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A CSR is the first step in getting an SSL certificate. It contains your public key and identifying information (domain name, organisation, location) signed with your private key. You send the CSR to a Certificate Authority (CA), they verify your identity and domain ownership, and they issue a certificate in return.
The format is PEM-encoded by default — Base64 text between `-----BEGIN CERTIFICATE REQUEST-----` and `-----END CERTIFICATE REQUEST-----` markers. The private key used to sign the CSR stays on your server. Only the public key travels in the CSR. This is cryptographically important: the CA never sees your private key.
Generating a CSR: `openssl req -new -newkey rsa:2048 -nodes -keyout server.key -out server.csr` creates both a private key and CSR in one command. Modern hosting providers (AWS, Cloudflare, Let's Encrypt with certbot) often handle CSR generation automatically, but understanding the process matters when debugging certificate issues.