Guard key material in RAM with memguard, add Markdown preview for message field

Reader now keeps the private key bytes and the per-submission ECDH/HKDF
secrets in memguard.LockedBuffer (mlock + guaranteed wipe) instead of a
manual zero loop. Added a regression test for the decrypt path.

Message field gets a self-hosted Markdown toggle preview (no external
library): headers, bold/italic, inline code, code blocks, blockquotes,
lists, links. Output is HTML-escaped and link schemes are whitelisted
before rendering. Submitted content is still the raw Markdown source,
unchanged on the wire.

1 files changed, 65 insertions, 0 deletions

diff --git a/cmd/nymdrop-reader/memguard_verify_test.go b/cmd/nymdrop-reader/memguard_verify_test.go
new file mode 100644
index 0000000..3ab5fce
--- /dev/null
+++ b/cmd/nymdrop-reader/memguard_verify_test.go
@@ -0,0 +1,65 @@
+package main
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/ecdh"
+	"crypto/rand"
+	"crypto/sha256"
+	"encoding/base64"
+	"encoding/hex"
+	"io"
+	"testing"
+
+	"golang.org/x/crypto/hkdf"
+)
+
+// Verifies the decryptMessage path still produces the correct plaintext after
+// wrapping the ECDH/HKDF secrets in memguard.LockedBuffer, using the exact
+// wire format the browser client (static/crypto.js) and relay produce.
+func TestDecryptMessageAfterMemguard(t *testing.T) {
+	readerPriv, err := ecdh.X25519().GenerateKey(rand.Reader)
+	if err != nil {
+		t.Fatal(err)
+	}
+	ephPriv, err := ecdh.X25519().GenerateKey(rand.Reader)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	shared, err := ephPriv.ECDH(readerPriv.PublicKey())
+	if err != nil {
+		t.Fatal(err)
+	}
+	aesKey := make([]byte, 32)
+	hkdfR := hkdf.New(sha256.New, shared, make([]byte, 32), []byte("nymdrop-v1"))
+	if _, err := io.ReadFull(hkdfR, aesKey); err != nil {
+		t.Fatal(err)
+	}
+	block, err := aes.NewCipher(aesKey)
+	if err != nil {
+		t.Fatal(err)
+	}
+	gcm, err := cipher.NewGCM(block)
+	if err != nil {
+		t.Fatal(err)
+	}
+	iv := make([]byte, 12)
+	if _, err := rand.Read(iv); err != nil {
+		t.Fatal(err)
+	}
+	want := "E2E memguard verification message"
+	ciphertext := gcm.Seal(nil, iv, []byte(want), nil)
+
+	packet := append(append(append([]byte{}, ephPriv.PublicKey().Bytes()...), iv...), ciphertext...)
+	noncePrefix := hex.EncodeToString(make([]byte, 16)) + ":"
+	dataB64 := base64.StdEncoding.EncodeToString(append([]byte(noncePrefix), packet...))
+
+	got, err := decryptMessage(dataB64, readerPriv)
+	if err != nil {
+		t.Fatalf("decryptMessage: %v", err)
+	}
+	if got != want {
+		t.Fatalf("plaintext mismatch: got %q want %q", got, want)
+	}
+}