Having trouble understanding SSL, PKI, certificates.

Hello,

I'm currently learning about firewalls and I'm having some trouble learning the details of SSL and Public and Private keys/ PKI/ certificates.

I have looked at Christof Pear's 1st and 24th lecture in youtube but I don't think I have time look at all his lectures and I also have his book, Understanding Cryptography but there just so much more topics about encryption that take time to understand. I have looked at the SEC+ book about PKI, countless cert authority websites that talk about how SSL works but It's still not clear for me.

Especially when authenticating clients with certificates, I'm having some trouble understanding that too.

Are there any courses in Udemy, or Linux Academy or an easier book to understand? Labs?

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Any resources you guys recommend?

Thank You.