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Recently, i have been looking at PKI and certificates, and TLS 1.2 as part of my study of cryptography. I have the understanding that the public key included in the certificate of a website is the key that is used to establish a session key between two parties.

But on certain websites, when i opened google chromes security overview by clicking on the green https padlock, chrome informed me that the key exchange was done by ECDHE with P-256 (Eliptic Curve Diffe Hellman, curve P-256). But, upon opening the full certificate details for the website, the public key in the certificate was an RSA 2048 bit public key. But, this confused me, as i thought it should be an ECDHE key and i am not sure whether the key exchange is done by ECDHE, as chrome says, or RSA, which is the public key in the certificate, which i thought was used in the key establishment.

Any help would be appreciated.

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You are mixing up key exchange and authentication. Key exchange is done so that both parties agree on the same encryption keys. Authentication of the server is done to make sure that the client is talking to the correct server and not to some man in the middle. The RSA/ECC key in the certificate is used for authentication. In case of RSA key exchange the RSA key in the certificate is also used of the key exchange but with DHE and ECDHE key exchanges the certificate key is not used. This means you can combine ECDHE key exchange with both RSA and ECC certificates.

For the deeper details see How does SSL/TLS work?

  • Oh thanks, just the book I am working from shows the public key in the certificate being used to compute the session key, that was why I was getting confused. – Sam Gregg Feb 4 '17 at 18:43
  • @SamGregg: that's RSA key exchange then not ECDHE. See edited response. – Steffen Ullrich Feb 4 '17 at 18:44
  • The book shows a DHKE key exchange with certificates (Understanding Cryptography) – Sam Gregg Feb 4 '17 at 18:46
  • @SamGregg: it is hard to tell what you are referring to but I guess that you refer to the very uncommon case of using DH with group parameters defined in the certificate. See security.stackexchange.com/a/14190/37315. I would actually recommend that if you are asking about TLS (i.e. what you see in google chrome) that you don't take a book as reference which does not specifically address how things are done in TLS. – Steffen Ullrich Feb 4 '17 at 19:54
  • Yes that is the case, probably used because the book is an introduction and not devoted to key establishment. – Sam Gregg Feb 4 '17 at 19:58
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Traditionally in SSL/TLS the client chose a master secret and encrypted it with the RSA key from the server's certificate. This is the approach used by the traditional RSA ciphersuites.

However the problem with this approach is that it if the server's long term private key is ever compromised then at attacker can passively decrypt the traffic. In particular they can go back to sessions that they recorded in the past and decrypt them retroactively.

So nowadays it is increasingly common to use "ECDHE-RSA" ciphersuites. In these the key exchange is performed by ephemeral elliptic curve diffe-hellman. The emphemeral bit means that the ECDH keys are only used for that one session. The RSA key from the certificate is used to sign the exchange to block man-in the middle attacks.

This means that someone who steals the server's long term private key is limited to active attacks on future sessions, they can't decrypt passively.

  • Actually DHE and then ECDHE became common 6 years ago, soon after Snowden. Today TLS1.3 is becoming common, and it no longer has keyexchange in the ciphersuite at all, and only has DHE (limited to rfc7719 groups and now fairly rare) or ECDHE keyexchange except for manually configured PSK (rare) or PSK that replaces session resumption. – dave_thompson_085 Aug 22 at 3:49

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