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So I'm looking at google.com's SSL certificate and do not understand a few things:

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I totally get that it ensures the identity of Google's webserver, but don't understand how it proves anything about my identity to the webserver?

Also I'm wondering why there is no "Key Encipherment" or "Key Agreement" purpose for this certificate? How can SSL perform a key exchange / Diffie-Hellman? Is the 2048 bit RSA key (visible under the details tab) used for encryption?

Bruno Rohée
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netik
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1 Answers1

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... how it proves anything about my identity to the webserver?

It does not. It only proves the identity of the server to you so that a man in the middle attack (where someone claims to be google.com) is not possible. If client identification is required (usually not) client certificates could be used.

"The certificate is intended... Proves your identity" does not mean that the certificate is actually used for this purposes. It only means that the certificate can be used for server authentication ("identity of a remote computer") and for client authentication ("your identity"). But in this case it is only used for server authentication.

Actually "your identity" is in fact confusing because it does not mean your identity at all. What this means is that if you would own this certificate (which you don't) then you could use it as a client certificate to prove your identity. Other certificate viewers (Chrome on Linux) show this Extended Key Usage in a less confusing way:

 TLS WWW Server Authentication (OID.1.3.6.1.5.5.7.3.1)
 TLS WWW Client Authentication (OID.1.3.6.1.5.5.7.3.2)

Also I'm wondering why there is no "Key Encipherment" or "Key Agreement" purpose for this certificate? Is the 2048 bit RSA key (visible under the details tab) used for encryption?

The certificate I get for google.com with Chrome has no RSA key but is an ECDSA certificate with a 256 bit key. But with some changes to the cipher preferences I can also get the 2048 bit RSA certificate.

According to Which key usages are required by each key exchange method? it needs digitalSignature in the ECDSA certificate and it has this. According to this Q&A it would also need "Key Encipherment" for ciphers like AES128-SHA (which google supports). But the 2048 bit certificate is missing key usage completely so in this case we refer to RFC 5280: 4.2.1.12. Extended Key Usage:

This extension indicates one or more purposes for which the certified public key may be used, in addition to or in place of the basic purposes indicated in the key usage extension.

Thus if no key usage is given but extended key usage we can imply the key usage from this. And in the same section of the RFC it then states that serverAuth implies digitalSignature, keyEncipherment or keyAgreement. Therefore we have the required keyEncipherment even if it was not explicitely given.

Apart from that neither RSA nor ECDSA keys are used for the encryption of the connection itself, only for authentication and with the ciphers inside the key exchange. Encryption of the traffic is done with symmetric encryption using a key created within the key exchange. See How does SSL/TLS work? for more details.

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Steffen Ullrich
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  • Your answer does not explain why the certificate states that it `proves your identity to a remote computer`, which I think was the OP's question. – SilverlightFox May 26 '16 at 13:48
  • @SilverlightFox: I now see why the OP was confused. He confused how a certificate **can be used** with how **it is used**. I've updated the answer. – Steffen Ullrich May 26 '16 at 13:52
  • The thing I still dont quite understand is why the "key exchange" usage is not required. Is it because the main purpose of ECDSA are digital signatures to proof the servers identity and not key exchange? but from the thread you mentioned I got that SSL will still perform a key exchange by encrypting a random secret with the RSA key right? but this is probably not a key exchange method that requires the keyexchange usage flag to be set? Thank you! – user66875 May 26 '16 at 14:27
  • @user66875: I've edited the answer to include the RSA certificate (which exists too) and also to show why the key usage you miss is implied by the extended key usage. – Steffen Ullrich May 26 '16 at 15:56