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My (bring your own device) school has recently implemented TLS interception for search engines and Youtube. We all installed a root certificate as a "Trusted Root CA."

Taking a look at the fake google certificate used for interception, it includes *.google.com, *.android.com, and every other google domain as alternate names.

The crux of the issue, however, is that every certificate in the district seems to use the same private key. I ran these commands:

diff -s <(openssl x509 -pubkey -noout -in root.crt) <(openssl x509 -pubkey -noout -in fake-google-signed-with-root.crt)

diff -s <(openssl x509 -pubkey -noout -in root.crt) <(openssl x509 -pubkey -noout -in fake-google-signed-with-root-2.crt)

and for both commands got "Files /dev/fd/63 and /dev/fd/62 are identical" as the output.

Does this mean the root certificate and the signed google certificate contain the same private key? Should each school in the district be using the same fake google certificate? What are the security implications of this?

  • The interception is operational so insecure. You do not need to worry about additional risks of key reuse. – eckes May 17 '17 at 22:33
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Let's start by being clear on what they are doing: forcing you to compromise the security of your device so that they can man-in-the-middle all of your HTTPS traffic, decrypt it, read it, then forward it on to the real server (presumably re-encrypted using proper HTTPS to the server, but you have no guarantee).

So they are using the same keypair in all of their fake certs. Points for you that you noticed this red flag and got curious about it. Let's take a closer look: In a normal TLS certificate setup, you get one cert per server and use a unique keypair fr each server. The reason is that private keys are supposed to be private, but now there are multiple copies of this private key on multiple machines so that a compromise of Server A also compromises Server B. Bad. In this case, all the certs belong to the TLS proxy machine, so whether all the certs were using the same key or not, hacking into that machine would give you all the keys anyway.

And finally, back to the opening point: why would they need to worry about doing things securely when the whole point is to break the security of your BYODs wide open?

  • the key is (hopefully) only on the gateway not on multiple machines. But it's still very critical. I would not dare to,do,that to non-company owned devices. – eckes May 17 '17 at 22:35
  • The key is likely only on gateways, but the same key seems to be on every gateway in the district. Considering that there are almost fifty schools in our district and each one has multiple boxes, the concern about a district-wide compromise rather than a per-school one is what concerns me. Also, the private key would give the attacker the ability to sign new certificates, which is problematic. – Spooky Mushroom May 17 '17 at 23:18
  • @mike-ounsworth While I agree that trusting a malicious CA breaks security, I trust the district to refrain from consciously stealing passwords. I am worried about security from individual attackers, should they steal the key. From what you said, it seems that using the same key over multiple gateways and in the root certificate increases the threat of such attacks considerably. Thanks for your response :) – Spooky Mushroom May 17 '17 at 23:23
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You should assume the school is filled with inept buffoons that would put all your information in a harddrive and hand deliver it to your worst enemy. No offense. That's just reasonable precautions.

Skip the school network. Setup a personal VPN server, and route your internet connection through a $50 a month unlimited data mobile device.

Since they are stripping TLS, the school server is a juicy target for a hacker that knows how to inject malicious packets. However, if you must connect through their network via your device - I'd suggest doing it with a virtual machine. Preferably, a live virtual machine. That way you don't have to worry about contaminating your network with whatever they have going on.

Sometimes it pays to wear the tinfoil hat.

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