On 2/26/2021 2:56 PM, Morten Minde Neergaard via curl-library wrote:
I'm using libcurl in a project I'm doing, and I'd like to specify some
extra flags to the SCHANNEL_CRED struct to enhance security and remove
potential error sources:
SCH_USE_STRONG_CRYPTO:
Disables some older cipher suites.
SCH_CRED_NO_DEFAULT_CREDS
Found a TODO about this flag at
https://curl.haxx.se/docs/todo.html#Add_option_to_disable_client_cer
I'm hoping to avoid forking curl to set the flags, and was basically
wondering how it would make sense to implement this.
The first thing that came to mind would be to add an option
CURLOPT_SSL_BACKEND_FLAGS where each backend could use these flags as
desired. The implementation-specific part of the patch would be like
this for SChannel:
--- a/lib/vtls/schannel.c
+++ b/lib/vtls/schannel.c
@@ -557,6 +557,8 @@ schannel_connect_step1(struct Curl_easy *data, struct
connectdata *conn,
"names in server certificates.\n"));
}
+ schannel_cred.dwFlags |= SSL_CONN_CONFIG(backend_flags);
+
switch(conn->ssl_config.version) {
case CURL_SSLVERSION_DEFAULT:
case CURL_SSLVERSION_TLSv1:
Now, I see that this isn't particularly pretty. Is such a patch likely
to be merged, and if not does anyone have a better way of solving this?
I've proposed two PRs to address the auto credentials issue. One would
leave auto credentials as the default and add an option to disable it
[1], and the other would disable auto credentials as the default
(breaking change) and add an option to enable it [2]. Please take any
discussion about it to the latter PR.
Regarding strong ciphers, CURLOPT_SSL_CIPHER_LIST [3] (--ciphers for the
curl tool [4]) can be used with Schannel to set some algorithms but
unlike other SSL backends it's relatively limited without ciphersuite
support or umbrella terms like "USE_STRONG_CRYPTO". We would consider a
patch for that to signal strong crypto.
[1]: https://github.com/curl/curl/pull/6672
[2]: https://github.com/curl/curl/pull/6673
[3]: https://curl.se/libcurl/c/CURLOPT_SSL_CIPHER_LIST.html
[4]: https://curl.se/docs/manpage.html#--ciphers
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