How to extract the Root CA and Subordinate CA from a certificate chain in Linux?

Question

I have an end-entity/server certificate which have an intermediate and root certificate. When I cat on the end-entity certificate, I see only a single BEGIN and END tag. It is the only the end-entity certificate.

Is there any way I can view the intermediate and root certificate content. I need only the content of BEGIN and END tag.

In Windows I can see the full cert chain from the "Certification Path". Below is the example for the Stack Exchange's certificate.

From there I can perform a View Certificate and export them. I can do that for both root and intermediate in Windows. I am looking for this same method in Linux.

Answer 1

From a web site, you can do:

openssl s_client -showcerts -verify 5 -connect stackexchange.com:443 < /dev/null

That will show the certificate chain and all the certificates the server presented.

Now, if I save those two certificates to files, I can use openssl verify:

$ openssl verify -show_chain -untrusted dc-sha2.crt se.crt 
se.crt: OK
Chain:
depth=0: C = US, ST = NY, L = New York, O = "Stack Exchange, Inc.", CN = *.stackexchange.com (untrusted)
depth=1: C = US, O = DigiCert Inc, OU = www.digicert.com, CN = DigiCert SHA2 High Assurance Server CA (untrusted)
depth=2: C = US, O = DigiCert Inc, OU = www.digicert.com, CN = DigiCert High Assurance EV Root CA

The -untrusted option is used to give the intermediate certificate(s); se.crt is the certificate to verify. The depth=2 result came from the system trusted CA store.

If you don't have the intermediate certificate(s), you can't perform the verify. That's just how X.509 works.

Depending on the certificate, it may contain a URI to get the intermediate from. As an example, openssl x509 -in se.crt -noout -text contains:

        Authority Information Access: 
            OCSP - URI:http://ocsp.digicert.com
            CA Issuers - URI:http://cacerts.digicert.com/DigiCertSHA2HighAssuranceServerCA.crt

That "CA Issuers" URI points to the intermediate cert (in DER format, so you need to use openssl x509 -inform der -in DigiCertSHA2HighAssuranceServerCA.crt -out DigiCertSHA2HighAssuranceServerCA.pem to convert it for further use by OpenSSL).

If you run openssl x509 -in /tmp/DigiCertSHA2HighAssuranceServerCA.pem -noout -issuer_hash you get 244b5494, which you can look for in the system root CA store at /etc/ssl/certs/244b5494.0 (just append .0 to the name).

I don't think there is a nice, easy OpenSSL command to do all that for you.

Answer 2

tl;dr - one liner bash magic to dump all certs in the chain

openssl s_client -showcerts -verify 5 -connect wikipedia.org:443 < /dev/null |
   awk '/BEGIN CERTIFICATE/,/END CERTIFICATE/{ if(/BEGIN CERTIFICATE/){a++}; out="cert"a".pem"; print >out}'
for cert in *.pem; do 
        newname=$(openssl x509 -noout -subject -in $cert | sed -nE 's/.*CN ?= ?(.*)/\1/; s/[ ,.*]/_/g; s/__/_/g; s/_-_/-/; s/^_//g;p' | tr '[:upper:]' '[:lower:]').pem
        echo "${newname}"; mv "${cert}" "${newname}" 
done

Explanation in 2 steps

To dump all certs in the chain to the current dir as cert${chain_number}.pem:

openssl s_client -showcerts -verify 5 -connect your_host:443 < /dev/null |
 awk '/BEGIN CERTIFICATE/,/END CERTIFICATE/{ if(/BEGIN CERTIFICATE/){a++}; out="cert"a".pem"; print >out}' 

The bonus-track to rename them to their common name:

for cert in *.pem; do 
   newname=$(openssl x509 -noout -subject -in $cert | sed -nE 's/.*CN ?= ?(.*)/\1/; s/[ ,.*]/_/g; s/__/_/g; s/_-_/-/; s/^_//g;p' | tr '[:upper:]' '[:lower:]').pem
   mv $cert $newname 
done

Answer 3

I found out that with the option -verify 5 openssl is going deep in the chain showing all the cert, even that not included in your certificate deployment.

If you really want to understand which chain is provided with your certificate you should run:

openssl s_client -showcerts -partial_chain -connect YOUR_ENDPOINT:443 < /dev/null |less

Answer 4

I also had the same challenge and next to that I discovered that openssl doesn't return the root ca. I have built an alternative for specifically for this purpose which might be useful for other developers, see here: GitHub - Certificate ripper

Usage

• Printing to the console

crip print --url=https://stackexchange.com --format=pem

• Exporting to a p12 trustore

crip export p12 --url=https://stackexchange.com

The p12 option can be replaced for pem or der if you want a different output.

Answer 5

The above failed to work for me when I was using a Lets Encrypt Free Wildcard Certificate.

To be more specific:
I have a Kubernetes Cluster + Ingress Controller that's configured using a Lets Encrypt Free Wildcard Certificate for *.mydomain.dev, it's hosting the following 2 domain names:

• grafana.mydomain.dev
• prometheus.mydomain.dev

I had to add the -servername flag following per this site: https://community.letsencrypt.org/t/where-can-i-download-the-trusted-root-ca-certificates-for-lets-encrypt/33241/2

export DOMAIN=grafana.mydomain.dev
openssl s_client -showcerts -verify 5 -connect $DOMAIN:443 -servername $DOMAIN < /dev/null 2> /dev/null | awk '/BEGIN/,/END/{ if(/BEGIN/){a++}; print}'

I also tweaked the chain so it gives me the full cert + intermediate + root to standard output & hide stderr noise.

Actually... When testing I found out this doesn't give me the CA... openssl s_client -showcerts -verify 5 -connect letsencrypt.org:443 < /dev/null 2> /dev/null | awk '/BEGIN/,/END/{ if(/BEGIN/){a++}; print}'

curl https://letsencrypt.org/certs/isrgrootx1.pem Gives a different value (and when testing some stuff this is the value that works.)