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I tried to find my root CA today and went through all my certificates using openssl x509 -in /path/to/certificate -purpose.

As I found what should be my root CA I got this output:

Certificate purposes:
SSL client : Yes
SSL client CA : Yes (WARNING code=3)
SSL server : Yes
SSL server CA : Yes (WARNING code=3)
Netscape SSL server : Yes
Netscape SSL server CA : Yes (WARNING code=3)
S/MIME signing : Yes
S/MIME signing CA : Yes (WARNING code=3)
S/MIME encryption : Yes
S/MIME encryption CA : Yes (WARNING code=3)
CRL signing : Yes
CRL signing CA : Yes (WARNING code=3)
Any Purpose : Yes
Any Purpose CA : Yes
OCSP helper : Yes
OCSP helper CA : Yes (WARNING code=3)
Time Stamp signing : No
Time Stamp signing CA : Yes (WARNING code=3)

It seems that some parts of linux or its tools are not as well documented as one thought. It could find no answer to this question not even this site could answered it three years ago (OpenSSL WARNINGS when using -purpose command).

I hope that times have changed and someone here is now capable of answering the question what that WARNING code=3 is all about. Maybe also supplying a list of all possible warning messages and their cause, if there is such.

share|improve this question
up vote 2 down vote accepted

Is that your own self-signed root CA, or an older Verisign V1 Root CA or similar? Because that would generate a warning. From the manual (emphasis mine)

The basicConstraints extension CA flag is used to determine whether the certificate can be used as a CA. If the CA flag is true then it is a CA, if the CA flag is false then it is not a CA. All CAs should have the CA flag set to true.

If the basicConstraints extension is absent then the certificate is considered to be a "possible CA" other extensions are checked according to the intended use of the certificate. A warning is given in this case because the certificate should really not be regarded as a CA: however it is allowed to be a CA to work around some broken software.

If the certificate is a V1 certificate (and thus has no extensions) and it is self signed it is also assumed to be a CA but a warning is again given: this is to work around the problem of Verisign roots which are V1 self signed certificates.

The code=3 might have to do with the fact that X509 v3 extensions are missing. At least that is what a quick scan of the code suggest: ./crypto/x509v3/v3_purp.c:

/*-
 * CA checks common to all purposes
 * return codes:
 * 0 not a CA
 * 1 is a CA
 * 2 basicConstraints absent so "maybe" a CA
 * 3 basicConstraints absent but self signed V1.
 * 4 basicConstraints absent but keyUsage present and keyCertSign asserted.
 */

Downloading a couple of the old VeriSign root certificates complete reproduces your warnings:

wget https://www.symantec.com/content/en/us/enterprise/verisign/roots/Class-3-Public-Primary-Certification-Authority-G2.pem 
openssl x509 -purpose -in Class-3-Public-Primary-Certification-Authority-G2.pem
Certificate purposes:
SSL client : Yes
SSL client CA : Yes (WARNING code=3)
SSL server : Yes
SSL server CA : Yes (WARNING code=3)
Netscape SSL server : Yes
Netscape SSL server CA : Yes (WARNING code=3)
S/MIME signing : Yes
S/MIME signing CA : Yes (WARNING code=3)
S/MIME encryption : Yes
S/MIME encryption CA : Yes (WARNING code=3)
CRL signing : Yes
CRL signing CA : Yes (WARNING code=3)
Any Purpose : Yes
Any Purpose CA : Yes
OCSP helper : Yes
OCSP helper CA : Yes (WARNING code=3)
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
share|improve this answer
    
It is my own self-signed certificate. I also have other self-signed CA certificates that do not produce a warning like that. I do not know why. I think that maybe this is about the second case that works around broken software, because I might be using this cert as a CA and a server cert. I am not 100% sure. I will check on that. Thanks for the quick and extensive answer. – func0der Jul 19 '15 at 16:29
    
It may have to with the version of OpenSSL and options you used when you generated them. The default settings that get shipped now, versus what was used a couple of years ago. The x509 man page for instance indicates that creating a v3 CA certificate requires an explicit switch, which I thought was not required anymore because of the defaults in opensssl.cnf: openssl x509 -req -in careq.pem -extfile openssl.cnf -extensions v3_ca -signkey key.pem -out cacert.pem - And on a somewhat rainy sunday afternoon digging a little deeper is fun! – HBruijn Jul 19 '15 at 16:37
    
Thank you for your comment. I will try to check that out. I would normally do so, but sometimes it is hard to find the right entry point for such searches. Also I was fighting with my openVPN configuration for a couple of hours already ^^ – func0der Jul 27 '15 at 8:10

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