HTTPS connections over proxy servers

TLS/SSL (The S in HTTPS) guarantees that there are no eavesdroppers between you and the server you are contacting, i.e. no proxies. Normally, you use CONNECT to open up a TCP connection through the proxy. In this case, the proxy will not be able to cache, read, or modify any requests/responses, and therefore be rather useless.

If you want the proxy to be able to read information, you can take the following approach:

Client starts HTTPS session Proxy transparently intercepts the connection and returns an ad-hoc generated(possibly weak) certificate Ka, signed by a certificate authority that is unconditionally trusted by the client. Proxy starts HTTPS session to target Proxy verifies integrity of SSL certificate; displays error if the cert is not valid. Proxy streams content, decrypts it and re-encrypts it with Ka Client displays stuff

An example is Squid's SSL bump. Similarly, burp can be configured to do this. This has also been used in a less-benign context by an Egyptian ISP.

Note that modern websites and browsers can employ HPKP or built-in certificate pins which defeat this approach.

The short answer is: It is possible, and can be done with either a special HTTP proxy or a SOCKS proxy.

First and foremost, HTTPS uses SSL/TLS which by design ensures end-to-end security by establishing a secure communication channel over an insecure one. If the HTTP proxy is able to see the contents, then it's a man-in-the-middle eavesdropper and this defeats the goal of SSL/TLS. So there must be some tricks being played if we want to proxy through a plain HTTP proxy.

The trick is, we turn an HTTP proxy into a TCP proxy with a special command named CONNECT. Not all HTTP proxies support this feature but many do now. The TCP proxy cannot see the HTTP content being transferred in clear text, but that doesn't affect its ability to forward packets back and forth. In this way, client and server can communicate with each other with help of the proxy. This is the secure way of proxying HTTPS data.

There is also an insecure way of doing so, in which the HTTP proxy becomes a man-in-the-middle. It receives the client-initiated connection, and then initiate another connection to the real server. In a well implemented SSL/TLS, the client will be notified that the proxy is not the real server. So the client has to trust the proxy by ignoring the warning for things to work. After that, the proxy simply decrypts data from one connection, reencrypts and feeds it into the other.

Finally, we can certainly proxy HTTPS through a SOCKS proxy, because the SOCKS proxy works at a lower level. You may think a SOCKS proxy as both a TCP and a UDP proxy.

as far as i can remember, you need to use a HTTP CONNECT query on the proxy. this will convert the request connection to a transparent TCP/IP tunnel.

so you need to know if the proxy server you use support this protocol.

If it's still of interest, here is an answer to a similar question: Convert HTTP Proxy to HTTPS Proxy in Twisted

To answer the second part of the question:

If yes, what kind of proxy server allows this?

Out of the box, most proxy servers will be configured to allow HTTPS connections only to port 443, so https URIs with custom ports wouldn't work. This is generally configurable, depending on the proxy server. Squid and TinyProxy support this, for example.

Here is my complete Java code that supports both HTTP and HTTPS requests using SOCKS proxy.

import java.io.IOException;
import java.net.InetSocketAddress;
import java.net.Proxy;
import java.net.Socket;
import java.nio.charset.StandardCharsets;

import org.apache.http.HttpHost;
import org.apache.http.client.methods.CloseableHttpResponse;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.client.protocol.HttpClientContext;
import org.apache.http.config.Registry;
import org.apache.http.config.RegistryBuilder;
import org.apache.http.conn.socket.ConnectionSocketFactory;
import org.apache.http.conn.socket.PlainConnectionSocketFactory;
import org.apache.http.conn.ssl.SSLConnectionSocketFactory;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;
import org.apache.http.impl.conn.PoolingHttpClientConnectionManager;
import org.apache.http.protocol.HttpContext;
import org.apache.http.ssl.SSLContexts;
import org.apache.http.util.EntityUtils;

import javax.net.ssl.SSLContext;

/**
    * How to send a HTTP or HTTPS request via SOCKS proxy.
    */
public class ClientExecuteSOCKS {

    public static void main(String[] args) throws Exception {
        Registry<ConnectionSocketFactory> reg = RegistryBuilder.<ConnectionSocketFactory>create()
            .register("http", new MyHTTPConnectionSocketFactory())
            .register("https", new MyHTTPSConnectionSocketFactory(SSLContexts.createSystemDefault
                ()))
            .build();
        PoolingHttpClientConnectionManager cm = new PoolingHttpClientConnectionManager(reg);
        try (CloseableHttpClient httpclient = HttpClients.custom()
            .setConnectionManager(cm)
            .build()) {
            InetSocketAddress socksaddr = new InetSocketAddress("mysockshost", 1234);
            HttpClientContext context = HttpClientContext.create();
            context.setAttribute("socks.address", socksaddr);

            HttpHost target = new HttpHost("www.example.com/", 80, "http");
            HttpGet request = new HttpGet("/");

            System.out.println("Executing request " + request + " to " + target + " via SOCKS " +
                "proxy " + socksaddr);
            try (CloseableHttpResponse response = httpclient.execute(target, request, context)) {
                System.out.println("----------------------------------------");
                System.out.println(response.getStatusLine());
                System.out.println(EntityUtils.toString(response.getEntity(), StandardCharsets
                    .UTF_8));
            }
        }
    }

    static class MyHTTPConnectionSocketFactory extends PlainConnectionSocketFactory {
        @Override
        public Socket createSocket(final HttpContext context) throws IOException {
            InetSocketAddress socksaddr = (InetSocketAddress) context.getAttribute("socks.address");
            Proxy proxy = new Proxy(Proxy.Type.SOCKS, socksaddr);
            return new Socket(proxy);
        }
    }

    static class MyHTTPSConnectionSocketFactory extends SSLConnectionSocketFactory {
        public MyHTTPSConnectionSocketFactory(final SSLContext sslContext) {
            super(sslContext);
        }

        @Override
        public Socket createSocket(final HttpContext context) throws IOException {
            InetSocketAddress socksaddr = (InetSocketAddress) context.getAttribute("socks.address");
            Proxy proxy = new Proxy(Proxy.Type.SOCKS, socksaddr);
            return new Socket(proxy);
        }
    }
}

You can accomplish this using man-in-the-middle techniques with dynamic SSL generation. Take a look at mitmproxy - it's a Python based, SSL-capable MITM proxy.

Tunneling (port forwarding) HTTPS through SSH (Linux version):

Turn off using 443 on localhost. Start tunneling as root: ssh -N login@proxy_server -L 443:target_ip:443 Adding 127.0.0.1 target_domain.com to /etc/hosts.

Everything you do on localhost.
Then: target_domain.com is accessible from localhost browser.

I don't think "have HTTPS connections over proxy servers" means the Man-in-the-Middle attack type of proxy server. I think it's asking whether one can connect to a http proxy server over TLS. And the answer is yes.

Is it possible to have HTTPS connections over proxy servers?

Yes, see my question and answer here. HTTPs proxy server only works in SwitchOmega

If yes, what kind of proxy server allows this?

The kind of proxy server deploys SSL certificates, like how ordinary websites do. But you need a pac file for the brower to configure proxy connection over SSL.

I had tried

start tunneling: ssh -N -D 12345 login@proxy_server

Setting the proxy in the firefox settings as localhost:12345 and ticking "use this proxy for all protocols"

and ticking "use this proxy for all protocols"

but this resulted in the error "Insecure connection" whenever I tried to connect to an https website.

The solution was to

"untick" the "use this proxy for all protocols"

set the proxy "localhost:12345" only as a SOCKS proxy

and leave the HTTP proxy, SSL proxy, FTP proxy blank

Reference from digital ocean documentation

How To Route Web Traffic Securely Without a VPN Using a SOCKS Tunnel