diff --git a/imptune/generators/intunewin_builder.py b/imptune/generators/intunewin_builder.py
new file mode 100644
index 0000000..d7c7b5e
--- /dev/null
+++ b/imptune/generators/intunewin_builder.py
@@ -0,0 +1,111 @@
+"""
+Python-native .intunewin file assembler.
+
+Generates valid .intunewin packages using AES-256-CBC encryption with HMAC-SHA256,
+producing the exact byte layout that Microsoft Intune expects.
+
+Encrypted blob layout (from svrooij.io reverse-engineering):
+    [0:32]   HMAC-SHA256 of the ciphertext (32 bytes, mac_key)
+    [32:48]  AES-256-CBC Initialization Vector (16 bytes — standard AES block size)
+    [48:]    AES-256-CBC ciphertext (PKCS7-padded to 16-byte boundary)
+
+IMPORTANT: IV is 16 bytes, NOT 32. STACK.md has a documentation error on this point.
+
+Outer ZIP structure:
+    IntuneWinPackage/
+    ├── Contents/
+    │   └── IntunePackage.intunewin    (the encrypted blob)
+    └── Metadata/
+        └── Detection.xml              (encryption metadata)
+"""
+import base64
+import hashlib
+import hmac
+import io
+import os
+import zipfile
+import xml.dom.minidom
+from xml.etree.ElementTree import Element, SubElement, tostring
+
+from Crypto.Cipher import AES
+from Crypto.Util.Padding import pad
+
+
+def build_intunewin(source_dir: str, setup_file: str, output_path: str) -> None:
+    """Build a .intunewin file from source_dir, with setup_file as entry point.
+
+    Args:
+        source_dir: Path to the directory containing files to package.
+        setup_file: Name of the setup/entry-point file (e.g., "install.ps1").
+                    Must be present in source_dir. Used in Detection.xml metadata.
+        output_path: Destination path for the generated .intunewin file.
+
+    Raises:
+        FileNotFoundError: If source_dir does not exist.
+        ValueError: If setup_file is empty.
+    """
+    # --- Step 1: Create inner ZIP (DEFLATE-compressed content) ---
+    inner_zip_buf = io.BytesIO()
+    with zipfile.ZipFile(inner_zip_buf, "w", compression=zipfile.ZIP_DEFLATED) as zf:
+        for root, dirs, files in os.walk(source_dir):
+            dirs.sort()   # deterministic ordering
+            for filename in sorted(files):
+                abs_path = os.path.join(root, filename)
+                arc_name = os.path.relpath(abs_path, source_dir)
+                # Normalise to forward slashes for cross-platform consistency
+                arc_name = arc_name.replace("\\", "/")
+                zf.write(abs_path, arc_name)
+    plaintext = inner_zip_buf.getvalue()
+
+    # --- Step 2: Generate random keys and IV ---
+    aes_key = os.urandom(32)   # 256-bit AES key
+    mac_key = os.urandom(32)   # 256-bit HMAC key (same size as AES key)
+    iv = os.urandom(16)        # 128-bit IV — standard AES-CBC block size (NOT 32 bytes)
+
+    # --- Step 3: Encrypt with AES-256-CBC (PKCS7 padding) ---
+    cipher = AES.new(aes_key, AES.MODE_CBC, iv)
+    ciphertext = cipher.encrypt(pad(plaintext, AES.block_size))
+
+    # --- Step 4: Compute HMAC-SHA256 over ciphertext (using mac_key) ---
+    mac_digest = hmac.new(mac_key, ciphertext, hashlib.sha256).digest()
+
+    # --- Step 5: Assemble encrypted blob: [HMAC(32)] + [IV(16)] + [ciphertext] ---
+    encrypted_blob = mac_digest + iv + ciphertext
+
+    # --- Step 6: Compute plaintext (inner ZIP) SHA256 digest for Detection.xml ---
+    file_digest = hashlib.sha256(plaintext).digest()
+
+    # --- Step 7: Build Detection.xml ---
+    app_info = Element(
+        "ApplicationInfo",
+        attrib={"xmlns": "http://schemas.microsoft.com/IntuneWin"},
+    )
+    SubElement(app_info, "Name").text = setup_file
+    SubElement(app_info, "UnencryptedContentSize").text = str(len(plaintext))
+    SubElement(app_info, "FileName").text = "IntunePackage.intunewin"
+    SubElement(app_info, "SetupFile").text = setup_file
+
+    enc_info = SubElement(app_info, "EncryptionInfo")
+    SubElement(enc_info, "EncryptionKey").text = base64.b64encode(aes_key).decode()
+    SubElement(enc_info, "MacKey").text = base64.b64encode(mac_key).decode()
+    SubElement(enc_info, "InitializationVector").text = base64.b64encode(iv).decode()
+    SubElement(enc_info, "Mac").text = base64.b64encode(mac_digest).decode()
+    SubElement(enc_info, "MacAlgorithm").text = "SHA256"
+    SubElement(enc_info, "ProfileIdentifier").text = "ProfileVersion1"
+    SubElement(enc_info, "FileDigest").text = base64.b64encode(file_digest).decode()
+    SubElement(enc_info, "FileDigestAlgorithm").text = "SHA256"
+
+    detection_xml = xml.dom.minidom.parseString(
+        tostring(app_info, encoding="unicode")
+    ).toprettyxml(indent="  ")
+
+    # --- Step 8: Build outer ZIP (STORED — no extra compression on encrypted content) ---
+    with zipfile.ZipFile(output_path, "w", compression=zipfile.ZIP_STORED) as outer:
+        outer.writestr(
+            "IntuneWinPackage/Contents/IntunePackage.intunewin",
+            encrypted_blob,
+        )
+        outer.writestr(
+            "IntuneWinPackage/Metadata/Detection.xml",
+            detection_xml.encode("utf-8"),
+        )