Why the extension with specific key declared can access chrome:// pages? - google-chrome-extension

Question

As we know, by default chrome extensions doesn't have access to chrome:// pages such as chrome://extensions and chrome://settings. ( Of course we can change chrome://flags/#extensions-on-chrome-urls flags however the following question is based on that we didn't change the default flags).
Recently I happen to find ChromeVox (offered by chrome.google.com) can work well in all pages including chrome:// pages. I checked the source code for this extension and find as long as we add the following line in manifest.json for any extension, the extension can work well in chrome:// pages.
"key": "MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDEGBi/oD7Yl/Y16w3+gee/95/EUpRZ2U6c+8orV5ei+3CRsBsoXI/DPGBauZ3rWQ47aQnfoG00sXigFdJA2NhNK9OgmRA2evnsRRbjYm2BG1twpaLsgQPPus3PyczbDCvhFu8k24wzFyEtxLrfxAGBseBPb9QrCz7B4k2QgxD/CwIDAQAB"
So it looks like chrome has something like whitelist to allow specific extensions to break the default restrictions. Am I right? Is there official guide to clarify this behavior?
Appendix:
The following is a sample extension, you will find with the key, console will output test even in chrome://extensions pages; however once removing the key, nothing happens.
manifest.json:
{
"manifest_version": 2,
"name": "Test",
"version": "1.0",
"content_scripts": [
{
"matches": [
"<all_urls>"
],
"js": [
"content.js"
]
}
],
"key": "MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDEGBi/oD7Yl/Y16w3+gee/95/EUpRZ2U6c+8orV5ei+3CRsBsoXI/DPGBauZ3rWQ47aQnfoG00sXigFdJA2NhNK9OgmRA2evnsRRbjYm2BG1twpaLsgQPPus3PyczbDCvhFu8k24wzFyEtxLrfxAGBseBPb9QrCz7B4k2QgxD/CwIDAQAB"
}
content.js:
console.log('test');

Answer 1

"key" property in manifest.json uniquely defines the extension's ID in encrypted form.
Some Google extensions are unfairly(?) whitelisted by ID in the source code of chromium.
In this case, ChromeVox:
scripting_whitelist_.push_back(extension_misc::kChromeVoxExtensionId);
And then this whitelist is checked to see whether an extension can run everywhere in PermissionsData::CanExecuteScriptEverywhere, which is accessed in CheckRestrictedUrls where we can see restricted schemes: chrome://, chrome-extension://, chrome-debugger://