Proposal: downloads.getFileHash()

proposals/downloads_get_file_hash.md

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+# Proposal: `downloads.getFileHash()`
+
+**Summary**
+
+Allow extensions to query hashes of contents of downloaded files
+programmatically.
+
+**Document Metadata**
+
+**Author:** @bershanskyi
+
+**Sponsoring Browser:** Chromium
+
+**Contributors:** @Rob--W
+
+**Created:** 2024-04-08
+
+**Related Issues:** [WECG GitHub issue 401](https://github.com/w3c/webextensions/issues/401)
+
+## Motivation
+
+### Objective
+
+This API will enable extensions to privately and efficiently scan user
+downloads for "Known Unsafe" contents and validate the integrity of "Known Safe"
+files without exposing the raw content of the files.
+
+#### Use Cases
+
+##### Filtering for unwanted downloads
+
+Many antivirus products detect previously seen "safe" and "unsafe" files by
+computing a file content hash and checking it against a set of known hashes.
+These checks require minimal effort on the client side, yet are powerful
+enough to limit the spread of known malware downloads since effectively every
+client gets "inoculated" against a particular download as soon as a single
+antivirus detects a malicious file.
+User-Agents may store file hashes even after the original file is deleted to
+facilitate tracing of malware exposure even if the file is deemed to be
+dangerous after the user initially downloads it.
+
+##### Download integrity assurance
+
+Files can get damaged while they are shared on a file hosting server or NAS.
+While some file formats explicitly support integrity checks like embedded
+checksums or even cryptographic signatures, other files do not. This API
+would allow file managers or extensions to programmatically check the
+integrity of downloaded files if the file source elects to provide them.
+
+### Known Consumers
+
+None at the moment.
+
+## Specification
+
+### Schema
+
+Schema in Chromium-style IDL:
+```idl
+namespace downloads {
+    callback GetFileHashCallback = void(DOMString hash);
+
+    [supportsPromises, permissions="downloads.getFileHash"]
+    static void getFileHash(
+        long downloadId,
+        [legalValues=("SHA-256")] DOMString algorithm,
+        GetFileHashCallback callback);
+}
+```
+
+Schema in TypeScript:
+```ts
+namespace downloads {
+  enum FileHashAlgorithm {
+    SHA256 = "SHA-256"
+  }
+
+  getFileHash: (
+    downloadId: number,
+    algorithm?: keyof downloads.FileHashAlgorithm)
+      => Promise<string>;
+}
+```
+
+### New Permissions
+
+No new permission. `downloads.getFileHash()` is honored only if extension has
+`"downloads"` permission and the host access for `DownloadItem.finalUrl`.
+
+### Manifest File Changes
+
+No changes besides the new optional permission `"downloads.getFileHash"`.
+
+## Security and Privacy
+
+### Exposed Sensitive Data
+
+Hashes (cryptographic) are considered one-way operations, but they can easily
+be reversed to the original input when the number of possible inputs is
+sufficiently small. So extension may be able to infer the raw contents of the
+file if the possible space of file contents is small.
+A malicious extension could:
+ - Observe hashes of downloaded files and send them to a home server
+ - Initiate a download with an arbitrary URL and obtain a hash of the response
+ - Initiate a download for a ranged HTTP request and obtain a hash of the
+   corresponding portion of the response
+
+### Abuse Mitigations
+
+The API method requires host access to the`DownloadItem.finalUrl`.
+
+### Additional Security Considerations
+
+User Agents must not provide obsolete hash algorithms (like MD5 and SHA-1).
+User Agents must expose only hashes of complete files and never provide
+partial hash results while the file is being downloaded. This is necessary to
+prevent malicious or compromised extensions from calculating incremental hashes
+and constructing a "hash oracle" to infer file contents one byte (or just a few
+bytes) at a time.
+
+## Alternatives
+
+### Existing Workarounds
+
+As of writing, extensions can calculate the digest of a download by
+implementing a native host component that would read the desired file from the
+disc directly. This approach is a significant elevation of privileges (requires
+unsandboxed application), largely expands the complexity of implementing such
+an extension (since the extension has to listen for download completion, notify
+the native component, and ensure that the file does not change or move before
+the hash is calculated) and raises friction for users (due to the complexity of
+installation). Additionally, calculating file content hash separately is
+slightly less efficient since it requires reading it back into memory.
+
+### Open Web API
+
+There are no equivalent APIs on the Open Web. Open Web has the concept of
+subresource integrity, but it applies only to page resources like images and
+other media and does not apply to downloaded files.
+
+## Implementation Notes
+
+ 1. Hash algorithm descriptor matching must be case-sensitive (which differs
+    from some other Open Web APIs).
+ 2. User Agent may throw an error upon a call to `downloads.getFileHash()`
+    if the required hash is not available. User Agent may compute the hash on
+    the fly if the original file is still available.
+ 3. User Agents must not support obsolete hash algorithms, see "Additional
+    Security Considerations" for more details.
+ 4. User Agents must encode (binary) hash digest into a hex string, using
+    lower-case letters.
+
+## Future Work
+
+In theory, some use cases could be served via a declarative API. For example,
+an antivirus extension could provide a set of hashes of known bad content for
+the browser to use directly. However, the development of such a schema is
+outside of the scope of this proposal.