X.509 has a structure which contains information like version number, serial number, signature algorithm, issuer, subject name, subject public key, signature of the issuer etc.
However, I'm having an argument between friends because according to them, the public key is the heart of the certificate.
On the other hand, I think issuer's signature (CA) is the most important attribute. This is because basically you can't do anything without having a valid certificate, whereas if public key for some reason is missing, I still think you can digitally sign documents (since it uses private key and not the public one).
Certificate here comes handy because you need the subject's name (signed by email@example.com") which you can access since you have a valid cert. If you don't have the CA's signature, you can't even digitally sign.
I need to know which is the right answer. Thanks!