SSL: How are certificates protected against man in the middle attacks?

14
votes

My question is about certificates specifically in ssl but I think the questions should apply to all certificates. I have included the SSL procedure for the sake of clarity.

In SSL this is what I understand the procedure is:

1)Client

  • sends supported crypto algorithms
  • sends client nonce
  1. Server
  • chooses (and sends) a
  • symmetric algorithm
  • a public key algorithm
  • a MAC algorithm
  • sends it's certificate
  • sends server nonce
  1. Client
  • verifies certificate
  • Extracts public key
  • Generates a pre-master secret key (pms)
  • encrypts with servers public key and sends
  1. Client and Server
  • compute master secrete (MS) from PMS and nonces
  • PMS sliced to generate two encryption & two mac keys
  1. Client
  • sends a mac of all handshakes (to ensure they were not previously modifide)
  1. Server
  • sends a mac of all handshakes

Question

What stops a man in the middle attack from happening at step two? Why can't a man in the middle, say trudy, capture the certificate sent by the server and change the public key in it (to something it has the private key to).

I assume that the certificate is encrypted somehow.

However the server cannot encrypt the certificate because the client does not have the public key yet. When the server gets the key from an authority (like veri-sign) would the key be pre-encrypted using verisign's public key? I think this should work because all web browsers should have the public keys of most authorities.

3
securitynetworkingsslcertificate
In depth description of how ssl (or TLS) works for anyone interested: moserware.com/2009/06/first-few-milliseconds-of-https.html – sixtyfootersdude

3 Answers

12
votes

No, the certificate is not encrypted. But it is signed by a certification authority (CA). Since those check the information included in the certificate (especially the URL to which the cert belongs), there shouldn't be a second valid certificate for a given URL.

The cert of the CA is checked against a trust store (e.g. in your browser). If this truststore is compromised, or if you trust not valid certificates, there is no protection against man in the middle attacks

4
votes

Certificates are signed by some trusted authority, such as Verisign.

The certificates for these root authorities are built right into the browsers when you download them. You can view the root certificates in Firefox, for example, by going to tools-->options-->advanced-->encryption-->view certificates-->authorities.

If any one of these root-certificate authorities is compromised, however, you are right that a certificate could be forged, making a man-in-the-middle attack possible.

2
votes

You actually pointed out a weak spot of PKI.

Say Trudy is in the middle of you and yourbank (bank.com). Trudy can change the public key at will at step 2 but the certificate's signature will be invalid. So Trudy has to find a way to generate the signature again. It's safe to say that the trusted CAs will not do this for him. So he has to sign with a fake CA, which is not trusted by your browser. This is still safe theoretically.

However, most browsers (especially IE 6) display a vague security warning and most people don't understand and just ignore, according to some tests.