C#•2mo ago

HTTPS payload decryption using root authorized private key

How could one decrypt HTTPS payload using a root authorized private-key in a .NET application?

20 Replies

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MODiX•2mo ago

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canton7•2mo ago

About the only thing I can say in response to a question that vague is ".NET provides classes to encrypt and decrypt data. Use them" Where did this encrypted payload come from? Why wasn't it decrypted automatically using TLS when you downloaded it? (For example, you rarely use asymmetric crypto to encrypt something big, like a HTTP payload. You use the asymmetric keys to share some private session-specific symmetric keys, and then use those, as symmetric crypto is a lot quicker. So the question itself doesn't really make sense anyway, as normally HTTPS payloads aren't encrypted with asymmetric keys)

Oliwer•2mo ago

HTTPS payloads captured using sharppcap. I have the private key from a trusted root certificate, I am just not sure of how to decrypt the array of bytes appropriately. I've tried researching for quite a while, but to no avail.

canton7•2mo ago

You can't, AFAIK. Well, unless you capture the TLS handshake

Oliwer•2mo ago

I got the handshake.

private static void Device_OnPacketArrival(object sender, PacketCapture e)
{
  if (!next)
  {
    return;
  }

  var capture = e.GetPacket();
  if (capture == null)
  {
    return;
  }

  var packet = capture.GetPacket();
  if (packet is not EthernetPacket ethernetPacket || !packet.HasPayloadPacket)
  {
    return;
  }

  var payloadPacket = packet.PayloadPacket;
  if (payloadPacket.PayloadPacket is TcpPacket tcp)
  {
    // HANDSHAKE
    if (tcp.PayloadData.Length > 0 && tcp.PayloadData[0] == 0x16)
    {
      // TODO
    }
  }
}

private static void Device_OnPacketArrival(object sender, PacketCapture e)
{
  if (!next)
  {
    return;
  }

  var capture = e.GetPacket();
  if (capture == null)
  {
    return;
  }

  var packet = capture.GetPacket();
  if (packet is not EthernetPacket ethernetPacket || !packet.HasPayloadPacket)
  {
    return;
  }

  var payloadPacket = packet.PayloadPacket;
  if (payloadPacket.PayloadPacket is TcpPacket tcp)
  {
    // HANDSHAKE
    if (tcp.PayloadData.Length > 0 && tcp.PayloadData[0] == 0x16)
    {
      // TODO
    }
  }
}

Here's my current situation. I'm assuming this (but a lot more complex) is how Fiddler does it, since they also use a trusted root certificate key.

canton7•2mo ago

IIRC fiddler MITM's the traffic?

Oliwer•2mo ago

It seems it does.

canton7•2mo ago

I have the private key from a trusted root certificate

So you have the actual private key that the server used for the TLS connection? But you don't have the private key which the browser used?

Oliwer•2mo ago

dangit I generated a self-signed one and added it to the trusted root certificates and do not have the browser one.

canton7•2mo ago

So the server was running on localhost, or...?

Oliwer•2mo ago

Yes.

canton7•2mo ago

Googling around, it seems the normal wya to do this is to configure the browser to create a key log file, which contains the various TLS session keys that it used. Wireshark at least can then use those to decrypt the traffic

Oliwer•2mo ago

I'll have a look at giving that a shot, thanks.

canton7•2mo ago

This looks like your question: https://security.stackexchange.com/questions/71309/it-is-possible-to-decrypt-https-with-the-private-public-pair-if-it-uses-dhe

Information Security Stack Exchange

it is possible to decrypt HTTPS with the (private, public) pair if ...

Assuming you have a PCAP file with HTTPS traffic and having the key pair (private & public), it is possible to decrypt the traffic if it uses Diffie Hellman Ephemeral? Using openSSL lib for exa...

Oliwer•2mo ago

Going out on a whim here, it's more or less the MITM route recommended? The only thing is, I can't do that cause it would slow down the networking so it wouldn't be able to run infinitely while doing other stuff. It's meant to be a user application.

canton7•2mo ago

I don't know if there is a "recommended" way to decrypt all encrypted traffic. That rather defeats the point of the encryption, no? 😛

Oliwer•2mo ago

Of course.

canton7•2mo ago

It's explicitly meant to be hard to decrypt it, so it's likely that there are no "good" options

Oliwer•2mo ago

That's unfortunate. I'll have to stick with the chrome extension route of capturing the requests. Thank you for your time once again, canton.

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HTTPS payload decryption using root authorized private key