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The following scenario:

We have a HTTP (not HTTPS, so unencrypted) server which contains an open (RSA) public key. The general sharing of the public key should not be a problem since it is the public key. My concerns are more about a MITM (man in the middle) attack. So let's assume that in our scenario the servers in the local network are well secured and there is no attack at the endpoints.

Now my question would be:

Is a MITM attack possible in the transfer through the public internet?

If not, would it be theoretically possible to have encrypted data transfer via HTTP with RSA encrypted POST requests and encrypted replies?

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  • I'm so confused. If the traffic is not secured, then, yes, a mitm attack is possible. How does the public key come into it since you say that the exposure of the key is not a problem anyway?
    – schroeder
    Nov 12 '21 at 9:40
  • Can you encrypt data before sending over an unencrypted line? Of course. But not if you are trying to secure at the network level. You can't secure a lower level of the stack by adding controls to the higher levels of the stack.
    – schroeder
    Nov 12 '21 at 9:42
  • @schroeder I mean, the first key exchange via HTTP would then be insecure - everything that follows is then secured via RSA. To be more precise, I want a service that can be set up quickly without having to request an SSL certificate first. There are no passwords or highly secure information exchanged in general, but I still want to encrypt the whole thing a little. Because of the first unsecure key exchange I am not sure if one could not hook in exactly there and then start a MITM attack.
    – scolastico
    Nov 12 '21 at 9:45
  • A small addition: I am of course aware that only the body of the http requests would be encrypted but since there is no information in the header (like cookies) this is not really bad.
    – scolastico
    Nov 12 '21 at 9:50
  • 1
    Ah, then this is a X/Y problem. You are trying to solve one problem, but asking about another thinking that the answer to that will solve the true problem. 1. Let's Encrypt solves your certificate problems. 2. you have a misunderstanding about how RSA encryption works and the impact of the public key being known.
    – schroeder
    Nov 12 '21 at 9:51
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TL;DR: No. "Secure" is not a binary property, and this is better than nothing, but it's very vulnerable to MitM.


What you're describing is effectively unauthenticated opportunistic encryption. It's not a terrible idea - it is better than plain text, in that it will defeat a passive eavesdropper - but it's not great. It would be trivial for a network attacker to defeat, either by substituting the public key for their own, or by modifying the HTML/javascript (assuming a browser-based client) that retrieves the key and performs the encryption. Even if the attacker misses the start of the communication and thus can't interfere with the key exchange, they can forcibly reset the communication between client and server, probably causing the page to get reloaded and giving themselves a new chance.

Incidentally, if you really think this is worth it, at least try to do it right. RSA is an OK but not great choice for exchanging a symmetric key but you definitely shouldn't try to encrypt the actual requests with it, and you couldn't even encrypt the responses with it unless the client also generated and send a public key. Instead you should use a hybrid cryptosystem, where a symmetric key (or secret that can be used to derive a key) is exchanged, and then further communication is secured by symmetric encryption (with either HMACs or AEAD for integrity too). And, if you're going to do hybrid crypto, it makes more sense to use Diffie-Hellman (or better yet, elliptic curve DH) for the key exchange, as the parameters are much lower-cost to generate than a strong RSA key so you can get forward secrecy and good cryptographic strength at a reasonable cost. ECDH is supported in the Web Crypto API, so you don't have to write any cryptographic primitives yourself.

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  • Thank you for your detailed answer. As a quick addendum: It is an API that is not called by normal browsers but only by two programs that are tuned to each other. So as you said, both logically generate a key set. In addition, the key local is stored by both for the duration of the session and then deleted by both.
    – scolastico
    Nov 12 '21 at 10:05
  • It's an API? Yeah, I really think you have a major X/Y Problem here. There are solutions to your problem, but you need to state the core problem not the problem with your proposed solution to the problem.
    – schroeder
    Nov 12 '21 at 11:38
  • As I said before, it is simply a matter of two programs communicating with each other in a "reasonably secure" manner (with the requirements already mentioned). I just don't feel like getting into the topic of tcp/udp connections and how to encrypt them. Instead I just had the idea to start a web server via the available libaray and check the body of post requests etc before and decrypt/encrypt them. The whole thing is not very elegant but is functional in my eyes.
    – scolastico
    Nov 12 '21 at 12:05
  • +1 Excellent answer by CBHacking as usual. OP, you mention two programs that are tuned to each other. I'm not sure what you mean by tuned, but if it's possible for you to pre-share keys between the two endpoints, then this greatly simplifies the problem, and eliminates the possibility of an MITM attack.
    – mti2935
    Nov 12 '21 at 13:06
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    Having said that, it really sounds like you are re-inventing the wheel here. Crypto is tricky, and one oversight can lead to a disastrous security vulnerability (as we've seen many times). If you can, use a tried and true protocol like IPSec, SSL/TLS, etc., instead of rolling your own crypto. See security.stackexchange.com/questions/18197/… for some interesting reading on this subject.
    – mti2935
    Nov 12 '21 at 13:08

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