What is a certificate?
A certificate is a digital ID card that proves a website or server is genuine, issued by a trusted authority.
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A certificate is a signed file that ties a name such as example.com to a public key. It is like a passport: anyone can print a card with their name, but only a passport office can issue one that border guards trust. In TLS the passport office is a certificate authority (CA), and your browser ships with a list of CAs it trusts.
Servers present their certificate when a connection starts. If the name does not match, the certificate expired, or the CA is unknown, the browser shows a scary warning. Free certificates from Let’s Encrypt last 90 days and are renewed automatically by tools such as certbot. Internal company systems often run their own CA and install its root on every laptop.
openssl x509 -in cert.pem -noout -subject -datesSee also: What is TLS? and What is a reverse proxy?
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See also
- 0277 What is TLS? TLS is the encryption layer that puts the S in HTTPS, protecting data between your browser and a server. Beginner · Explains an idea · Any system Article TLS & certificates 15 Oct 2026
- 0307 What is encryption? Encryption scrambles data with a key so only someone with the right key can read it again. Beginner · Explains an idea · Any system Article TLS & certificates 15 Oct 2026
- 0325 Fix: SSL certificate problem: unable to get local issuer certificate curl, git, pip or npm cannot chain the server's certificate to a root they trust. Usual causes: a corporate proxy that re-signs… Intermediate · Fixes an error · Linux, macOS, Windows Article Common errors · Making requests · TLS & certificates 15 Oct 2026
- 0355 TLS certificates explained What a certificate proves, who signs it, what the chain and expiry mean, and how to read any site's certificate from the… Beginner · Explains an idea · Any system Article TLS & certificates 15 Oct 2026
- 0356 Free HTTPS with Let’s Encrypt and certbot Install certbot, get a trusted certificate for your domain and let it configure nginx or Apache, or just obtain the files for… Beginner · Step by step · Linux Article TLS & certificates 15 Oct 2026
Repos in the wild
- certbot: the Let’s Encrypt client certbot/certbot The EFF's ACME client that obtains free TLS certificates from Let's Encrypt, can configure nginx and Apache for HTTPS automatically and renews everything on a timer. GitHub
- step-ca: run your own certificate authority for internal TLS and SSH smallstep/certificates step-ca from Smallstep is an open source certificate authority that issues TLS and SSH certificates for your own infrastructure, with ACME support so internal services get certificates the way public ones do. GitHub
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- Dominik +800 chars