[openssl-users] keyusage digitalSignature in CA certs

[Erwann Abalea]

Bonjour,

> Le 17 août 2017 à 15:20, Robert Moskowitz <rgm at htt-consult.com> a écrit :
> 
> Should digitalSignature be included in keyusage in CA certs?

It depends on what you plan to do with the corresponding private key.
If you want this private key to sign messages other than certificates and CRLs (such as OCSP responses), then yes.

> 
> https://jamielinux.com/docs/openssl-certificate-authority/create-the-root-pair.html
> 
> Includes it.
> 
> https://stackoverflow.com/questions/21297139/how-do-you-sign-certificate-signing-request-with-your-certification-authority/21340898#21340898
> 
> Does not include it.
> 
> It seems to make a root or intermediate CA be able to have more purposes than it should?  e.g.
> 
> SSL client : Yes
> SSL server : Yes
> S/MIME signing : Yes

This is the result of an analysis of the keyUsage *and* the extendedKeyUsage extensions (and maybe obsolete Netscape proprietary ones).

> So which is the right for a CA's key usage?

That really depends on what you want it to be valid for.

keyUsage=keyCertSign is fine for certificate signing
keyUsage=cRLSign is fine for CRL signing
keyUsage=digitalSignature is fine for OCSP signing

The other bits are not that common for a CA.

You can achieve the capabilities with different certificates.

For example, a keyCertSign-only CA cert can self-issue a cRLSign certificate in order to produce CRLs and a digitalSignature certificate to sign OCSP responses, or an issuing CA can issue different certificates for the same CA (they all have the same Subject, which is different from the issuing’s Subject) but for different purposes (and thus different keyUsage bits).

Cordialement,
Erwann Abalea