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So another engineer buddy of mine and I were having a drink the other night. He mentioned that you're allowed to use personal devices on the office wifi, but that they install a custom certificate so they can MITM your traffic.

Neither of us are security experts, but I know a little bit about the HTTP/TLS handshake protocol to question whether this is the case.

As far as I understand it (please forgive me if I butcher it):

• Client-Server initiate handshake, and exchange certificate from signing authority + public key + random string.




• Public key is used to decrypt a random string of characters, which is fed into a hashing algorithm and reveals a private key.




• Private key is used to decrypt the traffic that follows

We were reading this article, about how companies sometimes install certificates to decrypt outgoing traffic.

If the blog-post case is true, then how does this work? Would they get the private key using their trusted-root all uses certificate? Assuming that works, that covers the windows use-case, but what about other platforms like OSX/iOS, linux, BSD etc.?

Are there other approaches that I'm not considering, where a certificate install could be used to MitM?

Yes, they can MitM the traffic this way, using an internal certificate authority. There are two primary ways in which the MitM can work.

The first is to simply turn the edge gateway into a proxy, whereby TLS connections are made from the gateway to the server, and the gateway then generates server certificates on the fly from an internal CA in order to impersonate the remote server. Your system trusts the CA, so it trusts the server certificate.

The second is a slightly different take on the first. The gateway proxies the traffic similarly to the first method, except it only advertises static RSA cipher suites to the remote server. The reason for doing this is performance. With a static RSA key exchange (i.e. not Diffie-Hellman) the gateway can split the handshake as before in order to provide the client with a certificate generated via the internal CA, but instead of decrypting the content on the gateway and then re-encrypting it before proxying, it simply passes the same session key between the client and server. This way the gateway only has to decrypt the traffic once, using the captured session key, and never needs to re-encrypt it in order to proxy the traffic between client and server. This trick no longer works in TLS 1.3 as static RSA key exchange was removed.

Generally speaking this kind of TLS inspection is fairly commonplace in large organisations, particularly financials. Deploying it on BYOD devices is somewhat common, although you should consider the privacy and security implications that might arise from installing your company's internal CA certificate on your device. You need to ask yourself whether you trust that your IT security team is likely to be able to protect the signing keys, because if not then your device is liable to be MitM'ed by an attacker.