Network Working Group J. Yasskin Internet-Draft Google Intended status: Informational 13 April 2021 Expires: 15 October 2021 Use Cases and Requirements for Web Packages draft-yasskin-wpack-use-cases-02 Abstract This document lists use cases for signing and/or bundling collections of web pages, and extracts a set of requirements from them. Discussion Venues This note is to be removed before publishing as an RFC. Discussion of this document takes place on the WPACK Working Group mailing list (wpack@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/wpack/. Source for this draft and an issue tracker can be found at https://github.com/WICG/webpackage. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 15 October 2021. Copyright Notice Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved. Yasskin Expires 15 October 2021 [Page 1] Internet-Draft Use Cases and Requirements for Web Packa April 2021 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Use cases . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1. Essential . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1.1. Offline installation . . . . . . . . . . . . . . . . 4 2.1.2. Offline browsing . . . . . . . . . . . . . . . . . . 6 2.1.3. Save and share a web page . . . . . . . . . . . . . . 6 2.1.4. Privacy-preserving prefetch . . . . . . . . . . . . . 7 2.2. Nice-to-have . . . . . . . . . . . . . . . . . . . . . . 7 2.2.1. Packaged Web Publications . . . . . . . . . . . . . . 8 2.2.2. Avoiding Censorship . . . . . . . . . . . . . . . . . 9 2.2.3. Third-party security review . . . . . . . . . . . . . 9 2.2.4. Building packages from multiple libraries . . . . . . 10 2.2.5. Cross-CDN Serving . . . . . . . . . . . . . . . . . . 10 2.2.6. Pre-installed applications . . . . . . . . . . . . . 11 2.2.7. Protecting Users from a Compromised Frontend . . . . 12 2.2.8. Installation from a self-extracting executable . . . 13 2.2.9. Packages in version control . . . . . . . . . . . . . 13 2.2.10. Subresource bundling . . . . . . . . . . . . . . . . 13 2.2.11. Archival . . . . . . . . . . . . . . . . . . . . . . 14 3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 15 3.1. Essential . . . . . . . . . . . . . . . . . . . . . . . . 15 3.1.1. Indexed by URL . . . . . . . . . . . . . . . . . . . 15 3.1.2. Request headers . . . . . . . . . . . . . . . . . . . 15 3.1.3. Response headers . . . . . . . . . . . . . . . . . . 15 3.1.4. Signing as an origin . . . . . . . . . . . . . . . . 15 3.1.5. Random access . . . . . . . . . . . . . . . . . . . . 16 3.1.6. Resources from multiple origins in a package . . . . 16 3.1.7. Cryptographic agility . . . . . . . . . . . . . . . . 16 3.1.8. Unsigned content . . . . . . . . . . . . . . . . . . 16 3.1.9. Certificate revocation . . . . . . . . . . . . . . . 16 3.1.10. Downgrade prevention . . . . . . . . . . . . . . . . 16 3.1.11. Metadata . . . . . . . . . . . . . . . . . . . . . . 17 3.1.12. Implementations are hard to get wrong . . . . . . . . 17 3.2. Nice to have . . . . . . . . . . . . . . . . . . . . . . 17 3.2.1. Streamed loading . . . . . . . . . . . . . . . . . . 17 3.2.2. Signing without origin trust . . . . . . . . . . . . 17 3.2.3. Additional signatures . . . . . . . . . . . . . . . . 17 Yasskin Expires 15 October 2021 [Page 2] Internet-Draft Use Cases and Requirements for Web Packa April 2021 3.2.4. Binary . . . . . . . . . . . . . . . . . . . . . . . 18 3.2.5. Deduplication of diamond dependencies . . . . . . . . 18 3.2.6. Old crypto can be removed . . . . . . . . . . . . . . 18 3.2.7. Compress transfers . . . . . . . . . . . . . . . . . 18 3.2.8. Compress stored packages . . . . . . . . . . . . . . 18 3.2.9. Subsetting and reordering . . . . . . . . . . . . . . 18 3.2.10. Packaged validity information . . . . . . . . . . . . 18 3.2.11. Signing uses existing TLS certificates . . . . . . . 18 3.2.12. External dependencies . . . . . . . . . . . . . . . . 19 3.2.13. Trailing length . . . . . . . . . . . . . . . . . . . 19 3.2.14. Time-shifting execution . . . . . . . . . . . . . . . 19 3.2.15. Service Worker integration . . . . . . . . . . . . . 19 4. Non-goals . . . . . . . . . . . . . . . . . . . . . . . . . . 19 4.1. Store confidential data . . . . . . . . . . . . . . . . . 19 4.2. Generate packages on the fly . . . . . . . . . . . . . . 20 4.3. Non-origin identity . . . . . . . . . . . . . . . . . . . 20 4.4. DRM . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 4.5. Ergonomic replacement for HTTP/2 PUSH . . . . . . . . . . 20 5. Security Considerations . . . . . . . . . . . . . . . . . . . 21 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21 7. Informative References . . . . . . . . . . . . . . . . . . . 21 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 23 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 23 1. Introduction People would like to use content offline and in other situations where there isn't a direct connection to the server where the content originates. However, it's difficult to distribute and verify the authenticity of applications and content without a connection to the network. The W3C has addressed running applications offline with Service Workers ([ServiceWorkers]), but not the problem of distribution. Previous attempts at packaging web resources (e.g. Resource Packages (https://www.mnot.net/blog/2010/02/18/resource_packages) and the W3C TAG's packaging proposal (https://w3ctag.github.io/packaging-on-the- web/)) were motivated by speeding up the download of resources from a single server, which is probably better achieved through other mechanisms like HTTP/2 PUSH, possibly augmented with a simple manifest of URLs a page plans to use (https://lists.w3.org/Archives/Public/public-web- perf/2015Jan/0038.html). This attempt is instead motivated by avoiding a connection to the origin server at all. It may still be useful for the earlier use cases, so they're still listed, but they're not primary. Yasskin Expires 15 October 2021 [Page 3] Internet-Draft Use Cases and Requirements for Web Packa April 2021 2. Use cases These use cases are in rough descending priority order. If use cases have conflicting requirements, the design should enable more important use cases. 2.1. Essential 2.1.1. Offline installation Alex can download a file containing a website (a PWA (https://developers.google.com/web/progressive-web-apps/checklist)) including a Service Worker from origin "O", and transmit it to their peer Bailey, and then Bailey can install the Service Worker with a proof that it came from "O". This saves Bailey the bandwidth costs of transferring the website. There are roughly two ways to accomplish this: 1. Package just the Service Worker Javascript and any other Javascript that it importScripts() (https://w3c.github.io/ ServiceWorker/#importscripts), with their URLs and enough metadata to synthesize a navigator.serviceWorker.register(scriptURL, options) call (https://w3c.github.io/ServiceWorker/#navigator-service-worker- register), along with an uninterpreted but signature-checked blob of data that the Service Worker can interpret to fill in its caches. 2. Package the resources so that the Service Worker can fetch() them to populate its cache. Associated requirements for just the Service Worker: * Indexed by URL: The "register()" and "importScripts()" calls have semantics that depend on the URL. * Signing as an origin: To prove that the file came from "O". * Signing uses existing TLS certificates: So "O" doesn't have to spend lots of money buying a specialized certificate. * Cryptographic agility: Today's algorithms will eventually be obsolete and will need to be replaced. * Certificate revocation: "O"'s certificate might be compromised or mis-issued, and the attacker shouldn't then get an infinite ability to mint packages. Yasskin Expires 15 October 2021 [Page 4] Internet-Draft Use Cases and Requirements for Web Packa April 2021 * Downgrade prevention: "O"'s site might have an XSS vulnerability, and attackers with an old signed package shouldn't be able to take advantage of the XSS forever. * Metadata: Just enough to generate the "register()" call, which is less than a full W3C Application Manifest. Additional associated requirements for packaged resources: * Indexed by URL: Resources on the web are addressed by URL. * Request headers: If Bailey's running a different browser from Alex or has a different language configured, the "accept*" headers are important for selecting which resource to use at each URL. * Response headers: The meaning of a resource is heavily influenced by its HTTP response headers. * Resources from multiple origins in a package: So the site can be built from multiple components (Section 2.2.4). * Metadata: The browser needs to know which resource within a package file to treat as its Service Worker and/or initial HTML page. 2.1.1.1. Online use Bailey may have an internet connection through which they can, in real time, fetch updates to the package they received from Alex. 2.1.1.2. Fully offline use Or Bailey may not have any internet connection a significant fraction of the time, either because they have no internet at all, because they turn off internet except when intentionally downloading content, or because they use up their plan partway through each month. Associated requirements beyond Offline installation: * Packaged validity information: Even without a direct internet connection, Bailey should be able to check that their package is still valid. Yasskin Expires 15 October 2021 [Page 5] Internet-Draft Use Cases and Requirements for Web Packa April 2021 2.1.2. Offline browsing Alex can download a file containing a large website (e.g. Wikipedia) from its origin, save it to transferrable storage (e.g. an SD card), and hand it to their peer Bailey. Then Bailey can browse the website with a proof that it came from "O". Bailey may not have the storage space to copy the website before browsing it. This use case is harder for publishers to support if we specialize Section 2.1.1 for Service Workers since it requires the publisher to adopt Service Workers before they can sign their site. Associated requirements beyond Offline installation: * Random access: To avoid needing a long linear scan before using the content. * Compress stored packages: So that more content can fit on the same storage device. 2.1.3. Save and share a web page Casey is viewing a web page and wants to save it either for offline use or to show it to their friend Dakota. Since Casey isn't the web page's publisher, they don't have the private key needed to sign the page. Browsers currently allow their users to save pages, but each browser uses a different format (MHTML, Web Archive, or files in a directory), so Dakota and Casey would need to be using the same browser. Casey could also take a screenshot, at the cost of losing links and accessibility. Associated requirements: * Unsigned content: A client can't sign content as another origin. * Resources from multiple origins in a package: General web pages include resources from multiple origins. * Indexed by URL: Resources on the web are addressed by URL. * Response headers: The meaning of a resource is heavily influenced by its HTTP response headers. Yasskin Expires 15 October 2021 [Page 6] Internet-Draft Use Cases and Requirements for Web Packa April 2021 2.1.4. Privacy-preserving prefetch Lots of websites link to other websites. Many of these source sites would like the targets of these links to load quickly. The source could use "" to prefetch the target of a link, but if the user doesn't actually click that link, that leaks the fact that the user saw a page that linked to the target. This can be true even if the prefetch is made without browser credentials because of mechanisms like TLS session IDs. Because clients have limited data budgets to prefetch link targets, this use case is probably limited to sites that can accurately predict which link their users are most likely to click. For example, search engines can predict that their users will click one of the first couple results, and news aggreggation sites like Reddit or Slashdot can hope that users will read the article if they've navigated to its discussion. Two search engines have built systems to do this with today's technology: Google's AMP (https://www.ampproject.org/) and Baidu's MIP (https://www.mipengine.org/) formats and caches allow them to prefetch search results while preserving privacy, at the cost of showing the wrong URLs for the results once the user has clicked. A good solution to this problem would show the right URLs but still avoid a request to the publishing origin until after the user clicks. Associated requirements: * Signing as an origin: To prove the content came from the original origin. * Streamed loading: If the user clicks before the target page is fully transferred, the browser should be able to start loading early parts before the source site finishes sending the whole page. * Compress transfers * Subsetting and reordering: If a prefetched page includes subresources, its publisher might want to provide and sign both WebP and PNG versions of an image, but the source site should be able to transfer only best one for each client. 2.2. Nice-to-have Yasskin Expires 15 October 2021 [Page 7] Internet-Draft Use Cases and Requirements for Web Packa April 2021 2.2.1. Packaged Web Publications The W3C's Publishing Working Group (https://www.w3.org/publishing/groups/publ-wg/), merged from the International Digital Publishing Forum (IDPF) and in charge of EPUB maintenance, wants to be able to create publications on the web and then let them be copied to different servers or to other users via arbitrary protocols. See their Packaged Web Publications use cases (https://www.w3.org/TR/pwp-ucr/#pwp) for more details. Associated requirements: * Indexed by URL: Resources on the web are addressed by URL. * Signing as an origin: So that readers can be sure their copy is authentic and so that copying the package preserves the URLs of the content inside it. * Downgrade prevention: An early version of a publication might contain incorrect content, and a publisher should be able to update that without worrying that an attacker can still show the old content to users. * Metadata: A publication can have copyright and licensing concerns; a title, author, and cover image; an ISBN or DOI name; etc.; which should be included when that publication is packaged. Other requirements are similar to those from Offline installation: * Random access: To avoid needing a long linear scan before using the content. * Compress stored packages: So that more content can fit on the same storage device. * Request headers: If different users' browsers have different capabilities or preferences, the "accept*" headers are important for selecting which resource to use at each URL. * Response headers: The meaning of a resource is heavily influenced by its HTTP response headers. * Signing uses existing TLS certificates: So a publisher doesn't have to spend lots of money buying a specialized certificate. * Cryptographic agility: Today's algorithms will eventually be obsolete and will need to be replaced. Yasskin Expires 15 October 2021 [Page 8] Internet-Draft Use Cases and Requirements for Web Packa April 2021 * Certificate revocation: The publisher's certificate might be compromised or mis-issued, and an attacker shouldn't then get an infinite ability to mint packages. 2.2.2. Avoiding Censorship Some users want to retrieve resources that their governments or network providers don't want them to see. Right now, it's straightforward for someone in a privileged network position to block access to particular hosts, but TLS makes it difficult to block access to particular resources on those hosts. Today it's straightforward to retrieve blocked content from a third party, but there's no guarantee that the third-party has sent the user an accurate representation of the content: the user has to trust the third party. With signed web packages, the user can re-gain assurance that the content is authentic, while still bypassing the censorship. Packages don't do anything to help discover this content. Systems that make censorship more difficult can also make legitimate content filtering more difficult. Because the client that processes a web package always knows the true URL, this forces content filtering to happen on the client instead of on the network. Associated requirements: * Indexed by URL: So the user can see that they're getting the content they expected. * Signing as an origin: So that readers can be sure their copy is authentic and so that copying the package preserves the URLs of the content inside it. 2.2.3. Third-party security review Some users may want to grant certain permissions only to applications that have been reviewed for security by a trusted third party. These third parties could provide guarantees similar to those provided by the iOS, Android, or Chrome OS app stores, which might allow browsers to offer more powerful capabilities than have been deemed safe for unaudited websites. Binary transparency for websites is similar: like with Certificate Transparency [RFC6962], the transparency logs would sign the content of the package to provide assurance that experts had a chance to audit the exact package a client received. Yasskin Expires 15 October 2021 [Page 9] Internet-Draft Use Cases and Requirements for Web Packa April 2021 Associated requirements: * Additional signatures 2.2.4. Building packages from multiple libraries Large programs are built from smaller components. In the case of the web, components can be included either as Javascript files or as "