schannel: add "best effort" revocation check option

- Implement new option CURLSSLOPT_REVOKE_BEST_EFFORT and
  --ssl-revoke-best-effort to allow a "best effort" revocation check.

A best effort revocation check ignores errors that the revocation check
was unable to take place. The reasoning is described in detail below and
discussed further in the PR.

---

When running e.g. with Fiddler, the schannel backend fails with an
unhelpful error message:

	Unknown error (0x80092012) - The revocation function was unable
	to check revocation for the certificate.

Sadly, many enterprise users who are stuck behind MITM proxies suffer
the very same problem.

This has been discussed in plenty of issues:
https://github.com/curl/curl/issues/3727,
https://github.com/curl/curl/issues/264, for example.

In the latter, a Microsoft Edge developer even made the case that the
common behavior is to ignore issues when a certificate has no recorded
distribution point for revocation lists, or when the server is offline.
This is also known as "best effort" strategy and addresses the Fiddler
issue.

Unfortunately, this strategy was not chosen as the default for schannel
(and is therefore a backend-specific behavior: OpenSSL seems to happily
ignore the offline servers and missing distribution points).

To maintain backward-compatibility, we therefore add a new flag
(`CURLSSLOPT_REVOKE_BEST_EFFORT`) and a new option
(`--ssl-revoke-best-effort`) to select the new behavior.

Due to the many related issues Git for Windows and GitHub Desktop, the
plan is to make this behavior the default in these software packages.

The test 2070 was added to verify this behavior, adapted from 310.

Based-on-work-by: georgeok <giorgos.n.oikonomou@gmail.com>
Co-authored-by: Markus Olsson <j.markus.olsson@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Closes https://github.com/curl/curl/pull/4981

2 files changed, 63 insertions, 0 deletions

diff --git a/tests/data/Makefile.inc b/tests/data/Makefile.inc
index 0e606e560..8451bb011 100644
--- a/tests/data/Makefile.inc
+++ b/tests/data/Makefile.inc
@@ -207,6 +207,7 @@ test2040 test2041 test2042 test2043 test2044 test2045 test2046 test2047 \
 test2048 test2049 test2050 test2051 test2052 test2053 test2054 test2055 \
 test2056 test2057 test2058 test2059 test2060 test2061 test2062 test2063 \
 test2064 test2065 test2066 test2067 test2068 test2069 \
+test2064 test2065 test2066 test2067 test2068 test2069 test2070 \
          test2071 test2072 test2073 test2074 test2075 test2076 test2077 \
 test2078 \
 test2080 \
diff --git a/tests/data/test2070 b/tests/data/test2070
new file mode 100644
index 000000000..4a21512ba
--- /dev/null
+++ b/tests/data/test2070
@@ -0,0 +1,62 @@
+<testcase>
+<info>
+<keywords>
+HTTPS
+HTTP GET
+PEM certificate
+</keywords>
+</info>type
+
+#
+# Server-side
+<reply>
+<data>
+HTTP/1.1 200 OK
+Date: Thu, 09 Nov 2010 14:49:00 GMT
+Server: test-server/fake
+Content-Length: 7
+
+MooMoo
+</data>
+</reply>
+
+#
+# Client-side
+<client>
+<features>
+WinSSL
+!MinGW
+</features>
+<server>
+https Server-localhost-sv.pem
+</server>
+ <name>
+Ignore certificate revocation "best effort" strategy
+ </name>
+  <setenv>
+# This test is pointless if we're not using the schannel backend
+CURL_SSL_BACKEND=schannel
+ </setenv>
+ <command>
+--cacert %SRCDIR/certs/EdelCurlRoot-ca.crt --ssl-revoke-best-effort https://localhost:%HTTPSPORT/2070
+</command>
+# Ensure that we're running on localhost because we're checking the host name
+<precheck>
+perl -e "print 'Test requires default test server host' if ( '%HOSTIP' ne '127.0.0.1' );"
+</precheck>
+</client>
+
+#
+# Verify data after the test has been "shot"
+<verify>
+<strip>
+^User-Agent:.*
+</strip>
+<protocol>
+GET /2070 HTTP/1.1

+Host: localhost:%HTTPSPORT

+Accept: */*

+

+</protocol>
+</verify>
+</testcase>