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Say, the goal is to facilitate secure communication between two parties. The definition of secure is that

  • both parties can be sure that they are actually talking to a trusted party, not somebody else
  • they can be sure that no third party is listening to the communication

Furthermore, I have a Certificate Authority (CA), and the two parties (rightly) trust all certificates coming from that CA. Only trusted parties have a certificate. They do not trust any other certificates. The possession of a valid certificate from CA is counted as evidence that the owner of that certificate is trustworthy.

Under these circumstances, i.e. TLS with mutual authentication with single, controlled CA, is a Man In The Middle attack possible without prior possession of a certificate from the CA? Is there any need for additional precautions, such as host-name verification?

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First, a brief on how TLS works and why it is used-

TLS is used to provide encryption to the data stream and form a secure channel between the user and the server. A TLS Handshake is used to authenticate the server and generate the session keys to be used in the encryption process. ​TLS uses a combination of symmetric and asymmetric cryptography and hence, ​a prerequisite for this is that the​ server and the user need to have public and private keys generated for themselves​. Additionally, The ​server needs to have a valid SSL/TLS certificate which it can send to the user, which the user can then use to verify the server’s identity (checking with the Certificate Authority that issued it) and also get the server’s public key. The user sends a premaster secret encrypted with the server’s public key and the server decrypts it using its private key. Both parties now generate the session key using this premaster secret and previously exchanged nonces.

As long as the Certificate for the user is valid, the data is encrypted, and hence secure in transit.

Without the Certificate from the CA, A MITM (Man in the middle) attack is very well possible as the public key received can not be verified. Even stolen certificates are a problem (Refer to the Stuxnet attack for more on Stolen digital certificates from Realtek and JMicron).

I hope I answered your question correctly :)

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  • Thanks for your answer. When you say "Without Certificate from the CA a MITM attack is very well possible", are you talking about client/server not using such a certificate, or are you saying a MITM can be performed by a third party even without having a certificate?
    – ig-dev
    Commented Oct 22, 2019 at 6:05
  • A MITM attack can not be performed by a third party without having a certificate , as all the communication between the client and the server will be encrypted. What I meant to say was that if the client/server does not use certificates, a MITM attack is possible as there will be no way to verify the public key of the communicating party.
    – Ayush Ambastha
    Commented Oct 25, 2019 at 17:08

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