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+<div class="section">
+
+Depthcharge payload {#pagetop}
+===================
+
+This section relates to the depthcharge payload used in libreboot.
+
+Or [Back to main index](../).
+
+-   [CrOS security model](#cros_security_model)
+-   [Developer mode screen](#developer_mode_screen)
+    -   Holding the developer mode screen
+    -   Booting normally
+    -   Booting from different mediums
+    -   Showing device information
+    -   Warnings
+-   [Recovery mode screen](#recovery_mode_screen)
+    -   [Recovering from a bad state](#recovering_bad_state)
+    -   [Enabling developer mode](#enabling_developer_mode)
+-   [Configuring verified boot
+    parameters](#configuring_verified_boot_parameters)
+
+</div>
+
+<div class="section">
+
+CrOS security model {#cros_security_model}
+===================
+
+CrOS (Chromium OS/Chrome OS) devices such as Chromebooks implement a
+strict security model to ensure that these devices do not become
+compromised, that is implemented as the verified boot (vboot) reference,
+most of which is executed within depthcharge. A detailed overview of the
+CrOS security model is available on the dedicated page.
+
+<div class="subsection">
+
+In spite of the CrOS security model, depthcharge won\'t allow booting
+kernels without verifying their signature and booting from external
+media or legacy payload unless explicitly allowed: see [configuring
+verified boot parameters](#configuring_verified_boot_parameters).
+
+</div>
+
+</div>
+
+<div class="section">
+
+Developer mode screen {#developer_mode_screen}
+=====================
+
+The developer mode screen can be accessed in depthcharge when developer
+mode is enabled.\
+Developer mode can be enabled from the [recovery mode
+screen](#recovery_mode_screen).
+
+It allows booting normally, booting from internal storage, booting from
+external media (when enabled), booting from legacy payload (when
+enabled), showing information about the device and disabling developer
+mode.
+
+<div class="subsection">
+
+Holding the developer mode screen {#holding_developer_mode_screen}
+---------------------------------
+
+As instructed on the developer mode screen, the screen can be held by
+pressing **Ctrl + H** in the first 3 seconds after the screen is shown.
+After that delay, depthcharge will resume booting normally.
+
+</div>
+
+<div class="subsection">
+
+Booting normally {#booting_normally}
+----------------
+
+As instructed on the developer mode screen, a regular boot will happen
+after **3 seconds** (if developer mode screen is not held).\
+The default boot medium (internal storage, external media, legacy
+payload) is shown on screen.
+
+</div>
+
+<div class="subsection">
+
+Booting from different mediums {#booting_different_mediums}
+------------------------------
+
+Depthcharge allows booting from different mediums, when they are allowed
+(see [configuring verified boot
+parameters](#configuring_verified_boot_parameters) to enable or disable
+boot mediums).\
+As instructed on the developer mode screen, booting from various mediums
+can be triggered by pressing various key combinations:
+
+-   Internal storage: **Ctrl + D**
+-   External media: **Ctrl + U** (when enabled)
+-   Legacy payload: **Ctrl + L** (when enabled)
+
+</div>
+
+<div class="subsection">
+
+Showing device information {#showing_device_information}
+--------------------------
+
+As instructed on the developer mode screen, showing device information
+can be triggered by pressing **Ctrl + I** or **Tab**.\
+Various information is shown, including vboot non-volatile data, TPM
+status, GBB flags and key hashes.\
+
+</div>
+
+<div class="subsection">
+
+Warnings
+--------
+
+The developer mode screen will show warnings when:
+
+-   Booting kernels without verifying their signature is enabled
+-   Booting from external media is enabled
+-   Booting legacy payloads is enabled
+
+</div>
+
+</div>
+
+<div class="section">
+
+Recovery mode screen {#recovery_mode_screen}
+====================
+
+The recovery mode screen can be accessed in depthcharge, by pressing
+**Escape + Refresh + Power** when the device is off.
+
+It allows recovering the device from a bad state by booting from a
+trusted recovery media. When accessed with the device in a good state,
+it also allows enabling developer mode.
+
+<div class="subsection">
+
+Recovering from a bad state {#recovering_bad_state}
+---------------------------
+
+When the device fails to verify the signature of a piece of the boot
+software or when an error occurs, it is considered to be in a bad state
+and will instruct the user to reboot to recovery mode.\
+Recovery mode boots using only software located in write-protected
+memory, that is considered to be trusted and safe.
+
+Recovery mode then allows recovering the device by booting from a
+trusted recovery media, that is automatically detected when recovery
+mode starts. When no external media is found or when the recovery media
+is invalid, instructions are shown on screen.\
+Trusted recovery media are external media (USB drives, SD cards, etc)
+that hold a kernel signed with the recovery key.
+
+Google provides images of such recovery media for Chrome OS (which are
+not advised to users as they contain proprietary software).\
+They are signed with Google\'s recovery keys, that are pre-installed on
+the device when it ships.
+
+When replacing the full flash of the device, the pre-installed keys are
+replaced. When the recovery private key is available (e.g. when using
+self-generated keys), it can be used to sign a kernel for recovery
+purposes.
+
+</div>
+
+<div class="subsection">
+
+Enabling developer mode {#enabling_developer_mode}
+-----------------------
+
+As instructed on the recovery mode screen, developer mode can be enabled
+by pressing **Ctrl + D**.\
+Instructions to confirm enabling developer mode are then shown on
+screen.
+
+</div>
+
+</div>
+
+<div class="section">
+
+Configuring verified boot parameters {#configuring_verified_boot_parameters}
+====================================
+
+Depthcharge\'s behavior relies on the verified boot (vboot) reference
+implementation, that can be configured with parameters stored in the
+verified boot non-volatile storage.\
+These parameters can be modified with the **crossystem** tool, that
+requires sufficient privileges to access the verified boot non-volatile
+storage.
+
+**crossystem** relies on **mosys**, that is used to access the verified
+boot non-volatile storage on some devices. **crossystem** and **mosys**
+are both free software and their source code is made available by
+Google:
+[crossystem](https://chromium.googlesource.com/chromiumos/platform/vboot_reference/).
+[mosys](https://chromium.googlesource.com/chromiumos/platform/mosys/).\
+These tools are not distributed along with Libreboot yet. However, they
+are preinstalled on the device, with ChromeOS.
+
+Some of these parameters have the potential of **weakening the security
+of the device**. In particular, disabling kernels signature
+verification, external media boot and legacy payload boot can weaken the
+security of the device.
+
+<div class="subsection">
+
+The following parameters can be configured:
+
+-   Kernels signature verification:
+    -   Enabled with:\
+        \# **crossystem dev\_boot\_signed\_only=1**
+    -   Disabled with:\
+        \# **crossystem dev\_boot\_signed\_only=0**
+-   External media boot:
+    -   Enabled with:\
+        \# **crossystem dev\_boot\_usb=1**
+    -   Disabled with:\
+        \# **crossystem dev\_boot\_usb=0**
+-   Legacy payload boot:
+    -   Enabled with:\
+        \# **crossystem dev\_boot\_legacy=1**
+    -   Disabled with:\
+        \# **crossystem dev\_boot\_legacy=0**
+-   Default boot medium:
+    -   Internal storage:\
+        \# **crossystem dev\_default\_boot=disk**
+    -   External media:\
+        \# **crossystem dev\_default\_boot=usb**
+    -   Legacy payload:\
+        \# **crossystem dev\_default\_boot=legacy**
+
+</div>
+
+</div>
+
+<div class="section">
+
+Copyright © 2015 Paul Kocialkowski &lt;contact@paulk.fr&gt;\
+Permission is granted to copy, distribute and/or modify this document
+under the terms of the Creative Commons Attribution-ShareAlike 4.0
+International license or any later version published by Creative
+Commons; A copy of the license can be found at
+[../cc-by-sa-4.0.txt](../cc-by-sa-4.0.txt)
+
+Updated versions of the license (when available) can be found at
+<https://creativecommons.org/licenses/by-sa/4.0/legalcode>
+
+UNLESS OTHERWISE SEPARATELY UNDERTAKEN BY THE LICENSOR, TO THE EXTENT
+POSSIBLE, THE LICENSOR OFFERS THE LICENSED MATERIAL AS-IS AND
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+OF ERRORS, WHETHER OR NOT KNOWN OR DISCOVERABLE. WHERE DISCLAIMERS OF
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+
+TO THE EXTENT POSSIBLE, IN NO EVENT WILL THE LICENSOR BE LIABLE TO YOU
+ON ANY LEGAL THEORY (INCLUDING, WITHOUT LIMITATION, NEGLIGENCE) OR
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