How does SSL use symmetric and asymmetric encryption? And how do I manage certificated for multiple sites on one host? [duplicate]

This question already has answers here: How does browser generate symmetric ke开发者_如何学JAVAy during SSL handshake (2 answers) Closed 2 years ago.

First, some quotation from Microsoft TechNet's Managing Microsoft Certificate Services and SSL:

To recap, secure SSL sessions are established using the following technique:

1. The user's Web browser contacts the server using a secure URL.

2. The IIS server sends the browser its public key and server certificate.

3. The client and server negotiate the level of encryption to use for the secure communications.

4. The client browser encrypts a session key with the server's public key and sends the encrypted data back to the server.

5. The IIS Server decrypts the message sent by the client using its private key, and the session is established.

6. Both the client and the server use the session key to encrypt and decrypt transmitted data.

So, basically speaking, the SSL use the asymmetric encryption (public/private key pair) to deliver the shared session key, and finally achieved a communication way with symmetric encryption.

Is this right?

Add - 1 - 5:55 PM 12/17/2010

I am using IIS to host my websites. Suppose I have multiple sites on my single machine, and I want the client brower to use SSL URL to connect my sites. How many certificates do I need? Which of the following approach should I take?

1 - Apply for a single certicate and associate it to my single server machine which hosts mutiple sites.

2 - Apply for several certificates and associate each of my sites with its own certificate.

In IIS7, it seems I could only do approach 1.

Update - 1 - 6:09 PM 12/17/2010

I figure it out. I could install mutiple certificates on my server machine and bind each site with seperate certificate as necessary.

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Yes, that's right. Asymmetric encryption is necessary to verify the others identity and then symmetric encryption gets used because it's faster.

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You're wrong at points 4 and 5. The server and client independently compute the same session key. It is never actually transmitted at all.

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I would suggest that you post your update as a separate question.

In any case - you will require multiple certificate - one per site. Remember that these certificates tie your machine to your address. Since each of the websites is going to have a different address (potentially) , you need different certs for each of the sites

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You can only have a single SSL cert per listening port on the server. This is because the very first thing that is sent is the server certificate (as in your timeline). This is before the HTTP request so if you try to host two domains on a single server (say foo.com and bar.com) there is no way for the server to know which certificate to send to the client.

There are a few different ways to solve this problem:

1. Host different domains on different servers
2. Host different domains on different ports (eg. foo.com is serverd from 443 and bar.com is served from 8443). If you put your host behind multiple load-balancers, you can have them service all the sites on 443.
3. If the different domains are all sub-domains of a single parent domain, you can get a wildcard certificate. (e.g. domains www.foo.com, bar.foo.com, and baz.foo.com can all use a certificate for *.foo.com)
4. Get a single certificate for one of the domains and have the other domains listed as AltNames. (e.g. both foo.com and bar.com can use a foo.com certificate with a bar.com AltName)

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The answer is both. You will find a nice explanation in 4 steps from digicert.com below:

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• Server sends a copy of its asymmetric public key.
• Browser creates a symmetric session key and encrypts it with the server's asymmetric public key. Then sends it to the server.
• Server decrypts the encrypted session key using its asymmetric private key to get the symmetric session key.
• Server and Browser now encrypt and decrypt all transmitted data with the symmetric session key. This allows for a secure channel because only the browser and the server know the symmetric session key, and the session key is only used for that session. If the browser was to connect to the same server the next day, a new session key would be created.

https://www.digicert.com/ssl-cryptography.htm

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The case where the session key is independently computed by the client and server without the key ever being transmitted is Diffie-Hellman key exchange: https://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exchange --- see the nice paint illustration PKI will exchange the encrypted session key between the client and server.

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The SSL client sends the random byte string that enables both the client and the server to compute the secret key to be used for encrypting subsequent message data. The random byte string itself is encrypted with the server's public key(Asymmetric).

SSL uses both Asymmetric and symmetric keys.