always have the relevant "issued to" CN subject, or that we should run through all certificates' verifiers until the first non-empty subject was found.