diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1e7f45b1..edf12a15 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,6 +12,7 @@
 - Fix `date` text field formatting emitting invalid JavaScript, so the format was never applied. Fixes #1546
 - Fix `indentAllLines` applying the indent again on every paragraph and every page break, and keep it applied across continued text. Fixes #1606
 - Fix a hole in a sparse array being skipped entirely, which shifted every later entry down one
+- Encrypt strings inside name trees. Fixes #1513
 
 ### [v0.19.1] - 2026-06-10
 
diff --git a/lib/object.js b/lib/object.js
index 7bccec45..7e7a61ad 100644
--- a/lib/object.js
+++ b/lib/object.js
@@ -101,9 +101,12 @@ class PDFObject {
       // Byte arrays are converted to PDF hex strings
     } else if (object instanceof Uint8Array) {
       return `<${bytesToHex(object)}>`;
+    } else if (object instanceof PDFTree) {
+      // Name trees hold PDF strings as keys, which have to be encrypted like
+      // any other string in the document
+      return object.toString(encryptFn);
     } else if (
       object instanceof PDFAbstractReference ||
-      object instanceof PDFTree ||
       object instanceof SpotColor
     ) {
       return object.toString();
diff --git a/lib/tree.js b/lib/tree.js
index 6634d8de..14dfa319 100644
--- a/lib/tree.js
+++ b/lib/tree.js
@@ -19,7 +19,7 @@ class PDFTree {
     return this._items[key];
   }
 
-  toString() {
+  toString(encryptFn = null) {
     // Needs to be sorted by key
     const sortedKeys = Object.keys(this._items).sort((a, b) =>
       this._compareKeys(a, b),
@@ -30,14 +30,15 @@ class PDFTree {
       const first = sortedKeys[0],
         last = sortedKeys[sortedKeys.length - 1];
       out.push(
-        `  /Limits ${PDFObject.convert([this._dataForKey(first), this._dataForKey(last)])}`,
+        `  /Limits ${PDFObject.convert([this._dataForKey(first), this._dataForKey(last)], encryptFn)}`,
       );
     }
     out.push(`  /${this._keysName()} [`);
     for (let key of sortedKeys) {
       out.push(
-        `    ${PDFObject.convert(this._dataForKey(key))} ${PDFObject.convert(
+        `    ${PDFObject.convert(this._dataForKey(key), encryptFn)} ${PDFObject.convert(
           this._items[key],
+          encryptFn,
         )}`,
       );
     }
diff --git a/tests/unit/tree.spec.js b/tests/unit/tree.spec.js
new file mode 100644
index 00000000..84b94842
--- /dev/null
+++ b/tests/unit/tree.spec.js
@@ -0,0 +1,94 @@
+import PDFDocument from '../../lib/document';
+import PDFNameTree from '../../lib/name_tree';
+import PDFNumberTree from '../../lib/number_tree';
+import PDFObject from '../../lib/object';
+import { fromBinaryString, toBinaryString } from '../../lib/binary';
+import { collectPdf } from './helpers';
+
+// Stand-in for a real cipher: uppercasing keeps the expected output readable
+// while still showing that every string went through it individually
+const encryptFn = (bytes) =>
+  fromBinaryString(toBinaryString(bytes).toUpperCase());
+
+describe.each([
+  [
+    'PDFNameTree',
+    {
+      Tree: PDFNameTree,
+      keys: ['a', 'b'],
+      plain: `<<
+  /Limits [(a) (b)]
+  /Names [
+    (a) (one)
+    (b) (two)
+]
+>>`,
+      // the keys are PDF strings, so they get encrypted just like the values
+      encrypted: `<<
+  /Limits [(A) (B)]
+  /Names [
+    (A) (ONE)
+    (B) (TWO)
+]
+>>`,
+    },
+  ],
+  [
+    'PDFNumberTree',
+    {
+      Tree: PDFNumberTree,
+      keys: [1, 2],
+      plain: `<<
+  /Limits [1 2]
+  /Nums [
+    1 (one)
+    2 (two)
+]
+>>`,
+      // the keys are numbers rather than strings, so they stay as they are
+      encrypted: `<<
+  /Limits [1 2]
+  /Nums [
+    1 (ONE)
+    2 (TWO)
+]
+>>`,
+    },
+  ],
+])('%s', (_treeName, { Tree, keys, plain, encrypted }) => {
+  let tree;
+
+  beforeEach(() => {
+    tree = new Tree();
+    tree.add(keys[0], new String('one'));
+    tree.add(keys[1], new String('two'));
+  });
+
+  test('is written in plain text without an encryption function', () => {
+    expect(PDFObject.convert(tree)).toEqual(plain);
+  });
+
+  test('is encrypted with an encryption function', () => {
+    expect(PDFObject.convert(tree, encryptFn)).toEqual(encrypted);
+  });
+});
+
+describe('name trees in a document', () => {
+  const names = ['(heading)', '(data.txt)', 'app.alert'];
+
+  const writeDocument = (options = {}) => {
+    const document = new PDFDocument(options);
+    document.text('link', { destination: 'heading' });
+    document.file(Buffer.from('example text'), { name: 'data.txt' });
+    document.addNamedJavaScript('hello', 'app.alert("hi")');
+    return collectPdf(document);
+  };
+
+  test.each(names)('writes %s in plain text when not encrypted', (name) => {
+    expect(writeDocument()).toContain(name);
+  });
+
+  test.each(names)('never writes %s when encrypted', (name) => {
+    expect(writeDocument({ userPassword: 'secret' })).not.toContain(name);
+  });
+});