Should X509 certificate have nonRepudiation bit set to check PKCS7 signature?

Question

X509 certificate has set of keyUsage bits. Two of them are digitalSignature
nonRepudiation (recent editions of X.509 have renamed this bit to contentCommitment).

I read X509 RFC (http://tools.ietf.org/html/rfc5280) and it talks about general usage of these bit.

And I read PKCS7 RFC (http://tools.ietf.org/html/rfc2315) and it talks about PKCS7 structure and so on and doesn't specify which bits needs to be set.

Is there any RFC or other specification which identifies whether one or both of them should be set?

Regards, Victor

Answer 1

A PKCS#7 file generally contains a chain of certificates. That is, a Root CA certificate, any intermediate CA certificates that apply, and then the endpoint certificate (SSL, e-mail, etc.) A PKCS#7 is typically used to bundle these up into a single file. It's useful, in that you can import the entire chain at once into a keystore or other relying application.

As to the key usage bits, those are set depending on the needs and purpose of a particular certificate. For example, a Root CA certificate would typically have both digital signature and non-repudiation set. For an SSL certificate, you may find key encipherment and digital signature. There's really no correlation between key usage and PKCS#7 files, unless you're talking about the CA certificates contained in the PKCS#7 file.

Answer 2

BTW, this bit violates the separation of concerns in its design. Non-repudiation is a legal issues negotiated on the business level. Using the bit at the certificate/signing level is irrelevant. See e.g. http://www-personal.umich.edu/~lsiden/tutorials/signed-applet/ShockingTruth.html