Recall that even the 'ca' certificate is ultimately self-signed. So your question is really about why some self-signed certificates are more trusted than others.

In some fashion you could ask this question about any typical 'brand name' store. Why is Store-X trusted more than Store-Y? Simply because more people (or at least the person in question) has more experience with Store-X. Similarly for any particular self-signed CA cert, although we replace experience here with 'it is already in my certificate store', it is more trusted if the client knows about it already.

Now what if Store-Y isn't a chain store. Instead it is a little local boutique? Perhaps there isn't a need/expectation that a brand name and national marketing campaign is required; but they'd still like people to recognize their letterhead. So a logo and a local 'brand' is all that is required.

Similarly if all I want is for people to recognize my self-signed certificate I don't really need a CA, a pki hierarchy and all that. One self signed certificate should be enough...

If I've made things confusing with my metaphor you could also just think about the model for ssh... it is often valuable just to be able to know you're communicating with the same person you communicated with last time.

        - max

On Oct 13, 2006, at 5:02 PM, Chong Peng wrote:

guys:

we all know that a ca-signed certificate can provide authentication because the ca is trustable, by using ca-signed certificate, one is saying "i am somebady because the ca says so". but it seems that a self-signed certificate _cannot_ provide any authentication at all, because by using self-signed certificate, one is saying "i am somebody because i say so".

if my understanding is correct, then why self-signed certificate is still used?

thanks.

chong peng
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