identity+crypto: atomic 0600 key file and zeroize secrets on drop

Cargo.toml

@@ -20,6 +20,7 @@ thiserror = "1"
 tokio = { version = "1" }
 
 # Crypto & Networking
+zeroize = { version = "1", features = ["derive"] }
 iroh = "0.97"
 iroh-base = "0.97"
 iroh-gossip = "0.97"

crates/crypto/Cargo.toml

@@ -18,6 +18,7 @@ sha2 = "0.10"
 rand = "0.8"
 serde = { workspace = true }
 thiserror = { workspace = true }
+zeroize = { workspace = true }
 
 [target.'cfg(target_arch = "wasm32")'.dependencies]
 getrandom_02 = { package = "getrandom", version = "0.2", features = ["js"] }

crates/crypto/src/lib.rs

@@ -21,6 +21,7 @@ use rand::RngCore;
 use serde::{Deserialize, Serialize};
 use sha2::Sha256;
 use x25519_dalek::{PublicKey as X25519Public, StaticSecret as X25519Secret};
+use zeroize::ZeroizeOnDrop;
 
 use willow_identity::Identity;
 
@@ -54,8 +55,11 @@ pub enum CryptoError {
 // ───── Types ────────────────────────────────────────────────────────────────
 
 /// A 256-bit symmetric key for encrypting a channel's messages.
-#[derive(Clone, PartialEq)]
-pub struct ChannelKey(pub(crate) [u8; 32]);
+///
+/// The wrapped bytes are zeroized when the value is dropped so secret
+/// material doesn't linger in freed memory.
+#[derive(Clone, PartialEq, ZeroizeOnDrop)]
+pub struct ChannelKey(#[zeroize] pub(crate) [u8; 32]);
 
 impl std::fmt::Debug for ChannelKey {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
@@ -749,6 +753,14 @@ mod tests {
         assert!(!debug.contains(&format!("{:02x}", key.as_bytes()[0])));
     }
 
+    /// Compile-time guarantee that [`ChannelKey`] zeroizes its secret
+    /// bytes on drop. See issue #127.
+    #[test]
+    fn channel_key_is_zeroize_on_drop() {
+        fn assert_zeroize_on_drop<T: zeroize::ZeroizeOnDrop>() {}
+        assert_zeroize_on_drop::<ChannelKey>();
+    }
+
     // ── Issue #110: ratchet counter DoS bound ──────────────────────
 
     /// Regression guard for issue #110: a sealed packet with a huge

crates/identity/Cargo.toml

@@ -13,10 +13,12 @@ serde = { workspace = true }
 thiserror = { workspace = true }
 chrono = { workspace = true }
 rand = "0.9"
+zeroize = { workspace = true }
 
 [target.'cfg(target_arch = "wasm32")'.dependencies]
 getrandom_02 = { package = "getrandom", version = "0.2", features = ["js"] }
 getrandom_03 = { package = "getrandom", version = "0.3", features = ["wasm_js"] }
 
 [dev-dependencies]
 tokio = { workspace = true }
+tempfile = "3"