I have embedded devices that are located in different cites and connected to my OpenVPN server. I initialize my devices before deploying them and put the certificate/key pair through local network. When the devices go online at the remote site they are connected to my openVPN server via private tunnel. I am using OpenVPN version:2.3.10 with EasyRSA on Ubuntu 16.04 LTS

Now the certificates are about to expire. How can I renew the certificates on these devices? I am thinking of 2 approaches.

Approach 1) A script on the device checking the validity of the certificate every month or so and when the certificate is about to expire it should send a certificate generation request(a standard https get request or similar). On the server end, only requests coming from private tunnel should be served. A certificate/key with same CN should be generated and transfered to the client.

Problems with apporach 1)

  • How should the cert/key pair be transferred securely ? i dont want to exchange clients private key via network.
  • The whole process should be automated so i can also not put the key manually.
  • How should the cert/key generated on the server? i also dont want the apache web user i.e www-data to have access to my easyRSA folder.

Approach 2) Generate Private key at the client end (my embedded device). Create a Certificate signing request (CSR) send it to OpenVPN server (which is also my CA). The CA generates a certificate and sends back the client certificate.

Problems with apporach 2)

  • How to authenticate CSR ? how can i know that the CSR is coming from my embedded device and not some attacker. The only way that i can assure is, if the CSR is coming from the private tunnel.

  • In case if the certificate on my embedded device expires before renewal then the private tunnel would not exist. How can i authenticate CSR in this case?. Any one with CA.crt (CAs public key) can generate a private key and send a CSR to CA.

I think approach 2 seems more reasonable and secure but i dont know the solution to the problems i am facing. I want to know if I have a correct understanding of security? or if there is something I am missing. Also if i go with approach 2, how would the implementation look like? i know the theory but i dont know how to create CSR and send it to CA. And on CA end how to receive a CSR and generate a cert and send it back.

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