Create Private Certificate Authority on Linux

April 16, 2013 by

This tutorial will show you how to create your own private CA or Certificate Authority. This will give you the opportunity to sign your own certificates without having to pay someone else. However, since your private CA will not be trusted by others it may prompt warnings when others use it. You will need to add your root certificate to the machines you want to trust your CA.

I had written a similar article in 2008 (Create a Certificate Authority and Certificates with OpenSSL) but this tutorial supersedes the instructions for creating CA in the older one.

Install Prerequisites

I wrote this tutorial using Fedora 18. The only prerequisite I needed was OpenSSL.

su -c 'yum install openssl'

Create Directory Structure

mkdir /home/cg/myca

cd /home/cg/myca/

mkdir private certs newcerts conf export csr

echo '01' > serial

touch index.txt

We will run all commands by default in the /home/cg/myca directory, unless stated otherwise.

Config File

vim /home/cg/myca/conf/caconfig.cnf

This file would serve as the default config file for the CA. It should look something like the following:

[ ca ]
default_ca = CA_default

[ CA_default ]
dir = /home/cg/myca/
certs = $dir/certs
crl_dir = $dir/crl
database = $dir/index.txt
new_certs_dir = $dir/newcerts
certificate = $dir/certs/cacert.pem
serial = $dir/serial
#crl = $dir/crl.pem
private_key = $dir/private/cakey.pem
#RANDFILE = $dir/private/.rand
x509_extensions = usr_cert
#crl_extensions = crl_ext
default_days = 3650
#default_startdate = YYMMDDHHMMSSZ
#default_enddate = YYMMDDHHMMSSZ
#default_crl_days= 30
#default_crl_hours = 24
default_md = sha1
preserve = no
#msie_hack
policy = policy_match

[ policy_match ]
countryName = match
stateOrProvinceName = match
localityName = match
organizationName = match
organizationalUnitName = optional
commonName = supplied
emailAddress = optional

[ req ]
default_bits = 4096 # Size of keys
default_keyfile = key.pem # name of generated keys
distinguished_name = req_distinguished_name
attributes = req_attributes
x509_extensions = v3_ca
#input_password
#output_password
string_mask = nombstr # permitted characters
req_extensions = v3_req

[ req_distinguished_name ]
countryName = Country Name (2 letter code)
countryName_default = US
countryName_min = 2
countryName_max = 2
stateOrProvinceName = State or Province Name (full name)
stateOrProvinceName_default = New York
localityName = Locality Name (city, district)
localityName_default = New York
organizationName = Organization Name (company)
organizationName_default = Code Ghar
organizationalUnitName = Organizational Unit Name (department, division)
organizationalUnitName_default = IT
commonName = Common Name (hostname, FQDN, IP, or your name)
commonName_max = 64
commonName_default = CGIT
emailAddress = Email Address
emailAddress_max = 40
emailAddress_default = codeghar@example.com

[ req_attributes ]
#challengePassword = A challenege password
#challengePassword_min = 4
#challengePassword_max = 20
#unstructuredName = An optional company name

[ usr_cert ]
basicConstraints= CA:FALSE
subjectKeyIdentifier=hash
authorityKeyIdentifier=keyid,issuer:always
#nsComment = ''OpenSSL Generated Certificate''
#nsCertType = client, email, objsign for ''everything including object signing''
subjectAltName=email:copy
issuerAltName=issuer:copy
#nsCaRevocationUrl = http://www.domain.dom/ca-crl.pem
#nsBaseUrl = 
#nsRenewalUrl =
#nsCaPolicyUrl = 
#nsSslServerName =

[ v3_req ]
basicConstraints = CA:FALSE
keyUsage = nonRepudiation, digitalSignature, keyEncipherment

[ v3_ca ]
subjectKeyIdentifier = hash
authorityKeyIdentifier = keyid:always,issuer:always
basicConstraints = CA:TRUE
#keyUsage = cRLSign, keyCertSign
#nsCertType = sslCA, emailCA
#subjectAltName=email:copy
#issuerAltName=issuer:copy
#obj=DER:02:03

[ crl_ext ]
#issuerAltName=issuer:copy
authorityKeyIdentifier=keyid:always,issuer:always

Thanks to http://wwwneu.secit.at/web/documentation/openssl/openssl_cnf.html for helping with this file.

Generate Root Certificate

You can use the config file (caconfig.cnf) we created in the previous step to answer a lot of the questions asked during certificate generation. Just run the following command and answer the questions. Most questions will have the default values provided in caconfig.cnf.

openssl req -new -x509 -days 3650 -config conf/caconfig.cnf -keyform PEM -keyout private/key.ca.cg.pem -outform PEM -out certs/crt.ca.cg.pem

Although we specified the default number of days in caconfig.cnf file, we still have to specify the days flag when using the x509 flag. If we don’t the certificate is created with a default value of 30 days. Thanks to Re: default_days problem and OpenSSL req(1).

If you want to provide your own custom values you may run the following command instead.

openssl req -new -x509 -days 3650 -newkey rsa:4096 -extensions v3_ca -keyform PEM -keyout private/key.ca.cg.pem -outform PEM -out certs/crt.ca.cg.pem

You will be asked for a passphrase. Make sure you use a secure passphrase and don’t forget it. You will also be asked other relevant questions. Following is an example output of the process.

Generating a 4096 bit RSA private key
..............................................................................++
...........................................................................................................................................................................................................................................++
writing new private key to 'private/key.ca.cg.pem'
Enter PEM pass phrase:
Verifying - Enter PEM pass phrase:
-----
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) [US]:
State or Province Name (full name) [New York]:
Locality Name (city, district) [New York]:
Organization Name (company) [Code Ghar]:
Organizational Unit Name (department, division) [IT]:
Common Name (hostname, FQDN, IP, or your name) [CGIT]:
Email Address [codeghar@example.com]:

Two files, key.ca.cg.pem and crt.ca.cg.pem, will be created in $dir/private and $dir/certs directories respectively. Make sure you keep these files in a secure place and make their backups.

crt.ca.cg.pem is your root certificate and will be used to sign all the other certificates.

Verify Root Certificate

You should verify that the certificate was created properly with accurate information.

openssl x509 -in certs/crt.ca.cg.pem -inform pem -noout -text

Export Root Certificate

Since this newly created CA and its root certificate are not recognized and trusted by any computer, you need to import the root certificate on all other computers. By default an OS will have a list of trusted CAs and you need to import your CA to that list. The process varies for different OSes.

Windows

The root certificate we created is in PEM encoded format. For Windows we need it to be in DER encoded format. A great resource on the differences between the two is DER vs. CRT vs. CER vs. PEM Certificates and How To Convert Them.

openssl x509 -in certs/crt.ca.cg.pem -outform der -out export/ca.cg.crt

Verify the certificate was created successfully.

openssl x509 -in export/ca.cg.crt -inform der -noout -text

Once you have the exported file, copy it to your Windows machine. You can follow the instructions provided by How To Import a Trusted Root Certification Authority In Windows to import the certificate to the Trusted Root Certification Authorities store on Local Computer.

You can also export the certificate to PKCS12 format. Thanks to Importing a User Certificate to the Windows Certificate Store for this information.

openssl pkcs12 -export -out export/ca.cg.p12 -in certs/crt.ca.cg.pem -inkey private/key.ca.cg.pem

You will be asked to provide the passphrase you used to create the root certificate. You will also be asked for a new “Export Password”.

Copy the .p12 file to Windows and double-click it. A wizard will open and guide you to install it.

Conclusion

The process to create a CA is very simple. Next I will write about signing a certificate request.

Further Reading

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