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Linux/Unix - Create a local Certificate Authority (CA)

I get these questions all the time - people know i have some runtime with certificates and such - one question is "Can't i just issue my own certs?" - and the answer of course is yes - but I always make sure to add that it won't be any use on a public web site since no-one will trust it.
So setting up your own CA is not "generally useful", it is more if you need some specific things, like issuing certificates with a single signing source for client logins or similar.

Most business will have a  couple of Windows Domain controllers, if you need to sign certs for a limited set of users, what you should do is make sure some system in your windows domain runs Certificate Services, then issue certs from there, make sure any non-domain-members has a trust for that CA.

If you actually do need to set up you own CA, here is one way to do it

Procedure to set up your own local CA

The common name for the CA cert must NOT be the same as a domain name or anything else you will need a cert for, I used   "myLocalCA"  for my cn in this example.
There is more than one way to do this, i recommend using a sequence file for serials - so this should work if you follow this setup.
I recommend using a local user which is not used for anything else - the below assumes you run as that user


  • Create a directly, set very private permissions, and go to it
    • mkdir ~/myLocalCA
    • chmod 0700 ~/myLocalCA
    • cd ~/myLocalCA
  • Create a protected CA key:  
    • openssl genrsa -des3 -out myLocalCA.key 4096
  • Create the CA cert valid for 10 years:
    • openssl req -new -x509 -days 3650 -key myLocalCA.key -out myLocalCA.cert
    • Enter Country, State, City, Org, dept
    • Enter common name as the permenanent name of your CA (example: myLocalCA)
  • Use a Sequence file for the CA certificates serial number - it is best to just let openssl create it when you issue your first cert, just make sure to always reference the same file. (Each signed cert must have a Unique-to-the-CA serial).
    • When you sign, use these options: 
      • -CAcreateserial -CAserial myLocalCA.serial.sequence

Procedure to process (Sign) CSR

To do this you first need a CSR, chances are you be making them yourself, but you may also get them from a device or person or similar. For information on how to create a Certificate Signing request, see https://www.saumgm.com/p/openssl-cheat-sheet.html

Log in as your CA processor user (Assuming you did as the example above)

  • cd ~/myLocalCA
  • openssl x509 -req -days 720 -in /path/to/myhost.csr  -out /path/to/myhost.cert -CA myLocalCA.cert -CAkey myLocalCA.key -CAcreateserial -CAserial myLocalCA.serial.sequence
That is it - then just send or install the cert file where it is needed.

Trusting the CA

In order for browsers and things to trust certificates issued by this CA, you need to install the root certificate, the myLocalCA.cert file in a local CA trust repository. (Never share the private key, only the certificate!)

WARNING: You should NOT do this in any home, business, or production network or systems. Once you do this, any cert signed with this key will be trusted, make sure you keep your CA privates private and secure, and do not sign requests you do not know and trust completely.

On Ubuntu you can copy the cert file to /usr/local/share/ca-certificates/myLocalCA/myLocalCA.cert, make sure it has 0644 permissions, then do sudo update-ca-certificate

On Windows, right click the file and select install, choose the Trust CA repository.


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