add fingerprint validation

in case we have no explicit fingerprint, we use openssl's regular "PEER"
verification. if we have a fingerprint, we ignore openssl verification
results altogether and just verify the fingerprint of the presented leaf
certificate, skipping the rest of the certificate chain (depth != 0).

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>

src/main.rs

@@ -134,9 +134,52 @@ impl CtrlTunnel {
         }
 
         let mut ssl_connector_builder = SslConnector::builder(SslMethod::tls())?;
-        if fingerprint.is_some() {
-            // FIXME actually verify fingerprint via callback!
-            ssl_connector_builder.set_verify(openssl::ssl::SslVerifyMode::NONE);
+        if let Some(expected) = fingerprint {
+            ssl_connector_builder.set_verify_callback(
+                openssl::ssl::SslVerifyMode::PEER,
+                move |_valid, ctx| {
+                    let cert = match ctx.current_cert() {
+                        Some(cert) => cert,
+                        None => {
+                            // should not happen
+                            eprintln!("SSL context lacks current certificate.");
+                            return false;
+                        }
+                    };
+
+                    // skip CA certificates, we only care about the peer cert
+                    let depth = ctx.error_depth();
+                    if depth != 0 {
+                        return true;
+                    }
+
+                    let fp = match cert.digest(openssl::hash::MessageDigest::sha256()) {
+                        Ok(fp) => fp,
+                        Err(err) => {
+                            // should not happen
+                            eprintln!("failed to calculate certificate FP - {}", err);
+                            return false;
+                        }
+                    };
+                    let fp_string = hex::encode(&fp);
+                    let fp_string = fp_string
+                        .as_bytes()
+                        .chunks(2)
+                        .map(|v| std::str::from_utf8(v).unwrap())
+                        .collect::<Vec<&str>>()
+                        .join(":");
+
+                    let expected = expected.to_lowercase();
+                    if expected == fp_string {
+                        true
+                    } else {
+                        eprintln!("certificate fingerprint does not match expected fingerprint!");
+                        eprintln!("expected:    {}", expected);
+                        eprintln!("encountered: {}", fp_string);
+                        false
+                    }
+                },
+            );
         } else {
             ssl_connector_builder.set_verify(openssl::ssl::SslVerifyMode::PEER);
         }