[openssl] Verify a certificate chain using openssl verify

I'm building a own certificate chain with following componenents:

Root Certificate - Intermediate Certificate - User Certificate

Root Cert is a self signed certificate, Intermediate Certificate is signed by Root and User by Intermediate.

Now I want to verify if a User Certificate has its anchor by Root Certificate.

With

openssl verify -verbose -CAfile RootCert.pem Intermediate.pem

the validation is ok. In the next step I validate the User Cert with

openssl verify -verbose -CAfile Intermediate.pem UserCert.pem

and the validation shows

error 20 at 0 depth lookup:unable to get local issuer certificate

What is wrong?

The problem is, that openssl -verify does not do the job.

As Priyadi mentioned, openssl -verify stops at the first self signed certificate, hence you do not really verify the chain, as often the intermediate cert is self-signed.

I assume that you want to be 101% sure, that the certificate files are correct before you try to install them in the productive web service. This recipe here performs exactly this pre-flight-check.

Please note that the answer of Peter is correct, however the output of openssl -verify is no clue that everything really works afterwards. Yes, it might find some problems, but quite not all.

Here is a script which does the job to verify a certificate chain before you install it into Apache. Perhaps this can be enhanced with some of the more mystic OpenSSL magic, but I am no OpenSSL guru and following works:

#!/bin/bash
# This Works is placed under the terms of the Copyright Less License,
# see file COPYRIGHT.CLL.  USE AT OWN RISK, ABSOLUTELY NO WARRANTY. 
#
# COPYRIGHT.CLL can be found at http://permalink.de/tino/cll
# (CLL is CC0 as long as not covered by any Copyright)

OOPS() { echo "OOPS: $*" >&2; exit 23; }

PID=
kick() { [ -n "$PID" ] && kill "$PID" && sleep .2; PID=; }
trap 'kick' 0

serve()
{
kick
PID=
openssl s_server -key "$KEY" -cert "$CRT" "$@" -www &
PID=$!
sleep .5    # give it time to startup
}

check()
{
while read -r line
do
    case "$line" in
    'Verify return code: 0 (ok)')   return 0;;
    'Verify return code: '*)    return 1;;
#   *)  echo "::: $line :::";;
    esac
done < <(echo | openssl s_client -verify 8 -CApath /etc/ssl/certs/)
OOPS "Something failed, verification output not found!"
return 2
}

ARG="${1%.}"
KEY="$ARG.key"
CRT="$ARG.crt"
BND="$ARG.bundle"

for a in "$KEY" "$CRT" "$BND"
do
    [ -s "$a" ] || OOPS "missing $a"
done

serve
check && echo "!!! =========> CA-Bundle is not needed! <========"
echo
serve -CAfile "$BND"
check
ret=$?
kick

echo
case $ret in
0)  echo "EVERYTHING OK"
    echo "SSLCertificateKeyFile $KEY"
    echo "SSLCertificateFile    $CRT"
    echo "SSLCACertificateFile  $BND"
    ;;
*)  echo "!!! =========> something is wrong, verification failed! <======== ($ret)";;
esac

exit $ret

Note that the output after EVERYTHING OK is the Apache setting, because people using NginX or haproxy usually can read and understand this perfectly, too ;)

There is a GitHub Gist of this which might have some updates

Prerequisites of this script:

• You have the trusted CA root data in /etc/ssl/certs as usual for example on Ubuntu
• Create a directory DIR where you store 3 files:
  • DIR/certificate.crt which contains the certificate
  • DIR/certificate.key which contains the secret key for your webservice (without passphrase)
  • DIR/certificate.bundle which contains the CA-Bundle. On how to prepare the bundle, see below.
• Now run the script: ./check DIR/certificate (this assumes that the script is named check in the current directory)
• There is a very unlikely case that the script outputs CA-Bundle is not needed. This means, that you (read: /etc/ssl/certs/) already trusts the signing certificate. But this is highly unlikely in the WWW.
• For this test port 4433 must be unused on your workstation. And better only run this in a secure environment, as it opens port 4433 shortly to the public, which might see foreign connects in a hostile environment.

How to create the certificate.bundle file?

In the WWW the trust chain usually looks like this:

• trusted certificate from /etc/ssl/certs
• unknown intermediate certificate(s), possibly cross signed by another CA
• your certificate (certificate.crt)

Now, the evaluation takes place from bottom to top, this means, first, your certificate is read, then the unknown intermediate certificate is needed, then perhaps the cross-signing-certificate and then /etc/ssl/certs is consulted to find the proper trusted certificate.

The ca-bundle must be made up in excactly the right processing order, this means, the first needed certificate (the intermediate certificate which signs your certificate) comes first in the bundle. Then the cross-signing-cert is needed.

Usually your CA (the authority who signed your certificate) will provide such a proper ca-bundle-file already. If not, you need to pick all the needed intermediate certificates and cat them together into a single file (on Unix). On Windows you can just open a text editor (like notepad.exe) and paste the certificates into the file, the first needed on top and following the others.

There is another thing. The files need to be in PEM format. Some CAs issue DER (a binary) format. PEM is easy to spot: It is ASCII readable. For mor on how to convert something into PEM, see How to convert .crt to .pem and follow the yellow brick road.

Example:

You have:

• intermediate2.crt the intermediate cert which signed your certificate.crt
• intermediate1.crt another intermediate cert, which singed intermediate2.crt
• crossigned.crt which is a cross signing certificate from another CA, which signed intermediate1.crt
• crossintermediate.crt which is another intermediate from the other CA which signed crossigned.crt (you probably will never ever see such a thing)

Then the proper cat would look like this:

cat intermediate2.crt intermediate1.crt crossigned.crt crossintermediate.crt > certificate.bundle

And how can you find out which files are needed or not and in which sequence?

Well, experiment, until the check tells you everything is OK. It is like a computer puzzle game to solve the riddle. Every. Single. Time. Even for pros. But you will get better each time you need to do this. So you are definitively not alone with all that pain. It's SSL, ya' know? SSL is probably one of the worst designs I ever saw in over 30 years of professional system administration. Ever wondered why crypto has not become mainstream in the last 30 years? That's why. 'nuff said.

From verify documentation:

If a certificate is found which is its own issuer it is assumed to be the root CA.

In other words, root CA needs to self signed for verify to work. This is why your second command didn't work. Try this instead:

openssl verify -CAfile RootCert.pem -untrusted Intermediate.pem UserCert.pem

It will verify your entire chain in a single command.

You can easily verify a certificate chain with openssl. The fullchain will include the CA cert so you should see details about the CA and the certificate itself.

openssl x509 -in fullchain.pem -text -noout

After breaking an entire day on the exact same issue , with no prior knowledge on SSL certificates, i downloaded the CERTivity Keystores Manager and imported my keystore to it, and got a clear-cut visualisation of the certificate chain.

Screenshot :

If you only want to verify that issuer of UserCert.pem is actually Intermediate.pem do the following (example uses: OpenSSL 1.1.1):

openssl verify -no-CAfile -no-CApath -partial_chain -trusted Intermediate.pem UserCert.pem

and you will get:

UserCert.pem: OK

or

UserCert.pem: verification failed

That's one of the few legitimate jobs for cat:

openssl verify -verbose -CAfile <(cat Intermediate.pem RootCert.pem) UserCert.pem

Update:

As Greg Smethells points out in the comments, this command implicitly trusts Intermediate.pem. I recommend reading the first part of the post Greg references (the second part is specifically about pyOpenSSL and not relevant to this question).

In case the post goes away I'll quote the important paragraphs:

Unfortunately, an "intermediate" cert that is actually a root / self-signed will be treated as a trusted CA when using the recommended command given above:

$ openssl verify -CAfile <(cat geotrust_global_ca.pem rogue_ca.pem) fake_sometechcompany_from_rogue_ca.com.pem fake_sometechcompany_from_rogue_ca.com.pem: OK

It seems openssl will stop verifying the chain as soon as a root certificate is encountered, which may also be Intermediate.pem if it is self-signed. In that case RootCert.pem is not considered. So make sure that Intermediate.pem is coming from a trusted source before relying on the command above.

I've had to do a verification of a letsencrypt certificate and I did it like this:

1. Download the root-cert and the intermediate-cert from the letsencrypt chain of trust.

2. Issue this command:

   $ openssl verify -CAfile letsencrypt-root-cert/isrgrootx1.pem.txt -untrusted letsencrypt-intermediate-cert/letsencryptauthorityx3.pem.txt /etc/letsencrypt/live/sitename.tld/cert.pem 
   /etc/letsencrypt/live/sitename.tld/cert.pem: OK