Why are SSL/TLS certificates self-signed if they have no real signature/CA?

Question

I understand that normally an SSL (or more precisely X.509) certificate is supposed to be signed by some certifying authority to assure that it is genuine.

In some cases no such signature exists, e.g. if you generate a cert for testing purposes, or if you are the certifying authority (root certificate). In these cases, self-signed certificates are used.

My question: Why use this weird construct of self-signing? Why can a certificate not simply not have any signature? What does including a self-signed signature gain?

Or is it just because it's technically easier (no special case for no signature) to always have a signature in every certificate, even if it's a meaningless signature?

Answer 1

A certificate has three main parts

1. identity information
2. a public key
3. a digital signature

A certificate is signed by encrypting the first two parts with a private key, then appending that encrypted information to the end of the certificate. If you can decrypt the signature with the public key contained in the certificate, then you know that certificate was signed by the person that holds the matching private key. The signature binds the identity information to the public key. I sign my certificate with my private key so that you know only I can read messages that you might encrypt with my public key.

Now, unless you've really met me in person and I hand you my certificate, you can't really know that the identity information is legitimately mine. The original purpose of certificates was to establish a web of trust by first obtaining the certificates of people that you have met in person, then trusting people that have certificates signed by those people, then those people...

Answer 2

If you self-sign the certificate, it proves to someone that you actually control the secret key to that signature - ie, it is your certificate.

Otherwise, you could just create a public key that is random numbers and conforms to the format of a certificate, but isn't a real certificate.