X509: What's the difference between digital signature and non-repudiation
I have to deal with certificates issued by the Swiss post office on USB tokens. There deliver two certificates on the same token. In their intended usage fields, one has "non repudiation" and the other "digital signature".
Now, I can't understand what the practical difference between the two are: I've always seen both in the same certificate, never two certs for the same identity each with one of the roles. In fact, I can't imagine a scenario where non-repudiation and digital signature aren't the same, for all practical matter, the same thing.
Could anyone explain to me what the difference is, please ? And if you had a suggestion about in what situation one should be picked over the other, that would help as well.s
Answer
I realise this question is a bit old, but I think I can shed some much-needed light on the question.
The non-repudiation value in the keyUsage attribute relates to the whole certificate, not any purpose in particular. The presence of the non-repudiation flag indicates that the private key has sufficient protections in place that the entity named in the certificate cannot later repudiate—deny—actions they take with the certificate. The presence of the flag doesn't prevent repudiation, rather it indicates that repudiation isn't likely to survive reasonable scrutiny.
So in this specific case, the CA is giving the user the option of a certificate that does or does not include the non-repudiation element. If you want to assert to those verifying the signature that you can't easily deny it was you who signed it (the USB token is the key enabler here), use the non-repudiation certificate. Otherwise, use the certificate marked for digital signatures. (Depending on the other attributes in the certificate, you may or may not be able to sign documents with either or both certificates.)
See Wikipedia: http://en.wikipedia.org/wiki/Non-repudiation
See also the relevant RFC: http://www.faqs.org/rfcs/rfc3280.html (section 4.2.1.3)