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, 149 insertions, 0 deletions

diff --git a/static/markdown.js b/static/markdown.js
new file mode 100644
index 0000000..bad1a6d
--- /dev/null
+++ b/static/markdown.js
@@ -0,0 +1,149 @@
+// NymDrop — minimal self-hosted Markdown preview for the message field.
+// No external library: a small, auditable subset is enough for a
+// submission form, and it avoids pulling a third-party JS dependency into
+// a page whose whole point is minimizing trust in client-side code.
+//
+// The rendered HTML never leaves the browser — the encrypted submission
+// still carries the raw Markdown source text untouched (see app.js). This
+// is purely a "what will this look like" aid for the person writing it.
+
+function mdEscapeHtml(str) {
+    return str
+        .replace(/&/g, "&")
+        .replace(/</g, "&lt;")
+        .replace(/>/g, "&gt;")
+        .replace(/"/g, "&quot;")
+        .replace(/'/g, "&#39;");
+}
+
+// Only allow link schemes that can't execute script in the rendered preview.
+function mdSafeHref(url) {
+    if (/^(https?:|mailto:)/i.test(url)) return mdEscapeHtml(url);
+    return "#";
+}
+
+function mdInline(text) {
+    let out = mdEscapeHtml(text);
+    out = out.replace(/`([^`]+)`/g, "<code>$1</code>");
+    out = out.replace(/\*\*([^*]+)\*\*|__([^_]+)__/g, (m, a, b) => `<strong>${a || b}</strong>`);
+    out = out.replace(/\*([^*]+)\*|_([^_]+)_/g, (m, a, b) => `<em>${a || b}</em>`);
+    out = out.replace(/\[([^\]]+)\]\(([^)\s]+)\)/g, (m, label, url) =>
+        `<a href="${mdSafeHref(url)}" target="_blank" rel="noopener noreferrer">${label}</a>`);
+    return out;
+}
+
+function renderMarkdown(source) {
+    const lines = source.replace(/\r\n/g, "\n").split("\n");
+    const html = [];
+    let i = 0;
+    let para = [];
+
+    function flushPara() {
+        if (para.length) {
+            html.push("<p>" + para.map(mdInline).join("<br>") + "</p>");
+            para = [];
+        }
+    }
+
+    while (i < lines.length) {
+        const line = lines[i];
+
+        if (/^```/.test(line)) {
+            flushPara();
+            const code = [];
+            i++;
+            while (i < lines.length && !/^```/.test(lines[i])) {
+                code.push(lines[i]);
+                i++;
+            }
+            html.push("<pre><code>" + mdEscapeHtml(code.join("\n")) + "</code></pre>");
+            i++;
+            continue;
+        }
+
+        const heading = line.match(/^(#{1,3})\s+(.*)$/);
+        if (heading) {
+            flushPara();
+            const level = heading[1].length;
+            html.push(`<h${level}>${mdInline(heading[2])}</h${level}>`);
+            i++;
+            continue;
+        }
+
+        if (/^(---|\*\*\*)\s*$/.test(line)) {
+            flushPara();
+            html.push("<hr>");
+            i++;
+            continue;
+        }
+
+        if (/^>\s?/.test(line)) {
+            flushPara();
+            const quote = [];
+            while (i < lines.length && /^>\s?/.test(lines[i])) {
+                quote.push(lines[i].replace(/^>\s?/, ""));
+                i++;
+            }
+            html.push("<blockquote>" + renderMarkdown(quote.join("\n")) + "</blockquote>");
+            continue;
+        }
+
+        if (/^[-*]\s+/.test(line)) {
+            flushPara();
+            const items = [];
+            while (i < lines.length && /^[-*]\s+/.test(lines[i])) {
+                items.push(`<li>${mdInline(lines[i].replace(/^[-*]\s+/, ""))}</li>`);
+                i++;
+            }
+            html.push("<ul>" + items.join("") + "</ul>");
+            continue;
+        }
+
+        if (/^\d+\.\s+/.test(line)) {
+            flushPara();
+            const items = [];
+            while (i < lines.length && /^\d+\.\s+/.test(lines[i])) {
+                items.push(`<li>${mdInline(lines[i].replace(/^\d+\.\s+/, ""))}</li>`);
+                i++;
+            }
+            html.push("<ol>" + items.join("") + "</ol>");
+            continue;
+        }
+
+        if (line.trim() === "") {
+            flushPara();
+            i++;
+            continue;
+        }
+
+        para.push(line);
+        i++;
+    }
+    flushPara();
+
+    return html.join("\n");
+}
+
+(function() {
+    const textarea = document.getElementById("message");
+    const preview = document.getElementById("message-preview");
+    const toggle = document.getElementById("preview-toggle");
+    if (!textarea || !preview || !toggle) return;
+
+    let showingPreview = false;
+
+    toggle.addEventListener("click", function() {
+        showingPreview = !showingPreview;
+        if (showingPreview) {
+            preview.innerHTML = renderMarkdown(textarea.value) ||
+                '<p class="md-empty">Nothing to preview yet.</p>';
+            textarea.hidden = true;
+            preview.hidden = false;
+            toggle.textContent = "Edit";
+        } else {
+            preview.hidden = true;
+            textarea.hidden = false;
+            toggle.textContent = "Preview";
+        }
+    });
+})();