Create the intermediate pair
An intermediate certificate authority (CA) is an entity that can sign certificates on behalf of the root CA. The root CA signs the intermediate certificate, forming a chain of trust.
The purpose of using an intermediate CA is primarily for security. The root key can be kept offline and used as infrequently as possible. If the intermediate key is compromised, the root CA can revoke the intermediate certificate and create a new intermediate cryptographic pair.
Prepare the directory
The root CA files are kept in ~/CA. Choose a different directory (~/CA/intermediate) to store the intermediate CA files.
➜ CA mkdir intermediate ➜ CA ll total 24 drwxr-xr-x 3 xinxingegeya staff 102B 1 3 12:31 certs drwxr-xr-x 2 xinxingegeya staff 68B 1 3 12:00 crl -rw-r--r-- 1 xinxingegeya staff 0B 1 3 12:00 index.txt drwxr-xr-x 2 xinxingegeya staff 68B 1 3 13:49 intermediate drwxr-xr-x 2 xinxingegeya staff 68B 1 3 12:00 newcerts -rw-r--r-- 1 xinxingegeya staff 4.1K 1 3 12:56 openssl.cnf drwx------ 3 xinxingegeya staff 102B 1 3 12:25 private -rw-r--r-- 1 xinxingegeya staff 5B 1 3 12:01 serial ➜ CA cd intermediate ➜ intermediate ll ➜ intermediate mkdir certs crl csr newcerts private ➜ intermediate chmod 700 private ➜ intermediate touch index.txt ➜ intermediate echo 1000 > serial
Add a crlnumber file to the intermediate CA directory tree. crlnumber is used to keep track of certificate revocation lists.
➜ intermediate echo 1000 > ~/CA/intermediate/crlnumber
➜ intermediate touch ~/CA/intermediate/openssl.cnf
Create the intermediate key
Create the intermediate key (intermediate.key.pem). Encrypt the intermediate key with AES 256-bit encryption and a strong password.
➜ CA openssl genrsa -aes256 -out intermediate/private/intermediate.key.pem 4096 Generating RSA private key, 4096 bit long modulus ................................................................................................................++ .................................++ e is 65537 (0x10001) Enter pass phrase for intermediate/private/intermediate.key.pem: Verifying - Enter pass phrase for intermediate/private/intermediate.key.pem: ➜ CA chmod 400 intermediate/private/intermediate.key.pem
Create the intermediate certificate
Use the intermediate key to create a certificate signing request (CSR). The details should generally match the root CA. The Common Name, however, must be different.
Make sure you specify the intermediate CA configuration file (intermediate/openssl.cnf).
➜ CA openssl req -config intermediate/openssl.cnf -new -sha256 -key intermediate/private/intermediate.key.pem -out intermediate/csr/intermediate.csr.pem Enter pass phrase for intermediate/private/intermediate.key.pem: You are about to be asked to enter information that will be incorporated into your certificate request. What you are about to enter is what is called a Distinguished Name or a DN. There are quite a few fields but you can leave some blank For some fields there will be a default value, If you enter '.', the field will be left blank. ----- Country Name (2 letter code) [CN]:CN State or Province Name [Beijing]:Beijing Locality Name :Beijing Organization Name [Usoft Ltd]:Usoft Ltd Organizational Unit Name :it Common Name :dev.usoft.com.cn Email Address :firstname.lastname@example.org
To create an intermediate certificate, use the root CA with the v3_intermediate_ca extension to sign the intermediate CSR. The intermediate certificate should be valid for a shorter period than the root certificate. Ten years would be reasonable.
This time, specify the root CA configuration file (~/CA/openssl.cnf).
➜ CA openssl ca -config openssl.cnf -extensions v3_intermediate_ca -days 3650 -notext -md sha256 -in intermediate/csr/intermediate.csr.pem -out intermediate/certs/intermediate.cert.pem Using configuration from openssl.cnf Enter pass phrase for /Users/xinxingegeya/CA/private/ca.key.pem: Check that the request matches the signature Signature ok Certificate Details: Serial Number: 4097 (0x1001) Validity Not Before: Jan 3 07:25:21 2016 GMT Not After : Dec 31 07:25:21 2025 GMT Subject: countryName = CN stateOrProvinceName = Beijing organizationName = Usoft Ltd organizationalUnitName = it commonName = dev.usoft.com.cn emailAddress = email@example.com X509v3 extensions: X509v3 Subject Key Identifier: 1A:29:51:6C:66:90:9D:57:8E:41:5E:19:2A:74:E1:D9:B0:8E:9D:8B X509v3 Authority Key Identifier: keyid:D6:68:19:A7:A7:ED:57:5A:FB:42:55:C7:93:A9:AB:C8:D5:34:9A:78 X509v3 Basic Constraints: critical CA:TRUE, pathlen:0 X509v3 Key Usage: critical Digital Signature, Certificate Sign, CRL Sign Certificate is to be certified until Dec 31 07:25:21 2025 GMT (3650 days) Sign the certificate? [y/n]:y 1 out of 1 certificate requests certified, commit? [y/n]y Write out database with 1 new entries Data Base Updated ➜ CA
The index.txt file is where the OpenSSL ca tool stores the certificate database. Do not delete or edit this file by hand. It should now contain a line that refers to the intermediate certificate.
V 251231072521Z 1001 unknown /C=CN/ST=Beijing/O=Usoft Ltd/OU=it/CN=dev.usoft.com.cn/emailAddressfirstname.lastname@example.org
Verify the intermediate certificate
As we did for the root certificate, check that the details of the intermediate certificate are correct.
➜ CA openssl x509 -noout -text -in intermediate/certs/intermediate.cert.pem
Verify the intermediate certificate against the root certificate. An OK indicates that the chain of trust is intact.
➜ CA openssl verify -CAfile certs/ca.cert.pem intermediate/certs/intermediate.cert.pem intermediate/certs/intermediate.cert.pem: OK
Create the certificate chain file
When an application (eg, a web browser) tries to verify a certificate signed by the intermediate CA, it must also verify the intermediate certificate against the root certificate. To complete the chain of trust, create a CA certificate chain to present to the application.
To create the CA certificate chain, concatenate the intermediate and root certificates together. We will use this file later to verify certificates signed by the intermediate CA.
➜ CA cat intermediate/certs/intermediate.cert.pem certs/ca.cert.pem > intermediate/certs/ca-chain.cert.pem
Our certificate chain file must include the root certificate because no client application knows about it yet. A better option, particularly if you’re administrating an intranet, is to install your root certificate on every client that needs to connect. In that case, the chain file need only contain your intermediate certificate.