How to Check the Expiry Date of an SSL/TLS Certificate

For a couple of years now, I use the fantastic (and free*) service by Let’s Encrypt in order to generate SSL/TLS certificates.

One of the main differences to paid certs is that the ones of Let’s Encrypt are only valid 90 days.

This sounds bad at first, but actually this is a good thing from a security perspective, and anyway, you do not create / renew certs manually, but one of the many clients do this for you automatically.

Today I got a notification from the Let's Encrypt Expiry Bot that one of my domains’ certificate is about to expire.

How to check the end date?

While for web services you can check the end date by clicking on the lock symbol in the browser, how do you this when you just have access to the certificate file, e.g. cert.pem?

Use the client tooling

I use Certbot to manage my certifications, so all I need to do is entering…

# certbot certificates

  Certificate Name: xxx.example.com
    Domains: abc.example.com def.example.com xxx.example.com
    Expiry Date: 2021-04-04 12:49:35+00:00 (VALID: 41 days)
    Certificate Path: /etc/letsencrypt/live/xxx.example.com/fullchain.pem
    Private Key Path: /etc/letsencrypt/xxx.example.com/privkey.pem

All good!

Use openssl

You can also use the swiss knife of SSL handling on Linux…

$ openssl x509 -enddate -noout -in cert/xxx.example.com/cert.pem 

notAfter=Apr  4 12:49:35 2021 GMT

All good!

So, it looks like this was a false alarm!

*) The service is free for all, but the organization has to pay its bills, the infrastructure cost and its employees.

I donated and you should probably, too - if you can afford it: Official donation page