The documentation for CURLOPT_SSLKEY contains the following note:
"(iOS and Mac OS X only) This option is ignored if curl was built
against Secure Transport. Secure Transport expects the private key to
be already present in the keychain or PKCS#12 file containing the
certificate."

From my experiments it seems that this is also true on Windows when
curl is built against schannel - if the certificate does not contain
the private key, attempting to AcquireCrerdentialsHandle fails with
SEC_E_NO_CREDENTIALS.

Does that agree with what others see and expect?

Happy to open issue/create a PR if it is.

-- 
Richard
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