Add adversarial tests for MTProto handshake and enhance masking functionality

- Introduced multiple adversarial tests for MTProto handshake to ensure robustness against replay attacks, invalid mutations, and concurrent flooding.
- Implemented a function to build proxy headers based on the specified version, improving the handling of masking protocols.
- Added tests to validate the behavior of the masking functionality under various conditions, including unknown proxy protocol versions and oversized payloads.
- Enhanced relay tests to ensure stability and performance under high load and half-close scenarios.

src/config/load.rs

@@ -334,6 +334,24 @@ impl ProxyConfig {
             ));
         }
 
+        let handshake_timeout_ms = config
+            .timeouts
+            .client_handshake
+            .checked_mul(1000)
+            .ok_or_else(|| {
+                ProxyError::Config(
+                    "timeouts.client_handshake is too large to validate milliseconds budget"
+                        .to_string(),
+                )
+            })?;
+
+        if config.censorship.server_hello_delay_max_ms >= handshake_timeout_ms {
+            return Err(ProxyError::Config(
+                "censorship.server_hello_delay_max_ms must be < timeouts.client_handshake * 1000"
+                    .to_string(),
+            ));
+        }
+
         if config.timeouts.relay_client_idle_soft_secs == 0 {
             return Err(ProxyError::Config(
                 "timeouts.relay_client_idle_soft_secs must be > 0".to_string(),
@@ -977,6 +995,10 @@ impl ProxyConfig {
 #[path = "load_idle_policy_tests.rs"]
 mod load_idle_policy_tests;
 
+#[cfg(test)]
+#[path = "load_security_tests.rs"]
+mod load_security_tests;
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/config/load_security_tests.rs

@@ -0,0 +1,84 @@
+use super::*;
+use std::fs;
+use std::path::PathBuf;
+use std::time::{SystemTime, UNIX_EPOCH};
+
+fn write_temp_config(contents: &str) -> PathBuf {
+    let nonce = SystemTime::now()
+        .duration_since(UNIX_EPOCH)
+        .expect("system time must be after unix epoch")
+        .as_nanos();
+    let path = std::env::temp_dir().join(format!("telemt-load-security-{nonce}.toml"));
+    fs::write(&path, contents).expect("temp config write must succeed");
+    path
+}
+
+fn remove_temp_config(path: &PathBuf) {
+    let _ = fs::remove_file(path);
+}
+
+#[test]
+fn load_rejects_server_hello_delay_equal_to_handshake_timeout_budget() {
+    let path = write_temp_config(
+        r#"
+[timeouts]
+client_handshake = 1
+
+[censorship]
+server_hello_delay_max_ms = 1000
+"#,
+    );
+
+    let err = ProxyConfig::load(&path)
+        .expect_err("delay equal to handshake timeout must be rejected");
+    let msg = err.to_string();
+    assert!(
+        msg.contains("censorship.server_hello_delay_max_ms must be < timeouts.client_handshake * 1000"),
+        "error must explain delay<timeout invariant, got: {msg}"
+    );
+
+    remove_temp_config(&path);
+}
+
+#[test]
+fn load_rejects_server_hello_delay_larger_than_handshake_timeout_budget() {
+    let path = write_temp_config(
+        r#"
+[timeouts]
+client_handshake = 1
+
+[censorship]
+server_hello_delay_max_ms = 1500
+"#,
+    );
+
+    let err = ProxyConfig::load(&path)
+        .expect_err("delay larger than handshake timeout must be rejected");
+    let msg = err.to_string();
+    assert!(
+        msg.contains("censorship.server_hello_delay_max_ms must be < timeouts.client_handshake * 1000"),
+        "error must explain delay<timeout invariant, got: {msg}"
+    );
+
+    remove_temp_config(&path);
+}
+
+#[test]
+fn load_accepts_server_hello_delay_strictly_below_handshake_timeout_budget() {
+    let path = write_temp_config(
+        r#"
+[timeouts]
+client_handshake = 1
+
+[censorship]
+server_hello_delay_max_ms = 999
+"#,
+    );
+
+    let cfg = ProxyConfig::load(&path)
+        .expect("delay below handshake timeout budget must be accepted");
+    assert_eq!(cfg.timeouts.client_handshake, 1);
+    assert_eq!(cfg.censorship.server_hello_delay_max_ms, 999);
+
+    remove_temp_config(&path);
+}