anchor.diff:  of same Yubikey ID for multiple users.
   
   Each record in the file should begin on a new line. The parameters in each
  -record are separated by “:” character similar to /etc/passwd. 
  +record are separated by `:` character similar to `/etc/passwd`. 
   
   The contents of this file are as follows:
   
  
positive.code:  paul:indvnvlcbdre:ldvglinuddek
  
  
  The .yubico/authorized_yubikeys file must be created/updated manually and must
  be placed inside user's home directory before configuration of Yubico PAM
  module for SSH authentication.
  
  
  ====== Configuration of modified pam_yubico.so module at user level ======
  
  Append the following line to the beginning of /etc/pam.d/sshd file:
  
   auth required pam_yubico.so id=16 debug
  
  After the above configuration changes, whenever a user connects to the server
  using any SSH client, the PAM authentication interface will pass the control
  to Yubico PAM module. The Yubico PAM module first verifies the username with
  corresponding Yubikey token id as configured in the .yubico/authorized_yubikeys
  file that present in the user's home directory who is trying to assess server
  through SSH. If valid, the Yubico PAM module extracts the OTP string and sends
  it to the Yubico authentication server or else it reports failure. After
  
negative.code: 
  ====== Configuration of modified pam_yubico.so module at administrative level ======
  
  Append the following line to the beginning of the `/etc/pam.d/sshd` file:
  
   auth required pam_yubico.so id=16 debug authfile=/path/to/mapping/file
  
  Make sure you set `id=16` to the correct API-id for the yubico validation server.
  
  After the above configuration changes, whenever a user connects to the server
  using any ssh client, the PAM authentication interface will pass the control to
  