apt-getアップデートの公開鍵の問題

私はLubuntu 16.04を持っています。私の問題は次のようになりますこの問題:

    sudo apt-get update
    Get:1 http://it.archive.ubuntu.com/ubuntu xenial InRelease [247 kB]
    Hit:2 http://ppa.launchpad.net/fkrull/deadsnakes/ubuntu xenial InRelease       
    Hit:3 http://it.archive.ubuntu.com/ubuntu xenial-updates InRelease             
    Hit:4 http://it.archive.ubuntu.com/ubuntu xenial-backports InRelease           
    Hit:5 http://www.bchemnet.com/suldr debian InRelease                           
    Hit:6 http://archive.canonical.com xenial InRelease                            
    Get:7 http://security.ubuntu.com/ubuntu xenial-security InRelease [93,3 kB]    
    Fetched 340 kB in 6s (52,6 kB/s)                                               
    Reading package lists... Done
    W: There is no public key available for the following key IDs:
    FB510D557CC3E840

提供された答えのどれも効果がありませんでした。

1. ~によるとこの質問への回答1:

   sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys FB510D557CC3E840
   Executing: /tmp/tmp.oqit2axEyE/gpg.1.sh --keyserver keyserver.ubuntu.com --recv-keys FB510D557CC3E840
   gpg: requesting key 7CC3E840 from hkp server keyserver.ubuntu.com
   gpgkeys: key FB510D557CC3E840 not found on keyserver
   gpg: no valid OpenPGP data found.
   gpg: Total number processed: 0
   gpg: keyserver communications error: key not found
   gpg: keyserver communications error: bad public key
   gpg: keyserver receive failed: bad public key
   

2. それともこれから出てきた同じ質問、回答＃2:

   sudo apt-get install debian-keyring debian-archive-keyring
   Reading package lists... Done
   Building dependency tree       
   Reading state information... Done
   debian-archive-keyring is already the newest version (2014.3).
   debian-keyring is already the newest version (2016.01.20).
   0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
   

3. それともこれから出てきた同じ質問、回答＃3:

   sudo apt-get install debian-archive-keyring
   Reading package lists... Done
   Building dependency tree       
   Reading state information... Done
   debian-archive-keyring is already the newest version (2014.3).
   0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
   

4. それともこれから出てきた同じ質問、回答＃4:

   for n in `ls /var/lib/apt/lists/*gpg`; do echo "$n" ; gpg --list-packets "$n" | grep FB510D557CC3E840; done
   ls: cannot access '/var/lib/apt/lists/*gpg': No such file or directory
   

5. それともこれから出てきた同じ質問、回答＃5:

   sudo apt-get install debian-keyring debian-archive-keyring
   Reading package lists... Done
   Building dependency tree       
   Reading state information... Done
   debian-archive-keyring is already the newest version (2014.3).
   debian-keyring is already the newest version (2016.01.20).
   0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
   

6. 別の試み、この質問への回答1:

   gpg --keyserver keyserver.ubuntu.com:80 --recv-keys 7CC3E840
   gpg: requesting key 7CC3E840 from keyserver.ubuntu.com:80
   gpgkeys: no keyserver host provided
   gpg: keyserver internal error
   gpg: keyserver receive failed: keyserver error
   

7. 同じ答え2質問:

   sudo gpg --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 7CC3E840 
   gpg: WARNING: unsafe ownership on configuration file `/home/username/.gnupg/gpg.conf'
   gpg: external program calls are disabled due to unsafe options file permissions
   gpg: keyserver communications error: general error
   gpg: keyserver receive failed: general error
   

8. 権限に問題がないことを確認してください。

   ls -l ~/.gnupg/gpg.conf
   -rw------- 1 username username 9398 mag 16 15:34 /home/username/.gnupg/gpg.conf
   

9. 前回から回答:

   apt-key adv --keyserver hkp://keyserver.ubuntu.com --recv-keys  7CC3E840
   Executing: /tmp/tmp.8iRzKu4RVN/gpg.1.sh --keyserver hkp://keyserver.ubuntu.com --recv-keys 7CC3E840
   gpg: requesting key 7CC3E840 from hkp server keyserver.ubuntu.com
   gpgkeys: key 7CC3E840 not found on keyserver
   gpg: no valid OpenPGP data found.
   gpg: Total number processed: 0
   gpg: keyserver communications error: key not found
   gpg: keyserver communications error: bad public key
   gpg: keyserver receive failed: bad public key