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Sign server and client certificates

We will be signing certificates using our intermediate CA. You can use these signed certificates in a variety of situations, such as to secure connections to a web server or to authenticate clients connecting to a service.

Note: The steps below are from your perspective as the certificate authority. A third-party, however, can instead create their own private key and certificate signing request (CSR) without revealing their private key to you. They give you their CSR, and you give back a signed certificate. In that scenario, skip the genrsa and req commands.

Create a key

Our root and intermediate pairs are 4096 bits. Server and client certificates normally expire after one year, so we can safely use 2048 bits instead.

Note: Although 4096 bits is slightly more secure than 2048 bits, it slows down TLS handshakes and significantly increases processor load during handshakes. For this reason, most websites use 2048-bit pairs.

If you’re creating a cryptographic pair for use with a web server (eg, Apache), you’ll need to enter this password every time you restart the web server. You may want to omit the -aes256 option to create a key without a password.

# cd /root/ca
# openssl genrsa -aes256 \
      -out intermediate/private/www.example.com.key.pem 2048
# chmod 400 intermediate/private/www.example.com.key.pem

Create a certificate

Use the private key to create a certificate signing request (CSR). The CSR details don’t need to match the intermediate CA. For server certificates, the Common Name must be a fully qualified domain name (eg, www.example.com), whereas for client certificates it can be any unique identifier (eg, an e-mail address). Note that the Common Name cannot be the same as either your root or intermediate certificate.

# cd /root/ca
# openssl req -config intermediate/openssl.cnf \
      -key intermediate/private/www.example.com.key.pem \
      -new -sha256 -out intermediate/csr/www.example.com.csr.pem

Enter pass phrase for www.example.com.key.pem: secretpassword
You are about to be asked to enter information that will be incorporated
into your certificate request.
-----
Country Name (2 letter code) [XX]:US
State or Province Name []:California
Locality Name []:Mountain View
Organization Name []:Alice Ltd
Organizational Unit Name []:Alice Ltd Web Services
Common Name []:www.example.com
Email Address []:

To create a certificate, use the intermediate CA to sign the CSR. If the certificate is going to be used on a server, use the server_cert extension. If the certificate is going to be used for user authentication, use the usr_cert extension. Certificates are usually given a validity of one year, though a CA will typically give a few days extra for convenience.

# cd /root/ca
# openssl ca -config intermediate/openssl.cnf \
      -extensions server_cert -days 375 -notext -md sha256 \
      -in intermediate/csr/www.example.com.csr.pem \
      -out intermediate/certs/www.example.com.cert.pem
# chmod 444 intermediate/certs/www.example.com.cert.pem

The intermediate/index.txt file should contain a line referring to this new certificate.

V 160420124233Z 1000 unknown ... /CN=www.example.com

Verify the certificate

# openssl x509 -noout -text \
      -in intermediate/certs/www.example.com.cert.pem

The Issuer is the intermediate CA. The Subject refers to the certificate itself.

Signature Algorithm: sha256WithRSAEncryption
    Issuer: C=GB, ST=England,
            O=Alice Ltd, OU=Alice Ltd Certificate Authority,
            CN=Alice Ltd Intermediate CA
    Validity
        Not Before: Apr 11 12:42:33 2015 GMT
        Not After : Apr 20 12:42:33 2016 GMT
    Subject: C=US, ST=California, L=Mountain View,
             O=Alice Ltd, OU=Alice Ltd Web Services,
             CN=www.example.com
    Subject Public Key Info:
        Public Key Algorithm: rsaEncryption
            Public-Key: (2048 bit)

The output will also show the X509v3 extensions. When creating the certificate, you used either the server_cert or usr_cert extension. The options from the corresponding configuration section will be reflected in the output.

X509v3 extensions:
    X509v3 Basic Constraints:
        CA:FALSE
    Netscape Cert Type:
        SSL Server
    Netscape Comment:
        OpenSSL Generated Server Certificate
    X509v3 Subject Key Identifier:
        B1:B8:88:48:64:B7:45:52:21:CC:35:37:9E:24:50:EE:AD:58:02:B5
    X509v3 Authority Key Identifier:
        keyid:69:E8:EC:54:7F:25:23:60:E5:B6:E7:72:61:F1:D4:B9:21:D4:45:E9
        DirName:/C=GB/ST=England/O=Alice Ltd/OU=Alice Ltd Certificate Authority/CN=Alice Ltd Root CA
        serial:10:00

    X509v3 Key Usage: critical
        Digital Signature, Key Encipherment
    X509v3 Extended Key Usage:
        TLS Web Server Authentication

Use the CA certificate chain file we created earlier (ca-chain.cert.pem) to verify that the new certificate has a valid chain of trust.

# openssl verify -CAfile intermediate/certs/ca-chain.cert.pem \
      intermediate/certs/www.example.com.cert.pem

www.example.com.cert.pem: OK

Deploy the certificate

You can now either deploy your new certificate to a server, or distribute the certificate to a client. When deploying to a server application (eg, Apache), you need to make the following files available:

  • ca-chain.cert.pem
  • www.example.com.key.pem
  • www.example.com.cert.pem
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