> For the complete documentation index, see [llms.txt](https://help.iodd.kr/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://help.iodd.kr/aes256-encryption-mini-pro-st-lk/admin/duress-pin.md).

# Duress PIN

The **Duress PIN** feature, also called a **decoy PIN**, lets an IODD **ST400** or **MINI PRO** present only a prepared `vhd4.vhd` virtual disk to the PC when the device is unlocked with the **User 4 PIN**.

This mode is useful when you need to expose a separate, shareable volume while keeping the normal storage area separate from the PC session. It can support a **plausibly deniable** access workflow by showing a prepared decoy volume instead of the normal physical partition.

{% hint style="warning" %}
**Supported Models and Firmware** Duress PIN with `vhd4.vhd` is supported on **ST400** and **MINI PRO** with firmware **version 82 or later**, updated using `iodd_firm_upd-0.8.2.0_xx.exe` or later.
{% endhint %}

***

### Terminology: Duress PIN, Decoy PIN, and Plausibly Deniable Access

In this guide, **Duress PIN** refers to the IODD feature name. **Decoy PIN** is a common search term for a PIN that opens a prepared alternate volume. **Plausibly deniable access** describes the user-facing result: the PC sees only the prepared `vhd4.vhd` disk during that session.

***

### How It Works

When the device is unlocked with a PIN other than **User 4**, the PC sees the normal physical partition. Use this state to create, delete, or update `vhd4.vhd`.

When the device is unlocked with the **User 4 PIN**, the PC sees only the disk mapped to `vhd4.vhd`. Other files from the physical partition are not shown in that session.

| Unlock Method         | Main Purpose                                                   | What the PC Sees          |
| --------------------- | -------------------------------------------------------------- | ------------------------- |
| PIN other than User 4 | File management, ISO/VHD management, `vhd4.vhd` setting update | Normal physical partition |
| User 4 PIN            | Duress PIN access                                              | Disk sized to `vhd4.vhd`  |

***

### Preparing `vhd4.vhd`

1. Unlock the device with a PIN other than the **User 4 PIN**.
2. On the PC, create a file named exactly `vhd4.vhd` in the **root folder** of the first partition.
3. Make sure the file is in the root folder, not in a subfolder.
4. Return to the root folder list on the device screen.
5. Rebuild the root folder list by pressing **Key 1**, rebooting the device, or entering a subfolder and returning to the root folder.
6. When `vhd4.vhd` appears in the list, the device updates the VHD setting.
7. Unlock the device again with the **User 4 PIN**.
8. The PC shows a disk whose size matches `vhd4.vhd`.

{% hint style="info" %}
When creating `vhd4.vhd`, use **VHD\_Tool++ v0.9.0.10 or later** from the [IODD Download Center](https://dir.iodd.kr/?dir=vhd_tools). VHD\_Tool++ is a Windows-only tool.
{% endhint %}

***

### `vhd4.vhd` Requirements

| Item                | Requirement                        |
| ------------------- | ---------------------------------- |
| File name           | `vhd4.vhd`                         |
| Location            | Root folder of the first partition |
| Format              | Fixed-size VHD file                |
| Unsupported formats | Dynamic VHD, VHDX                  |
| Placement           | Contiguous, without fragmentation  |

Select `vhd4.vhd` on the device and press **Key 5** to display the file information message. The `fragment` value must be `1`.

{% hint style="danger" %}
A fragmented `vhd4.vhd` cannot be used. If the LCD shows a defragmentation or `DEFRAG` message, recreate `vhd4.vhd` as a contiguous fixed-size VHD file, then unlock with a PIN other than User 4 and rebuild the root folder list.
{% endhint %}

***

### Updating the VHD Setting

The device checks whether `vhd4.vhd` exists when it builds the root folder list.

| State                                   | Device Behavior                                     |
| --------------------------------------- | --------------------------------------------------- |
| `vhd4.vhd` exists and is not fragmented | Updates the VHD setting for use with the User 4 PIN |
| `vhd4.vhd` exists but is fragmented     | Does not apply the VHD setting                      |
| `vhd4.vhd` does not exist               | Deletes the saved VHD setting                       |

{% hint style="warning" %}
Rebuild the root folder list only while unlocked with a PIN other than the **User 4 PIN**. A user with disk write protection cannot update the `vhd4.vhd` setting.
{% endhint %}

***

### Troubleshooting

#### The VHD Disk Does Not Appear After Unlocking With the User 4 PIN

1. Unlock the device with a PIN other than the User 4 PIN.
2. Check that `vhd4.vhd` exists in the root folder of the first partition.
3. Rebuild the root folder list by pressing **Key 1**, rebooting the device, or entering a subfolder and returning to the root folder.
4. Check that `vhd4.vhd` is not fragmented.
5. Unlock again with the User 4 PIN.

If the PIN used to update the setting has disk write protection enabled, the VHD setting cannot be updated.

#### The Device Does Not Recognize `vhd4.vhd`

* Make sure the file name is exactly `vhd4.vhd`.
* Make sure the file is in the root folder of the first partition.
* Make sure the root folder list was rebuilt after creating the file.
* Make sure the file is a fixed-size VHD, not a dynamic VHD or VHDX file.
* Make sure the file is not fragmented.

#### A Defragmentation or `DEFRAG` Message Appears

`vhd4.vhd` is not stored contiguously on the disk. Delete and recreate `vhd4.vhd` on the PC, or arrange it so it is stored in a contiguous area. Then unlock with a PIN other than the User 4 PIN and rebuild the root folder list.