How to verify a signature of one entity by another using OpenSSL CLI?

All we need is an easy explanation of the problem, so here it is.

I’m teaching a class about certificate chains, so I download the chain from www.google.com which has 4 certs. I want to demonstrate how the dependencies work from issuer to subject by showing that each entity signs the cert of the one below it.

Call the certs g3, g2, g1 and g0 where g3 is a self-signed root, g2 and g1 are intermediate CAs and g0 is a leaf cert. I try this:

$ openssl verify g0
error 20 at 0 depth lookup:unable to get local issuer certificate

This makes sense: the verify fails because this is a leaf cert with no CA given. So next I try this:

$ openssl verify -CAfile g1 g0
g0: OK

Great! With the intermediate, it now works! Continuing up the chain:

$ openssl verify -CAfile g2 g1
g1: OK

Awesome! Looks like g1 is properly signed by g2. But this is actually not what’s being verified. Because g1 is also apparently signed by g3:

$ openssl verify -CAfile g3 g1
g1: OK

And in fact, g1 doesn’t even need a CA:

$ openssl verify g1
OK

So it would appear that any cert with CA set to True in Basic Constraints will always pass verification!

Question: How do I verify that g2 actually signed g1 using OpenSSL from the CLI? And is this yet another OpenSSL bug? Or just counterintuitive behavior?

How to solve :

I know you bored from this bug, So we are here to help you! Take a deep breath and look at the explanation of your problem. We have many solutions to this problem, But we recommend you to use the first method because it is tested & true method that will 100% work for you.

Method 1

I believe certs g2 and g3 are already present in the default trusted directory (/etc/ssl/certs/) which openssl uses to build the chain of trust and find the ‘trust anchor’.

In my setup:

g2 = GeoTrust Global CA
g3 = Equifax Secure Certificate Authority

Both of these certs are present in the /etc/ssl/certs/ directory.

If you try out the same experiment with a self-generated chain of certs, you will find the behaviour that you expect.

Note: Use and implement method 1 because this method fully tested our system.
Thank you 🙂

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0

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