What is CSR? A CSR or Certificate Signing request is a block of encoded text that is given to a Certificate Authority when applying for an SSL Certificate. It is usually generated on the server where the certificate will be installed and contains information that will be included in the certificate such as the organization name, common name (domain name), locality, and country. It also contains the public key that will be included in the certificate.
A private key is usually created at the same time that you create the CSR, making a key pair. A CSR is generally encoded using ASN.1 according to the PKCS #10 specification.
A certificate authority will use a CSR to create your SSL certificate, but it does not need your private key. You need to keep your private key secret. The certificate created with a particular CSR will only work with the private key that was generated with it. So if you lose the private key, the certificate will no longer work.
The bit-length of a CSR and private key pair determine how easily the key can be cracked using brute force methods. As of 2016, a key size of fewer than 2048 bits is considered weak and could potentially be broken in a few months or less with enough computing power. If a private key is broken, all the connections initiated with it would be exposed to whoever had the key.
The Extended Validation guidelines that SSL certificate providers are required to follow, require that all EV certificates use a 2048-bit key size to ensure their security well into the future. Because of this, most providers encourage 2048-bit keys on all certificates whether they are EV or not.