SSL Authentication with Certificates: Should the C

Question

Quick Version of Question

Gmail, TD (Canadian Bank), Royal Bank (Canadian Bank) all use ssl. When you inspect their certificates they all have

Common Name (CN)   mail.google.com

Or more generally:

Common Name (CN)   <url>

Is this needed to prevent man in the middle attacks?

Summary

JBoss allows clients and servers to authenticate using certificates and ssl. One thing that seems strange is that you are not required to give your hostname on the certificate.

I think that this means if Server B is in your truststore, Sever B can pretend to be any server that they want.

(And likewise: if Client B is in your truststore...)

Am I missing something here?

Authentication Steps

(Summary of Wikipeida Page)

Client                                                  Server
=================================================================================================
1) Client sends Client Hello
        ENCRIPTION: None
        - highest TLS protocol supported
        - random number
        - list of cipher suites
        - compression methods

                                                        2) Sever Hello
                                                                ENCRIPTION: None
                                                                - highest TLS protocol supported
                                                                - random number
                                                                - choosen cipher suite
                                                                - choosen compression method

                                                        3) Certificate Message
                                                                ENCRIPTION: None
                                                                -

                                                        4) ServerHelloDone
                                                                ENCRIPTION: None

5) Certificate Message
        ENCRIPTION: None

6) ClientKeyExchange Message
        ENCRIPTION: server's public key => only server can read
                => if sever can read this he must own the certificate
        - may contain a PreMasterSecerate, public key or nothing (depends on cipher)

7) CertificateVerify Message
        ENCRIPTION: clients private key
        - purpose is to prove to the server that client owns the cert


                        8) BOTH CLIENT AND SERVER:
                                - use random numbers and PreMasterSecret to compute a common secerate


9) Finished message
        - contains a has and MAC over previous handshakes
                (to ensure that those unincripted messages did not get broken)


                                                        10) Finished message
                                                                - samething

Sever Knows

• The client has the public key for the sent certificate (step 7)

• The client's certificate is valid because either:

  • it has been signed by a CA (verisign)
  • it has been self-signed BUT it is in the server's truststore

• It is not a replay attack because presumably the random number (step 1 or 2) is sent with each message

Client Knows

• The server has the public key for the sent certificate (step 6 with step 8)

• The server's certificate is valid because either:

  • it has been signed by a CA (verisign)
  • it has been self-signed BUT it is in the client's truststore

• It is not a replay attack because presumably the random number (step 1 or 2) is sent with each message

Potential Problem

• Suppose the client's truststore has certs in it:

  • Server A
  • Server B (malicous)

• Server A has hostname www.A.com

• Server B has hostname www.B.com

• Suppose: The client tries to connect to Server A but Server B launches a man in the middle attack.

• Since server B:

  • has a public key for the certificate that will be sent to the client
  • has a "valid certificate" (a cert in the truststore)
• And since:
  • certificates do not have a hostname feild in them

It seems like Server B can pretend to be Server A easily.

Is there something that I am missing?

Answer 1

Can you point to some text that says JBoss doesn't need a hostname in the cert, or is it simply your observation? I assume by 'hostname' you mean the Common Name (CN) or Distinguished Name (DN)??

Normally an application should check an X.509 cert for:

Valid date range
Usage (eg; server auth)
Chains to a trusted root
CN == DNS name of target host (it might be another name, not just DNS)
Not revoked (using a CRL or OCSP)

Technically, an app can choose to ignore any of these and simply indicate that all is well with the cert.... but that's bad :)

Answer 2

I think you're missing something, but I'm not sure if I understand your reasoning.

However, when server B tries to launch a man in the middle attack, you say that it has a public key. This is true, but to setup a ssl connection, you should also have a private key belonging to that public key. Moreover, the certificate used is coupled to the dns name (in case of https). So a client tries to connect to A, he types in www.a.com. Since we assume that B does not know the private key of A, he will have another keypair. He could never receive a valid (i.e. trusted) certificate from a major CA that is coupled to a domain he does not own.

So B could never get a certificate with common name www.A.com, for this reason, B could not perform a man in the middle attack.