How to set up Signed Exchanges using Web Packager

This content originally appeared on web.dev and was authored by Katie Hempenius

A signed exchange (SXG) is a delivery mechanism that makes it possible to authenticate the origin of a resource independently of how it was delivered. The following instructions explain how to set up Signed Exchanges using Web Packager. Instructions are included for both self-signed and CanSignHttpExchanges certificates.

Serve SXGs using a self-signed certificate #

Using a self-signed certificate to serve SXGs is primarily used for demonstration and testing purposes. SXGs signed with a self-signed certificate will generate error messages in the browser when used outside of testing environments and should not be served to crawlers.

Prerequisites #

To follow these instructions you will need to have openssl and Go installed in your development environment. You will also need an existing HTTPS site.

Architecture #

These instructions use the following architecture to serve SXGs:

Following these instructions verbatim will setup a webpackager instance that packages and serves content from example.com as a SXG. To generate SXGs for a different site, replace the mentions of example.com in these instructions with the site of your choice. In production environments you will only be able to generate SXGs for sites that you own.

Generate a self-signed certificate #

This section explains how to generate a self-signed certificate that can be used with signed exchanges.

Instructions #

1. Generate a private key.

   openssl ecparam -out priv.key -name prime256v1 -genkey

   The private key will be saved as a file named priv.key.

2. Create a certificate signing request (CSR).

   openssl req -new -sha256 -key priv.key -out cert.csr -subj '/O=Web Packager Demo /CN=example.com'

   A certificate signing request is a block of encoded text that conveys the information necessary to request a certificate from a certificate authority(CA). Although you will not be requesting a certificate from a CA, it is still necessary to create a certificate signing request.

   The command above creates a certificate signing request for an organization named Web Packager Demo that has the common name example.com. The common name should be the fully qualified domain name of the site that contains the content that you want to package as SXG.

   In a production SXG setup, this would be a site that you own. However, in a testing environment like the one described in these instructions, it can be any site.

3. Create a certificate that has the CanSignHttpExchanges extension.

   openssl x509 -req -days 90 -in cert.csr -signkey priv.key -out cert.pem -extfile <(echo -e "1.3.6.1.4.1.11129.2.1.22 = ASN1:NULL\nsubjectAltName=DNS:example.com")

   This command uses the private key and the CSR created in steps 1 and 2 to create the certificate file cert.pem. The -extfile flag associates the certificate with the CanSignHttpExchanges certificate extension (1.3.6.1.4.1.11129.2.1.22 is the object identifier for the CanSignHttpExchanges extension). In addition, the -extfile flag also defines example.com as a Subject Alternative Name.

   If you are curious about the contents of cert.pem, you can view them using the following command:

   openssl x509 -in cert.pem -noout -text

   You are done creating private keys and certificates. You will need the priv.key and cert.pem files in the next section.

Setup the Web Packager server for testing #

Prerequisites #

1. Install Web Packager.

   git clone https://github.com/google/webpackager.git

2. Build webpkgserver.

   cd webpackager/cmd/webpkgserver
   go build .

   webpkgserver is a specific binary within the Web Packager project.

3. Verify that webpkgserver has been installed correctly.

   webpkgserver --help

   This command should return information about the usage of webpkgserver. If this does not work, a good first troubleshooting step is to verify that your GOPATH is configured correctly.

Instructions #

1. Navigate to the webpackager directory (you might already be in this directory).

   cd /path/to/cmd/webpkgserver

2. Create a webpkgsever.toml file by copying the example.

   cp ./webpkgserver.example.toml ./webpkgserver.toml

   This file contains the configuration options for webpkgserver.

3. Open webpkgserver.toml with an editor of your choice and make the following changes:

   • Change the line #AllowTestCert = false to AllowTestCert = true.
   • Change the line PEMFile = 'path/to/your.pem' to reflect the path to the PEM certificate, cert.pem, that you created. Do not change the line mentioning TLS.PEMFile—this is a different configuration option.
   • Change the line KeyFile = 'priv.key' to reflect the path of the private key, priv.key, that you created. Do not change the line mentioning TLS.KeyFile—this is a different configuration option.
   • Change the line Domain = 'example.org' to reflect the domain that you created a certificate for. If you have followed the instructions in this article verbatim, this should be changed to example.com. webpkgserver will only fetch content from the domain indicated by webpkgserver.toml. If you try to fetch pages from a different domain without updating webpkgserver.toml, the webpkgserver logs will show the error message URL doesn't match the fetch targets.
   • (Optional) To preload subresources, change the line #PreloadCSS = false to PreloadCSS = true. In addition, change the line #PreloadJS = false to PreloadJS = true. For more information about subresource substitution, check out the explainer.

4. Start webpkgserver.

   webpkgserver

   If the server has started successfully, you should see the message Listening at 127.0.0.1:8080. If you do not see this message, a good first troubleshooting step is to double-check webpkgserver.toml.

   Next, in order to test SXGs that use a self-signed certificate, you will need to enable the Allow Signed HTTP Exchange certificates without extension flag in Chrome.

5. Open Chrome, go to chrome://flags, and then set Allow Signed HTTP Exchange certificates without extension to Enabled. Then click the Relaunch button to have these changes take effect.

6. Open the DevTools Network tab, then visit the following URL: http://localhost:8080/priv/doc/https://example.com.

   This makes a request to the webpackager instance running at http://localhost:8080 for a SXG containing the contents of https://example.com. /priv/doc/ is the default API endpoint used by webpackager.

A resource with the type signed-exchange should be listed in the Network tab. If you don't see this resource, try clearing the cache, then reloading http://localhost:8080/priv/doc/https://example.com.

DevTools highlights the SXG in red because the SXG has errors associated with it. To view these errors and other information about the SXG, click on the SXG, then click Preview.

The Preview tab displays information about the Signed Exchange and its signature. At the top of the Preview tab you should see the error Failed to fetch certificate. The browser displays this error when it is unable to load a certificate from the Certificate URL indicated in the signature. The next section explains how to fix this error by uploading a certificate.

Without the certificate, the browser is unable to authenticate the SXG and it falls back to loading the resource without using SXG. This is why there is an additional request to example.com listed in the Network panel.

Upload the self-signed certificate #

To establish the authenticity of a SXG, the browser must be able to load the certificate that was used to sign the SXG from the Certificate URL indicated in the signature. If the browser is unable to load this certificate, it will request that the content be delivered without using SXG.

These instructions explain how to serve a certificate from an existing HTTPS environment.

Prerequisites #

The instructions in this section assume that you have the ability to upload a certificate to an existing HTTPS site. In addition, you should be comfortable adjusting the server configuration of this site.

Instructions #

1. In DevTools, locate the Certificate URL indicated in the Signature of the SXG. Copy the hash that is located at the end of this string.

   This hash is an identifier that corresponds to the certificate. If you were to regenerate the SXG using a different certificate, the Certificate URL listed in the Signature would be different.

   

2. Create a copy of cert.pem. The filename of the new version should match the hash you just copied—for example, dKqTlYij_pSjvADDzlMTv4MBF6lUcGR2vaY1ZbfNKww.

   cp cert.pem dKqTlYij_pSjvADDzlMTv4MBF6lUcGR2vaY1ZbfNKww

3. Upload the renamed certificate to your site. The particular directory that you upload the certificate to does not matter.

4. Open webpkgserver.toml with the editor of your choice and make the following changes:

   • Uncomment and change the line #CertURLBase = '/webpkg/cert' to match the deployed location of your certificate. For most people, this location will be similar to https://mysite.com/. If you get the error CertURLBase: must be set non-empty after starting webpkgserver, try adding a / at the end of the URL.
   • Uncomment and change #CertPath = '/webpkg/cert' to match the deployed location of your certificate. For example, if the certificate will be served from your root directory, change this value to /.

5. Restart webpkgserver.

   webpkgserver

6. Visit http://localhost:8080/priv/doc/https://example.com

The Network panel shows that the SXG and its certificate were loaded with no errors.

Serve signed exchanges using a CanSignHttpExchanges certificate #

The instructions in this section explain how to serve SXGs using a CanSignHttpExchanges certificate. Production use of SXGs requires a CanSignHttpExchanges certificate.

These instructions are fairly similar to those for serving SXGs with a self-signed certificate. For the sake of brevity, these instructions are written with the assumption that you understand the concepts discussed in the Setup Signed Exchanges using a self-signed certificate section.

Prerequisites #

You have a CanSignHttpExchanges certificate. This page lists the CAs that offer this type of certificate.

Generate a CanSignHttpExchanges certificate #

Prerequisites #

1. Install the gen-certurl tool.

   go get -v -u github.com/WICG/webpackage/go/signedexchange/cmd/gen-certurl

   gen-certurl is a tool that converts certificates to the certificate format used by signed exchanges.

2. Verify that gen-certurl has been installed correctly.

   gen-certurl --help

   This command should return information about its usage.

Instructions #

1. Follow the steps 1 through 3 of Generate a self-signed certificate.

2. Convert cert.pem to the application/cert-chain+cbor format.

   gen-certurl -pem cert.pem -ocsp <(echo ocsp) > cert.cbor

   Certificates come in many formats. cert.pem is in the PEM format. Certificates for signed exchanges must be in the application/cert-chain+cbor format.

   When using a self-signed certificate, webpkgserver automatically converts the certificate indicated by the PEMFile option in webpkgserver.toml to the application/cert-chain+cbor format. However, if you are using a CanSignHttpExchanges certificate, you must generate the CBOR-encoded certificate yourself.

Setup up Web Packager for production use #

1. Follow steps 1 and 2 of Setup the Web Packager Server for testing.

2. Open webpkgserver.toml with the editor of your choice and make the following changes:

   • Change the line PEMFile = cert.pem to PEMFile = cert.cbor.

Upload the CanSignHttpExchanges certificate #

1. Follow steps 1 through 4 of Upload the self-signed certificate to upload cert.cbor.

2. Adjust your server config to serve cert.cbor using the Content-Type: application/cert-chain+cbor response header.

   If this header is not set, you will see the following error when you inspect the SXG in DevTools: Content type of cert-url must be application/cert-chain+cbor. Actual content type: text/html.

3. Restart webpkgserver.

   webpkgserver

This content originally appeared on web.dev and was authored by Katie Hempenius

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