public TlsCertificate issuer { get; construct; }

A TlsCertificate representing the entity that issued this certificate.

If null, this means that the certificate is either self-signed, or else the certificate of the issuer is not available.

Beware the issuer certificate may not be the same as the certificate that would actually be used to construct a valid certification path during certificate verification. RFC 4158 explains why an issuer certificate cannot be naively assumed to be part of the the certification path (though GLib's TLS backends may not follow the path building strategies outlined in this RFC). Due to the complexity of certification path building, GLib does not provide any way to know which certification path will actually be used. Accordingly, this property cannot be used to make security-related decisions. Only GLib itself should make security decisions about TLS certificates.