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AMP Packager

AMP Packager is a tool to improve AMP URLs. By running it in a proper configuration, web publishers may (eventually) have origin URLs appear in AMP search results.

The AMP Packager works by creating Signed HTTP Exchanges (SXGs) containing AMP documents, signed with a certificate associated with the origin, with a maximum lifetime of 7 days. In the future, the Google AMP Cache will fetch, cache, and serve them, similar to what it does for normal AMP HTML documents. When a user loads such an SXG, Chrome validates the signature and then displays the certificate's domain in the URL bar instead of google.com, and treats the web page as though it were on that domain.

The packager is an HTTP server that sits behind a frontend server; it fetches and signs AMP documents as requested by the AMP Cache.

Packager/Signer

How to use

In all the instructions below, replace amppackageexample.com with a domain you own and can obtain certificates for.

Development server

Manual installation
  1. Install Go version 1.10 or higher. Optionally, set $GOPATH to something (default is ~/go) and/or add $GOPATH/bin to $PATH.

  2. go get -u github.com/ampproject/amppackager/cmd/amppkg

    Optionally, move the built ~/go/bin/amppkg wherever you like.

  3. Create a file amppkg.toml. A minimal config looks like this:

    LocalOnly = true
    CertFile = 'path/to/fullchain.pem'
    KeyFile = 'path/to/privkey.pem'
    OCSPCache = '/tmp/amppkg-ocsp'
    
    [[URLSet]]
      [URLSet.Sign]
        Domain = "amppackageexample.com"
    

    More details can be found in amppkg.example.toml.

  4. amppkg -development

    If amppkg.toml is not in the current working directory, pass -config=/path/to/amppkg.toml.

Docker

Follow the instructions here on how to deploy a local Docker container.

Test your config

  1. Run Chrome with the following commandline flags:
    --user-data-dir=/tmp/udd
    --ignore-certificate-errors-spki-list=$(openssl x509 -pubkey -noout -in path/to/fullchain.pem | openssl pkey -pubin -outform der | openssl dgst -sha256 -binary | base64)
    --enable-features=SignedHTTPExchange
    'data:text/html,<a href="https://localhost:8080/priv/doc/https://amppackageexample.com/">click me'
    
  2. Open DevTools. Check 'Preserve log'.
  3. Click the click me link.
  4. Watch the URL transmogrify! Verify it came from an SXG by switching DevTools to the Network tab and looking in the Size column for (from signed-exchange) and in the Type column for signed-exchange. Click on that row and then click on the Preview tab, to see if there are any errors.

Demonstrate privacy-preserving prefetch

This step is optional; just to show how privacy-preserving prefetch works with SXGs.

  1. go get -u github.com/ampproject/amppackager/cmd/amppkg_dl_sxg.
  2. amppkg_dl_sxg https://localhost:8080/priv/doc/https://amppackageexample.com/
  3. Stop amppkg with Ctrl-C.
  4. go get -u github.com/ampproject/amppackager/cmd/amppkg_test_cache.
  5. amppkg_test_cache
  6. Open Chrome and DevTools, as above.
  7. Visit https://localhost:8000/. Observe the prefetch of /test.sxg.
  8. Click the link. Observe that the cached SXG is used.

Productionizing

For now, productionizing is a bit manual. The minimum steps are:

  1. Don't pass -development flag to amppkg. This causes it to serve HTTP rather than HTTPS, among other changes.
  2. Don't expose amppkg to the outside world; keep it on your internal network.
  3. Configure your TLS-serving frontend server to conditionally proxy to amppkg:
    1. If the URL starts with /amppkg/, forward the request unmodified.
    2. If the URL points to an AMP page and the AMP-Cache-Transform request header is present, rewrite the URL by prepending /priv/doc and forward the request.
    3. If at all possible, don't send URLs of non-AMP pages to amppkg; its transforms may break non-AMP HTML.
    4. DO NOT forward /priv/doc requests; these URLs are meant to be generated by the frontend server only.
  4. For HTTP compliance, ensure the Vary header set to AMP-Cache-Transform, Accept for all URLs that point to an AMP page, irrespective of whether the response is HTML or SXG. (SXG responses that come from amppkg will have the appropriate Vary header set, so it may only be necessary to explicitly set the Vary header for HTML responses.)
  5. Get an SXG cert from your CA. It must use an EC key with the prime256v1 algorithm, and it must have a CanSignHttpExchanges extension. One provider of SXG certs is DigiCert. You MUST use this in amppkg.toml, and MUST NOT use it in your frontend.
  6. Every 90 days or sooner, renew your SXG cert (per WICG/webpackage#383) and restart amppkg (per #93).
  7. Keep amppkg updated from releases (the default branch, so go get works) about every ~2 months. The details of this release cadence are still being worked out, but they will be signaled by Googlebot changing its AMP-Cache-Transform header from google;v=N to google;v=N..{N+1} and then ~2 months later to google;v={N+1}. (Or perhaps Google will always allow at least 2 versions; TBD.) You can use various tools to subscribe to releases.

You may also want to:

  1. Launch amppkg as a restricted user.
  2. Save its stdout to a rotated log somewhere.
  3. Use the provided tools to verify that your published AMP documents are valid, for instance just before publication, or with a regular audit of a sample of documents. The transforms are designed to work on valid AMP pages, and may break invalid AMP in small ways.

Once you've done the above, you should be able to test by launching Chrome without any comamndline flags; just make sure chrome://flags/#enable-signed-http-exchange is enabled. To test by visiting the packager URL directly, first add a Chrome extension to send an AMP-Cache-Transform: any request header. Otherwise, follow the above "Demonstrate privacy-preserving prefetch" instructions.

Security Considerations

Signed exchanges come with some security considerations that publishers should consider. A starting list of recommendations based on that:

  • Use different keys for the signed exchange cert and the TLS cert.
  • Only sign public content that's OK to be shared with crawlers.
  • Don't sign personalized content. (It's OK to sign content that includes static JS that adds personalization at runtime.)
  • Be careful when signing inline JS; if it includes a vulnerability, it may be possible for attackers to exploit it without intercepting the network path, for up to 7 days.

Testing productionization without a valid certificate

It is possible to test an otherwise fully production configuration without obtaining a certificate with the CanSignHttpExchanges extension. amppkg still needs to perform OCSP verification, so the Issuer CA must be valid (i.e. no self-signed certificates). e.g. You can use a certificate from Let's Encrypt.

Running amppkg with the -invalidcert flag will skip the check for CanSignHttpExchanges. This flag is not necessary when using the -development flag.

Chrome can be configured to allow these invalid certificates with the Allow Signed HTTP Exchange certificates without extension experiment: chrome://flags/#allow-sxg-certs-without-extension

Redundancy

If you need to load balance across multiple instances of amppkg, you'll want your OCSPCache to be backed by a shared storage device (e.g. NFS). It doesn't need to be shared among all instances globally, but perhaps among all instances per datacenter. The reason for this is to reduce the number of OCSP requests amppkg needs to make, per OCSP stapling recommendations.

How will these web packages be discovered by Google?

Googlebot makes requests with an AMP-Cache-Transform header. Responses that are acceptable AMP SXGs will be eligible for display to SXG-supporting browsers, and the HTML payload will be extracted and eligible for use in the AMP viewer in other browsers.

Limitations

Currently, the packager will refuse to sign any AMP documents larger than 4 MB. Patches that allow for streamed signing are welcome.

The packager refuses to sign any URL that results in a redirect. This is by design, as neither the original URL nor the final URL makes sense as the signed URL.

To account for possible clock skew in user agents, the packager back-dates packages by 24h, which means they effectively last only 6 days for most users.

This tool only packages AMP documents. To sign non-AMP documents, look at the commandline tools on which this was based, at https://github.com/WICG/webpackage/tree/master/go/signedexchange.

Local Transformer

The local transformer is a library within the AMP Packager that transforms AMP HTML for security and performance improvements. Ports of or alternatives to the AMP Packager will need to include these transforms.

More info here.

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