Thanks for the info. I think I understand now. A plain old digital signature will not have the cert info, but a PKCS7 container does. I think that is what I have, if I followed the example correctly. Am I correct in saying that a PKCS7 container contains a digital signature + certificate information?
In the verification process, it only verifies that the certificate was signed by a "trusted" CA. But I would like to know who this trusted CA is. The reason is, this...I have a piece of data that I want to sign. I give this piece of data along with my application to someone else. When this person runs the app, I want to verify that he is using the data that I gave him. So I want to make sure the data he uses is signed by me. To do that, I run the verification on the signed data (the PCKS7 container). But the certificate validation only tells me if it was signed by a trusted CA. But there is nothing to prevent this person from creating their own CA, making it a trusted CA on their system, creating their own data file, and signing it. That's why I want to make sure the data was signed by a certificate that was issued to me, and that the root CA is the well known, trusted CA that I had sign my cert; not his own CA that he made "trusted" on his system. Ed > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of > Bernhard Froehlich > Sent: Friday, March 04, 2005 12:31 AM > To: openssl-users@openssl.org > Subject: Re: Questions about cert verification > > Edward Chan wrote: > > > I've been trying to follow the examples in "Network Security with > > OpenSSL". But I just don't get it. I know, I'm an idiot. Can > > somebody point me in the right direction with the > appropriate API's to > > use for doing the following: > > > > I have a digital signature that I want to verify. As part of my > > verification, I want to > > > > 1. get the certificate information from the signature. I > want to know > > who created the signature (so I want to look at the cert > that was used > > to create the signature). I also want to know the the root CA who > > signed this cert. > > > > 2. how do I get an X509* to these certs? > > > > 3. how do I verify that the root cert is that of a specific > CA. For > > this, can I simply compare the public key in this root cert > with the > > public key that is known for the CA of interest. Is that enough to > > determine identity of the root cert? Nobody else can create a self > > signed cert with the same public key can they? > > > > Does this make any sense? I'm not sure if I'm explaining myself > > correctly, so this may seem like jibberish. If so, please let me > > know. And thanks for any help you can give. > > > > Thanks, > > > > Ed > > > I'm giving this a try, but I'm not very sure of myself in > this area. If something sounds not plausible (or you know > that it's wrong) please correct me. > > The "digital signature" in a technical sense (like it is used > in Chapter > 8 of the O'Reilly book) consists of the encrypted checksum of > the data. > So it does not contain a certificate or even an ID of the key > used to generate it. You have to know the key in advance > before you can check the signature. > Applications which digitally sign data (like S/MIME) usually > transfer additional information to specify the used key and > its certificates. In the case of S/MIME a PKCS#7 container is > used to this effect. > So you cannot verify a "naked" signature without knowing the > public key. > On the other hand, if you have a PKCS#7 container you can use > the approach described in the book's Chapter 10 > (PKCS7_verify) or use other > PKCS7 APIs to extract public key and certificate from the container. > To be sure a certificate is issued by a specific CA you > should use this CA's certificate as the only trusted CA in > the verification process. If you don't trust the CA for > issuing Sub-CA-certs you can additionally check the length of > the certificat chain. This should take care of the issue in > the most appropriate way. > Comparing public keys may also work, but why to repeat the > job that has already be done by OpenSSL in the verification process? > > Hope it helps > Ted > ;) > > -- > PGP Public Key Information > Download complete Key from http://www.convey.de/ted/tedkey_convey.asc > Key fingerprint = 31B0 E029 BCF9 6605 DAC1 B2E1 0CC8 70F4 7AFB 8D26 > > ______________________________________________________________________ OpenSSL Project http://www.openssl.org User Support Mailing List openssl-users@openssl.org Automated List Manager [EMAIL PROTECTED]
