[U-Boot] Secure booting

Can I obtain information as how secure booting works.How the keys and certificates are generated and also encryption and decryption steps involved in the authentication process. i would like to know just the theory behind it not and commands or technical details. I have attached pictorial representation of 3 consecutive steps involved in secure booting but while I tried to understanding the working though the diagram I felt that authentication might never succeed as no decryption is taking place.Can anyone please inform how the steps are taking place? I have also attached the documentation related to high assurance booting. Thanks in Advance!