[SRI Message Signatures] Enforce signature matching.

This patch teaches the network service's `URLLoader` how to evaluate the
SRI-valid subset of HTTP Message Signatures, blocking mismatched
responses once headers are received and processed.

This check is implemented behind a new feature flag, which is disabled
by default. End-to-end tests live in web platform tests under
//web_tests/virtual/sri-message-signatures that enables the flag.

This is part of a chain of CLs implementing this feature (#2 from
https://wicg.github.io/signature-based-sri/#overview):

1.  [Parsing] https://crrev.com/c/6020612
2.  [Validation 1] https://crrev.com/c/6030571
3.  [Validation 2] https://crrev.com/c/6032589
4.  [Enforcement] https://crrev.com/c/6038714 [You are here]

    `url_loader.cc` are the only meaningful changes in behavior
    reported as undercovered. These are tested through the WPT
    included in this CL.

Bug: 379534943
Low-Coverage-Reason: COVERAGE_UNDERREPORTED The changes to
Change-Id: I6ece80da25ed4329a6f976c2c74c639c2799b856
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6038714
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: Camille Lamy <clamy@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1389294}

subresource-integrity/signatures/tentative/fetch.any.js

@@ -0,0 +1,51 @@
+// META: global=window,dedicatedworker,sharedworker
+
+// Given `{ digest: "...", body: "...", cors: true, type: "..." }`:
+function resourceURL(data) {
+  let params = new URLSearchParams(data);
+  return "./resource.py?" + params.toString();
+}
+
+// HTTP/1.1 200 OK
+// Date: Tue, 20 Apr 2021 02:07:56 GMT
+// Content-Type: application/json
+// Identity-Digest: sha-256=:X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=:
+// Content-Length: 18
+// Signature-Input:
+// signature=("identity-digest";sf);alg="ed25519";keyid="JrQLj5P/89iXES9+vFgrI \
+//           y29clF9CC/oPPsw3c5D0bs=";tag="sri"
+// Signature: signature=:H7AqWWgo1DJ7VdyF9DKotG/4hvatKDfRTq2mpuY/hvJupSn+EYzus \
+//            5p24qPK7DtVQcxJFhzSYDj4RBq9grZTAQ==:
+//
+// {"hello": "world"}
+//
+promise_test(test => {
+  const data = {
+    body: `{"hello": "world"}`
+  };
+  return fetch(resourceURL(data)).then(r => {
+    assert_equals(r.status, 200, "Response status is 200.");
+  });
+}, "No signature: loads.");
+
+promise_test(test => {
+  const data = {
+    body: `{"hello": "world"}`,
+    digest: `sha-256=:X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=:`,
+    signature: `signature=:H7AqWWgo1DJ7VdyF9DKotG/4hvatKDfRTq2mpuY/hvJupSn+EYzus5p24qPK7DtVQcxJFhzSYDj4RBq9grZTAQ==:`,
+    signatureInput: `signature=("identity-digest";sf);alg="ed25519";keyid="JrQLj5P/89iXES9+vFgrIy29clF9CC/oPPsw3c5D0bs=";tag="sri"`
+  };
+  return fetch(resourceURL(data)).then(r => {
+    assert_equals(r.status, 200, "Response status is 200.");
+  });
+}, "Valid signature: loads.");
+
+promise_test(test => {
+  const data = {
+    body: `{"hello": "world"}`,
+    digest: `sha-256=:X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=:`,
+    signature: `signature=:AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA==:`,
+    signatureInput: `signature=("identity-digest";sf);alg="ed25519";keyid="JrQLj5P/89iXES9+vFgrIy29clF9CC/oPPsw3c5D0bs=";tag="sri"`
+  };
+  return promise_rejects_js(test, TypeError, fetch(resourceURL(data)));
+}, "Non-matching signature: blocked.");

subresource-integrity/signatures/tentative/resource.py

@@ -0,0 +1,38 @@
+'''
+SRI Message Signature helper, generating responses that:
+
+*   Include or exclude an `Integrity-Digest` header depending on the request's
+    `digest` parameter.
+*   Include or exclude an `Signature` header depending on the request's
+    `signature` parameter.
+*   Include or exclude an `Signature-Input` header depending on the request's
+    `signatureInput` parameter.
+
+*   Include or exclude `Access-Control-Allow-Origin: *` depending on the
+    request's `cors` parameter.
+
+*   Sets a `Content-Type` header from the request's `type` parameter.
+
+*   Echos the `body` parameter into the response body.
+'''
+def main(request, response):
+  digest = request.GET.first(b'digest', b'')
+  signature = request.GET.first(b'signature', b'')
+  signatureInput = request.GET.first(b'signatureInput', b'')
+  if digest:
+    response.headers.set(b'identity-digest', digest)
+  if signature:
+    response.headers.set(b'signature', signature)
+  if signatureInput:
+    response.headers.set(b'signature-input', signatureInput)
+
+
+  cors = request.GET.first(b'cors', '')
+  if cors:
+    response.headers.set(b'access-control-allow-origin', b'*')
+
+  response.headers.set(b'content-type',
+                       request.GET.first(b'type', b'text/plain'))
+
+  response.status_code = 200
+  response.content = request.GET.first(b'body', '')