fix(hub): clear parser-owned session key bytes

internal/hubruntime/config.go

@@ -549,7 +549,13 @@ func loadSessionPrivateKey(path string) (ed25519.PrivateKey, error) {
 	if !ok || len(key) != ed25519.PrivateKeySize {
 		return nil, fmt.Errorf("load hub configuration: session private key is not Ed25519")
 	}
-	return append(ed25519.PrivateKey(nil), key...), nil
+	return copyAndClearSessionPrivateKey(key), nil
+}
+
+func copyAndClearSessionPrivateKey(key ed25519.PrivateKey) ed25519.PrivateKey {
+	copied := append(ed25519.PrivateKey(nil), key...)
+	clear(key)
+	return copied
 }
 
 func readMountedFile(label, path string, maxBytes int) ([]byte, error) {

internal/hubruntime/runtime_test.go

@@ -3,6 +3,7 @@
 package hubruntime
 
 import (
+	"bytes"
 	"context"
 	"crypto/ed25519"
 	"crypto/rand"
@@ -306,6 +307,33 @@ func TestLoadSessionPrivateKeyRequiresReadOnlyPKCS8Ed25519(t *testing.T) {
 	}
 }
 
+func TestCopyAndClearSessionPrivateKeyTransfersOwnership(t *testing.T) {
+	publicKey, parsedKey, err := ed25519.GenerateKey(rand.Reader)
+	if err != nil {
+		t.Fatal(err)
+	}
+	expected := append(ed25519.PrivateKey(nil), parsedKey...)
+	returned := copyAndClearSessionPrivateKey(parsedKey)
+	t.Cleanup(func() {
+		clear(expected)
+		clear(returned)
+	})
+
+	if &parsedKey[0] == &returned[0] {
+		t.Fatal("returned private key aliases the parser-owned key")
+	}
+	if !bytes.Equal(parsedKey, make([]byte, ed25519.PrivateKeySize)) {
+		t.Fatal("parser-owned private key was not cleared")
+	}
+	if !bytes.Equal(returned, expected) {
+		t.Fatal("returned private key changed during ownership transfer")
+	}
+	message := []byte("sith session key ownership transfer")
+	if !ed25519.Verify(publicKey, message, ed25519.Sign(returned, message)) {
+		t.Fatal("returned private key cannot produce a valid signature")
+	}
+}
+
 func runtimeTestTLS(t *testing.T) (*tls.Config, *tls.Config) {
 	t.Helper()
 	publicKey, privateKey, err := ed25519.GenerateKey(rand.Reader)