Flush-response experiments

Co-Authored-By: brekotis <93345790+brekotis@users.noreply.github.com>

src/protocol/constants.rs

@@ -159,10 +159,13 @@ pub const MAX_TLS_CHUNK_SIZE: usize = 16384 + 256;
 /// Generate padding length for Secure Intermediate protocol.
 /// Total (data + padding) must not be divisible by 4 per MTProto spec.
 pub fn secure_padding_len(data_len: usize, rng: &SecureRandom) -> usize {
-    if data_len % 4 == 0 {
-        (rng.range(3) + 1) as usize // 1-3
-    } else {
-        rng.range(4) as usize // 0-3
+    let rem = data_len % 4;
+    match rem {
+        0 => (rng.range(3) + 1) as usize,           // {1, 2, 3}
+        1 => rng.range(3) as usize,                 // {0, 1, 2}
+        2 => [0usize, 1, 3][rng.range(3) as usize], // {0, 1, 3}
+        3 => [0usize, 2, 3][rng.range(3) as usize], // {0, 2, 3}
+        _ => unreachable!(),
     }
 }
 
@@ -332,4 +335,24 @@ mod tests {
         assert_eq!(TG_DATACENTERS_V4.len(), 5);
         assert_eq!(TG_DATACENTERS_V6.len(), 5);
     }
+
+    #[test]
+    fn secure_padding_never_produces_aligned_total() {
+        let rng = SecureRandom::new();
+        for data_len in 0..1000 {
+            for _ in 0..100 {
+                let padding = secure_padding_len(data_len, &rng);
+                assert!(
+                    padding <= 3,
+                    "padding out of range: data_len={data_len}, padding={padding}"
+                );
+                assert_ne!(
+                    (data_len + padding) % 4,
+                    0,
+                    "invariant violated: data_len={data_len}, padding={padding}, total={}",
+                    data_len + padding
+                );
+            }
+        }
+    }
 }