CSR Generator

How to use it

1. Enter the main host name as the common name, for example www.example.com.
2. Add every other name the certificate must cover, such as example.com without www, under subject alternative names.
3. Choose a key type. ECDSA P-256 is small and fast and works with every current browser; choose RSA 2048 if an old client or a device such as a load balancer needs RSA.
4. Fill in the organization details only if you order an organization-validated (OV) or extended-validation (EV) certificate. Domain-validated certificates ignore them.
5. Select “Generate CSR”. Send the CSR to your certificate authority (CA) and store the private key on the server where the certificate will be installed.

If you prefer to create the key on the server itself, which is the better practice for production, copy the OpenSSL command the tool shows. It produces an equivalent key and CSR with the same names.

Worked example

For www.example.com and example.com, with the organization Example Inc in the US and an ECDSA P-256 key, the tool shows this command:

openssl req -new \
  -newkey ec -pkeyopt ec_paramgen_curve:P-256 \
  -nodes -keyout private-key.pem \
  -out request.csr \
  -sha256 \
  -subj '/C=US/O=Example Inc/CN=www.example.com' \
  -addext 'subjectAltName=DNS:www.example.com,DNS:example.com'

Decoded with openssl req -in request.csr -noout -text -verify, the CSR from the tool and the one from the command have the same fields; only the key and the signature differ, because each run creates a new key:

Certificate request self-signature verify OK
Certificate Request:
    Data:
        Version: 1 (0x0)
        Subject: C=US, O=Example Inc, CN=www.example.com
        Subject Public Key Info:
            Public Key Algorithm: id-ecPublicKey
                Public-Key: (256 bit)
                ASN1 OID: prime256v1
                NIST CURVE: P-256
        Attributes:
            Requested Extensions:
                X509v3 Subject Alternative Name:
                    DNS:www.example.com, DNS:example.com
    Signature Algorithm: ecdsa-with-SHA256

The common name www.example.com also appears in the list of alternative names. The tool adds it there because CAs and browsers use only that list for host names.

What is inside a CSR

A CSR is a PKCS #10 certification request (RFC 2986): the subject name, the public key and a list of attributes, signed with the private key that belongs to the public key. The signature proves to the CA that whoever sent the request holds the key; the CA does not need the key itself.

The key pair comes from the Web Crypto interface of your browser and its cryptographically secure random generator. The private key is exported unencrypted in PKCS #8 form, the form openssl req writes as well, and exists only in this page until you copy or download it.

For publicly trusted TLS certificates the CA/Browser Forum Baseline Requirements allow RSA keys of at least 2048 bits and ECDSA keys on P-256, P-384 or P-521 (section 6.1.5). They forbid the organizational unit (OU) in subscriber certificates, and a domain-validated certificate may carry only the country and the common name in its subject (section 7.1.2.7). A CA may therefore drop fields you enter.

Common mistakes

• Losing the private key. The CA never sees it and cannot restore it; without it the issued certificate is useless and you must start again with a new CSR.
• Sending the private key to the CA or a reseller. Only the CSR goes to them. Anyone who asks for the key is not following the process.
• Listing only www.example.com. The certificate then does not cover example.com, and visitors who type the short name see a warning.
• Abbreviating the state (CA instead of California) or entering a country name instead of the two-letter code. OV and EV orders are checked against these fields.
• Reusing an old CSR after the key was exposed. A new certificate for an exposed key is exposed as well; generate a new key.

Sources

• RFC 2986: PKCS #10 certification request syntax
• RFC 5280: X.509 certificate profile
• RFC 5758: ECDSA signature algorithm identifiers
• CA/Browser Forum Baseline Requirements
• OpenSSL req command