apt-get update failed. "Certificate verification failed: The certificate is NOT trusted. The certificate issuer is unknown. Could not handshake:

Debian apt 源更新， https 错误。

apt-get install ca-certificates

Ign:1 https://mirrors.tuna.tsinghua.edu.cn/debian stretch InRelease
Ign:2 https://mirrors.tuna.tsinghua.edu.cn/debian stretch-updates InRelease          
Hit:4 http://cdn-fastly.deb.debian.org/debian buster InRelease                                                             
Hit:5 http://cdn-fastly.deb.debian.org/debian buster-updates InRelease                                                     
Hit:3 http://security-cdn.debian.org/debian-security buster/updates InRelease                                              
Ign:6 https://mirrors.tuna.tsinghua.edu.cn/debian stretch-backports InRelease                        
Ign:7 https://mirrors.tuna.tsinghua.edu.cn/debian-security stretch/updates InRelease
Err:8 https://mirrors.tuna.tsinghua.edu.cn/debian stretch Release
  Certificate verification failed: The certificate is NOT trusted. The certificate issuer is unknown.  Could not handshake: Error in the certificate verification. [IP: 101.6.8.193 443]
Err:9 https://mirrors.tuna.tsinghua.edu.cn/debian stretch-updates Release
  Certificate verification failed: The certificate is NOT trusted. The certificate issuer is unknown.  Could not handshake: Error in the certificate verification. [IP: 101.6.8.193 443]
Err:10 https://mirrors.tuna.tsinghua.edu.cn/debian stretch-backports Release
  Certificate verification failed: The certificate is NOT trusted. The certificate issuer is unknown.  Could not handshake: Error in the certificate verification. [IP: 101.6.8.193 443]
Err:11 https://mirrors.tuna.tsinghua.edu.cn/debian-security stretch/updates Release
  Certificate verification failed: The certificate is NOT trusted. The certificate issuer is unknown.  Could not handshake: Error in the certificate verification. [IP: 101.6.8.193 443]

You can add [trusted=yes] in the sources.list. For example:

deb [trusted=yes] http://ppa.launchpad.net/repo_name/pkg/ubuntu vivid main
deb-src [trusted=yes] http://ppa.launchpad.net/repo_name/pkg/ubuntu vivid main

askubuntu

deb [trusted=yes] https://mirrors.tuna.tsinghua.edu.cn/debian/ buster main contrib non-free
# deb-src [trusted=yes] https://mirrors.tuna.tsinghua.edu.cn/debian/ buster main contrib non-free
deb [trusted=yes] https://mirrors.tuna.tsinghua.edu.cn/debian/ buster-updates main contrib non-free
# deb-src [trusted=yes] https://mirrors.tuna.tsinghua.edu.cn/debian/ buster-updates main contrib non-free
deb [trusted=yes] https://mirrors.tuna.tsinghua.edu.cn/debian/ buster-backports main contrib non-free
# deb-src [trusted=yes] https://mirrors.tuna.tsinghua.edu.cn/debian/ buster-backports main contrib non-free
deb [trusted=yes] https://mirrors.tuna.tsinghua.edu.cn/debian-security buster/updates main contrib non-free
# deb-src [trusted=yes] https://mirrors.tuna.tsinghua.edu.cn/debian-security buster/updates main contrib non-free