How to create my own certificate chain?

I would like to set up my own OCSP Responder for testing purposes, and this requires me to have a Root certificate with a few certificates generated from it.

I've managed to create a self-signed certificate using openssl, and I want to use it as the Root certificate. The next step would be to create the derived certificates, however, I can't seem to find the documentation on how to do this. Does anyone know where I can find this information?

• Edit:
  In retrospect, my question is not yet completely answered, and to clarify the problem, I'll represent my certificate chain like this: Root > A > B > C > ...
  
  

I am currently able to create the Root and A certificates via the below, but I haven't found how to make a longer chain:

# Root certificate is created like this:
  openssl req -new -newkey rsa:1024 -nodes -out ca.csr -keyout ca.key
  openssl x509 -trustout -signkey ca.key -days 365 -req -in ca.csr -out ca.pem

# Certificate A is created like this:
  openssl genrsa -out client.key 1024
  openssl req -new -key client.key -out client.csr
  openssl ca -in client.csr -out client.cer

• This command implicitly depends on the root certificate, for which it finds the required info within the OpenSSL configuration file, however, certificate B must only rely on A, which is not registered in the config file, so the previous command won't work here.
  
  

What command should I use to create certificates B and beyond?

• Edit:
  I found the answer in this article: Certificate B (chain A -> B) can be created with these two commands and this approach seems to be working well.:

  # Create a certificate request
  openssl req -new -keyout B.key -out B.request -days 365
  
  # Create and sign the certificate
  openssl ca -policy policy_anything -keyfile A.key -cert A.pem -out B.pem -infiles B.request
  

  I also changed the openssl.cnf file:

  [ usr_cert ]
  basicConstraints=CA:TRUE # prev value was FALSE
  

You can use OpenSSL directly.

1. Create a Certificate Authority private key (this is your most important key):

   openssl req -new -newkey rsa:1024 -nodes -out ca.csr -keyout ca.key
   

2. Create your CA self-signed certificate:

   openssl x509 -trustout -signkey ca.key -days 365 -req -in ca.csr -out ca.pem
   

3. Issue a client certificate by first generating the key, then request (or use one provided by external system) then sign the certificate using private key of your CA:

   openssl genrsa -out client.key 1024
   openssl req -new -key client.key -out client.csr
   openssl ca -in client.csr -out client.cer
   

(You may need to add some options as I am using these commands together with my openssl.conf file. You may need to setup your own .conf file first.)

Once you have created your CA, you can use it to sign certs:

• Create a key:

  openssl genrsa -out key_A.key  1024
  

• Create a CSR:

  openssl req -new -key key_A.key -out csr_A.csr
    # You are about to be asked to enter information etc....
  

• Sign it:

  openssl x509 -req -days 365 -in csr_A.csr -CA CA_certificate_you_created.crt \
  -CAkey CA_key_you_created.key -set_serial 01 -out crt_A.crt
  

  And so on, replacing A with B, CA_certificate_you_created.crt with crt_A.crt, and CA_key_you_created.key with key_A.key

Changing the below means that the certificates you issue can be used to sign other certificates:

basicConstraints=CA:TRUE  # prev value was FALSE

OpenSSL comes with a Perl script CA.pl to help you create a self-signed root CA cert, along with the matching private key, plus a few simple files and directories to help keep track of any future certs you sign (a.k.a. issue) with that root CA. It also helps you generate other key pairs and certificate signing requests (CSRs) and helps you process those CSRs (that is, issue certs for them), and more.

Note that many products require CA certs to contain a certain attribute marking them as CA certs, or they won't be accepted as valid signers/issuers of other certs. If the self-signed cert you created does not contain that attribute, you might have trouble getting other software to treat it like a valid root CA cert.

If I recall correctly, the syntax goes something like this:

CA.pl -newca    # Create a new root CA  

CA.pl -newreq   # Create a new CSR

CA.pl -sign     # Sign a CSR, creating a cert  

CA.pl -pkcs12   # Turn an issued cert, plus its matching private key and trust chain, 
                # into a .p12 file you can install on another machine