[DO NOT UPSTREAM] Add support for ssh_keylog

This allows decrypting SSH traffic in Wireshark, which will likely be
useful during debugging.

Author: bjorn3

.gitignore

@@ -1,2 +1,3 @@
 /target
 ssh_host_ed25519_key
+ssh_keylog

src/key_exchange.rs

@@ -1,4 +1,4 @@
-use std::str;
+use std::{io::Write, str};
 
 use aws_lc_rs::{
     agreement::{self, EphemeralPrivateKey, UnparsedPublicKey, X25519},
@@ -15,6 +15,7 @@ use crate::{
 };
 
 pub(crate) struct EcdhKeyExchange {
+    cookie: [u8; 16],
     /// The current session id or `None` if this is the initial key exchange.
     session_id: Option<digest::Digest>,
 }
@@ -138,6 +139,30 @@ impl EcdhKeyExchange {
             return Err(());
         }
 
+        if std::env::var("OXISH_ENABLE_KEYLOG").as_deref() == Ok("1") {
+            assert!(cfg!(debug_assertions));
+            eprintln!("Logging shared secret to ssh_keylog file!");
+            std::fs::OpenOptions::new()
+                .create(true)
+                .append(true)
+                .write(true)
+                .open("ssh_keylog")
+                .unwrap()
+                .write_all(
+                    format!(
+                        "{:032x} SHARED_SECRET {}\n",
+                        u128::from_be_bytes(self.cookie),
+                        shared_secret
+                            .iter()
+                            .map(|&byte| format!("{byte:02X}"))
+                            .collect::<Vec<_>>()
+                            .join("")
+                    )
+                    .as_bytes(),
+                )
+                .unwrap();
+        }
+
         // The first exchange hash is used as session id.
         let session_id = self.session_id.as_ref().unwrap_or(&exchange_hash);
         let derivation = KeyDerivation {
@@ -325,6 +350,8 @@ impl KeyExchange {
             }
         };
 
+        let cookie = key_exchange_init.cookie;
+
         if let Err(error) = conn
             .stream_write
             .write_packet(&key_exchange_init, |kex_init_payload| {
@@ -354,6 +381,7 @@ impl KeyExchange {
         }
 
         Ok(EcdhKeyExchange {
+            cookie,
             session_id: self.session_id,
         })
     }