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                          Secure Shell Developer Guide


Secure Shell is a Chrome App (currently a “v1.5” app, soon to become a “v2” or Platform App) that combines hterm with a NaCl build of OpenSSH to provide a PuTTY-like app for Chrome users.

See /HACK.md for general information about working with the source control setup.

Building the dependencies

The Secure Shell app depends on some library code from libapps/libdot/ and the hterm terminal emulator from in libapps/hterm/. To build these external dependencies, run...

nassh$ ./bin/mkdeps.sh

This will create the nassh/js/nassh_deps.concat.js file containing all of the necessary libdot and hterm source. If you find yourself changing a lot of libdot or hterm code and testing those changes in Secure Shell you can run this script with the “--forever” (aka -f) option. When run in this manner it will automatically re-create nassh_deps.concat.js file whenever one of the source files is changed.

The NaCl plugin dependency

Secure Shell depends on a NaCl (Native Client) plugin to function. This plugin is a port of OpenSSH. You'll have to find or create a version of this plugin, and copy it into libapps/nassh/plugin/.

Your options are (pick one):

  1. Build it yourself from ssh_client. This can take some time, but once it's finished:
# In the ssh_client/ directory.
$ cp -a output/hterm/plugin/ ../nassh/
  1. Grab an existing release. For example:
# In the nassh/ directory.
$ wget https://commondatastorage.googleapis.com/chromeos-localmirror/secureshell/releases/0.8.39.tar.xz
$ tar --strip-components=1 -xf 0.8.39.tar.xz hterm/plugin
  1. Copy the plugin/ directory from the latest version of Secure Shell. If you have Secure Shell installed, the plugin can be found in your profile directory, under Default/Extensions/pnhechapfaindjhompbnflcldabbghjo/<version>/plugin/.

Dev-Test cycle

Loading Unpacked Extensions

Loading directly from the checked out nassh directory is the normal way of testing. It will use the dev extension id to avoid conflicts with the stable extension id, although it will still conflict if you have the dev version installed from the Chrome Web Store (CWS).

You will need to manually select the variant you want to work on. The extension is defined by the manifest_ext.json file while the app is defined by the manifest_v1.5.json file. Simply symlink it to manifest.json:

nassh$ ln -s manifest_ext.json manifest.json

For details on the different manifests and modes, see the next section (and the FAQ). You probably want to start with the extension version if you're not going to be hacking on Chrome OS features.

The extension id is controlled by the key field in the manifest.json. See the manifest key docs for more details.

Adding To Chrome

Now that your checkout is ready, you can load it into Chrome.

  1. Navigate to the chrome://extensions page.
  2. Turn on Developer Mode (look for the toggle in the upper right or bottom right of the page depending on Chrome version).
  3. Click Load Unpacked Extension and navigate to the nassh/ directory.

If you‘re not running on Chrome OS device, and loading the app, you might see warnings right away about certain permissions (see the whitelisted sections below). You can ignore those. It’s unfortunate they show up with the same level/color as legitmate errors.

* 'file_system_provider_capabilities' is only allowed for extensions and packaged apps, but this is a legacy packaged app.
* 'terminalPrivate' is not allowed for specified platform.
* 'fileSystemProvider' is not allowed for specified platform.


There are three manifest files in here currently. The manifest_v1.5.json is used to build the Secure Shell App and has been what we've used for the longest time, but is largely for Chrome OS only now. The manifest_ext.json is used to build the Secure Shell Extension which works on all platforms (but lacks any Chrome OS specific features).

The manifest_v2.json is not used currently. Some day we might finish the migration and replace manifest_v1.5.json with it so we only have one app manifest. Today it is not often tested.

The “v1.5” and “v2” app formats should not be confused with the “v1” and “v2” manifest formats. Secure Shell uses the legacy/deprecated “v1.5” app style to launch itself rather than the “v2” style. It means that, in many ways, the “v1.5” app behaves more like an extension (e.g. it shares cookies with your main browser instance and can run inside a tab) rather than an app (e.g. it doesn't get access to many newer chrome.app.* APIs).

See the FAQ for more details on the differences between the extension & app.

If you‘re updating these files, you’ll sometimes also need to update the manifest for crosh which lives in the Chromium tree.

Whitelisted Permissions

Using the dev extension id is necessary in order to access some APIs that are whitelisted only for Secure Shell. If you don't need these features, you can get by with using a different id (and delete the settings from the manifest_v1.5.json for the app to avoid warnings at runtime). These settings are already removed from the manifest_ext.json for the extension.

  • Access to crosh under Chrome OS (terminalPrivate). (1) (2) (3)
  • Access to raw sockets under NaCl. This allows connecting directly to SSH servers (e.g. port 22). (1)
    Note: Making connections over https using relay servers will still work though. See the FAQ for more details.
  • SFTP backend for Chrome OS (fileSystemProvider and file_system_provider_capabilities). (1)
    Note: Normal extensions/apps can use these features, but since Secure Shell is still a “legacy packaged app”, we had to whitelist access.

To double check what & where things are whitelisted, search the Chromium code base for our extension ids:

Stable Appcs/pnhechapfaindjhompbnflcldabbghjocs/0EA6B717932AD64C469C1CCB6911457733295907
Dev Appcs/okddffdblfhhnmhodogpojmfkjmhinfpcs/58B0C2968C335964D5433E89CA4D86628A0E3D4B
Stable Extensioncs/iodihamcpbpeioajjeobimgagajmlibdcs/3BC1ED0B3E6EFDC7BD4D3D1D75D44B52DEE0A226
Dev Extensioncs/algkcnfjnajfhgimadimbjhmpaeohhlncs/38C361D4A0726CE45D3572D65071B6BDB3092371

The hashes are the SHA1's of the (lower case) extension id.

$ ext_id_hash() { printf "$1" | tr '[:upper:]' '[:lower:]' | sha1sum | tr '[:lower:]' '[:upper:]'; }
$ ext_id_hash pnhechapfaindjhompbnflcldabbghjo

Partner Extensions

There are a few extensions we talk to for various services at runtime which need to whitelist our extension ids (for source verification). If you don't need any of these services, then you can ignore it.

Stable Extension

If you try to load an unpacked extension using the stable extension id, you might run into problems if your administrator installs it via enterprise policy. If you see the error below, you won't be able to bypass it. Just use the dev extension id instead.

Secure Shell (extension ID "pnhechapfaindjhompbnflcldabbghjo") is blocked by the administrator.


While most of the UI code for crosh lives here (e.g. HTML/CSS/JS), the backend code and manifest lives in Chrome.

Coding Style

See the libapps hacking document for details.

Source Layout

Keep in mind that the NaCl ssh_client code does not live here.

The vast majority of the code here lives under js/.

JavaScript Source Layout

There are a few specialized modules that are not relevant to the core Secure Shell logic.

NaCl/JS Life cycle

When the extension is launched (e.g. a new connection is opened), the background page is automatically created. This is used to monitor global state like extension updates and coordinate SFTP mounts. The logic lives in nassh_background.js and takes care of creating a new instance of nassh.App which it saves in the background page‘s app variable. If you aren’t looking at the SFTP logic, you can probably ignore this code.

When the extension is run, a new nassh.html window is shown. If no connection info is provided via the URL, then an iframe is created to show nassh_connect_dialog.html. Here the user manages their saved list of connections and picks the one they want to connect to. This logic is in nassh_connect_dialog.js. Once the user makes a selection (either connecting or mounting), a message is sent to nassh_command_instance.js. There the connection dialog is closed, the NaCl plugin is loaded, and the streams are connected to hterm.


Here is the API that the JS code uses to communicate with the NaCl ssh_client module.

The nassh.CommandInstance.prototype.sendToPlugin_ function in nassh_command_instance.js is used to package up and make all the calls. Helper functions are also provided in that file to avoid a JS API to callers.

At the lowest level, we pass a JSON string to the plugin. It has two fields, both of which must be specified (even if arguments is just []).

  • name: The function we want to call (as a string).
  • arguments: An array of arguments to the function.

The name field can be any one of:

Function nameDescriptionArguments
startSessionStart a new ssh connection!(object session)
onOpenFileOpen a new file.(int fd, bool success, bool is_atty)
onOpenSocketOpen a new socket.(int fd, bool success, bool is_atty)
onReadSend new data to the plugin.(int fd, base64 data)
onWriteAcknowledgeTell plugin we've read data.(int fd, int count)
onCloseClose an existing fd.(int fd)
onReadReadyNotify plugin data is available.(int fd, bool result)
onResizeNotify terminal size changes.(int width, int height)
onExitAcknowledgeUsed to quit the plugin.()

The session object currently has these members:

  • str username: Username for accessing the remote system.
  • str host: Hostname for accessing the remote system.
  • int port: Port number for accessing the remote system.
  • int terminalWidth: Initial width of the terminal window.
  • int terminalHeight: Initial height of the terminal window.
  • bool useJsSocket: Whether to use JS for network traffic.
  • object environment: A key/value object of environment variables.
  • array arguments: Extra command line options for ssh.
  • int writeWindow: Size of the write window.
  • str authAgentAppID: Extension id to use as the ssh-agent.
  • str subsystem: Which subsystem to launch.


Here is the API that the NaCl ssh_client code uses to communicate with the JS layers.

At the lowest level, we pass a JSON string to the JS code. It has two fields, both of which must be specified (even if arguments is just []).

  • name: The function we want to call (as a string).
  • arguments: An array of arguments to the function.

The name field can be any one of:

Function nameDescriptionArguments
openFilePlugin wants to open a file.(int fd, str path, int mode)
openSocketPlugin wants to open a socket.(int fd, str host, int port)
readPlugin wants to read data.(int fd, int count)
writePlugin wants to write data.(int fd, base64 data)
closePlugin wants to close an fd.(int fd)
isReadReadyPlugin wants to know read status.(int fd)
exitThe plugin is exiting.(int code)
printLogSend a string to console.log.(str str)

SFTP Support

On Chrome OS, it is possible to mount remote paths via SFTP and the Files app. We currently support version 3 of the protocol.

External API

The extension provides an external API to allow other apps/extensions to invoke SFTP mount using Chrome messaging. The nassh background page adds a listener for chrome.runtime.onMessageExternal which can be invoked by other apps / extensions by calling chrome.runtime.sendMessage with the following fields in message:

Field nameTypeDescription
command!stringCommand to run. Currently only mount supported


Field nameTypeDescription
knownHosts!stringFile contents of known_hosts to be used for connection. e.g. output from ssh-keyscan <ssh-server>
identityFile!stringFile contents of private key identity_file (e.g. contents of id_rsa file -----BEGIN RSA PRIVATE KEY----- ...)
username!stringUsername for connection
hostname!stringHostname or IP address for connection
portnumber(optional) Port, default is 22
fileSystemId!stringID used for Chrome OS mounted filesystem
displayName!stringDisplay name in Chrome OS Files.app for mounted filesystem


Here's a random list of documents which would be useful to people.