SECURITY: First 6 bytes of payload should be printable characters

Observation shows that prepending 6 bytes of printable characters to random payload will exempt it from blocking.
by 2022-01-13 gfw.report et al.

crates/shadowsocks/Cargo.toml

@@ -48,6 +48,7 @@ spin = { version = "0.9", features = ["std"], optional = true }
 pin-project = "1.0"
 bloomfilter = { version = "1.0.8", optional = true }
 thiserror = "1.0"
+rand = "0.8"
 
 serde = { version = "1.0", features = ["derive"] }
 serde_urlencoded = "0.7"

crates/shadowsocks/src/context.rs

@@ -4,6 +4,7 @@ use std::{io, net::SocketAddr, sync::Arc};
 
 use byte_string::ByteStr;
 use log::warn;
+use rand::Rng;
 
 use crate::{
     config::{ReplayAttackPolicy, ServerType},
@@ -66,6 +67,20 @@ impl Context {
         loop {
             random_iv_or_salt(nonce);
 
+            // SECURITY: First 6 bytes of payload should be printable characters
+            // Observation shows that prepending 6 bytes of printable characters to random payload will exempt it from blocking.
+            // by 2022-01-13 gfw.report et al.
+            const SECURITY_PRINTABLE_PREFIX_LEN: usize = 6;
+            if nonce.len() >= SECURITY_PRINTABLE_PREFIX_LEN {
+                // Printable characters follows definition of isprint in C/C++
+                static ASCII_PRINTABLE_CHARS: &[u8] = br##"0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz!"#$%&'()*+,-./:;<=>?@[\]^_`{|}~ "##;
+
+                let mut rng = rand::thread_rng();
+                for b in nonce.iter_mut().take(SECURITY_PRINTABLE_PREFIX_LEN) {
+                    *b = ASCII_PRINTABLE_CHARS[rng.gen_range::<usize, _>(0..ASCII_PRINTABLE_CHARS.len())];
+                }
+            }
+
             // Salt already exists, generate a new one.
             if unique && self.check_nonce_and_set(nonce) {
                 continue;