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 smime.p7s Description: S/MIME Cryptographic Signature
[Fwd: Delivery problems: Re: Questions about cert verification]
Hi list-admin / Bert Koster, I keep receiving NDRs like the one attached for every mail I send to this list. Looks like someone using the email [EMAIL PROTECTED] or [EMAIL PROTECTED] has an invalid forward on his account. If you are using this eMail maybe you should check your settings. Otherwise I'd ask for that adress to be removed from the mailing list. 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 ---BeginMessage--- The recipient is unknown Warning, delivery failure! This is a status message indicating that a message could not be delivered to 1 or more recipients. Original message subject: Re: Questions about cert verification Date received: 04-Mar-2005 10:02:44 +0100 Recipients and delivery history [EMAIL PROTECTED] Transcript of session follows --- 04-Mar-2005 10:02:44 +0100 Received via SMTP from MMX1.ENGELSCHALL.COM 04-Mar-2005 10:02:50 +0100 Forwarding from [EMAIL PROTECTED] to [EMAIL PROTECTED] 04-Mar-2005 10:03:08 +0100 [EMAIL PROTECTED] is unknown Reporting-MTA: dns;hemelwijs.aweka.org. Final-Recipient: rfc822;sysadm1@aweka.org Action: failed Status: 5.0.0 (permanent failure) ---BeginMessage--- This is a cryptographically signed message in MIME format. 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 smime.p7s Description: S/MIME Cryptographic Signature ---End Message--- ---End Message--- smime.p7s Description: S/MIME Cryptographic Signature
Re: [Fwd: Delivery problems: Re: Questions about cert verification]
On Fri, Mar 04, 2005, Bernhard Froehlich wrote: Hi list-admin / Bert Koster, I keep receiving NDRs like the one attached for every mail I send to this list. Looks like someone using the email [EMAIL PROTECTED] or [EMAIL PROTECTED] has an invalid forward on his account. If you are using this eMail maybe you should check your settings. Otherwise I'd ask for that adress to be removed from the mailing list. Solved. Ralf S. Engelschall [EMAIL PROTECTED] www.engelschall.com __ OpenSSL Project http://www.openssl.org User Support Mailing Listopenssl-users@openssl.org Automated List Manager [EMAIL PROTECTED]
RE: Questions about cert verification
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 Listopenssl-users@openssl.org Automated List Manager [EMAIL PROTECTED]
Re: Questions about cert verification
Edward Chan wrote: 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? a pkcs7 object may contain the signer certificate (and other), but it's not mandatory 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. you specify the trusted certs through supplying the X509_STORE of trusted certificates 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. if the attacker has control over the application you can't really prevent this. The only thing you can do is to make things more difficult for the attacker for example by using hard coded or encrypted certificates but this will not stop a skilled attacker. Nils __ OpenSSL Project http://www.openssl.org User Support Mailing Listopenssl-users@openssl.org Automated List Manager [EMAIL PROTECTED]
