validation.pem
During the first chef-client run, client.pem private key does not exist. Instead, the chef-client will attempt to use the private key assigned to the chef-validator, located in /etc/chef/validation.pem.

client.pem
Every request made by the chef-client to the Chef server must be an authenticated request using the Chef server API and a private key. When the chef-client makes a request to the Chef server, the chef-client authenticates each request using a private key located in /etc/chef/client.pem. During the initial chef-client run, the chef-client will register with the Chef server using the private key assigned to the chef-validator, after which the chef-client will obtain a client.pem private key for all future authentication requests to the Chef server.

Rajesh Kumar

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