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+SSL Certificate Verification
+============================
+
+SSL is TLS
+----------
+
+SSL is the old name. It is called TLS these days.
+
+
+Native SSL
+----------
+
+If libcurl was built with Schannel or Secure Transport support (the native SSL
+libraries included in Windows and Mac OS X), then this does not apply to
+you. Scroll down for details on how the OS-native engines handle SSL
+certificates. If you're not sure, then run "curl -V" and read the results. If
+the version string says "WinSSL" in it, then it was built with Schannel
+support.
+
+It is about trust
+-----------------
+
+This system is about trust. In your local CA certificate store you have certs
+from *trusted* Certificate Authorities that you then can use to verify that the
+server certificates you see are valid. They're signed by one of the CAs you
+trust.
+
+Which CAs do you trust? You can decide to trust the same set of companies your
+operating system trusts, or the set one of the known browsers trust. That's
+basically trust via someone else you trust. You should just be aware that
+modern operating systems and browsers are setup to trust *hundreds* of
+companies and recent years several such CAs have been found untrustworthy.
+
+Certificate Verification
+------------------------
+
+libcurl performs peer SSL certificate verification by default.  This is done
+by using a CA certificate store that the SSL library can use to make sure the
+peer's server certificate is valid.
+
+If you communicate with HTTPS, FTPS or other TLS-using servers using
+certificates that are signed by CAs present in the store, you can be sure
+that the remote server really is the one it claims to be.
+
+If the remote server uses a self-signed certificate, if you don't install a CA
+cert store, if the server uses a certificate signed by a CA that isn't
+included in the store you use or if the remote host is an impostor
+impersonating your favorite site, and you want to transfer files from this
+server, do one of the following:
+
+ 1. Tell libcurl to *not* verify the peer. With libcurl you disable this with
+    `curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, FALSE);`
+
+    With the curl command line tool, you disable this with -k/--insecure.
+
+ 2. Get a CA certificate that can verify the remote server and use the proper
+    option to point out this CA cert for verification when connecting. For
+    libcurl hackers: `curl_easy_setopt(curl, CURLOPT_CAPATH, capath);`
+
+    With the curl command line tool: --cacert [file]
+
+ 3. Add the CA cert for your server to the existing default CA certificate
+    store. The default CA certificate store can changed at compile time with the
+    following configure options:
+
+    --with-ca-bundle=FILE: use the specified file as CA certificate store. CA
+    certificates need to be concatenated in PEM format into this file.
+
+    --with-ca-path=PATH: use the specified path as CA certificate store. CA
+    certificates need to be stored as individual PEM files in this directory.
+    You may need to run c_rehash after adding files there.
+
+    If neither of the two options is specified, configure will try to auto-detect
+    a setting. It's also possible to explicitly not hardcode any default store
+    but rely on the built in default the crypto library may provide instead.
+    You can achieve that by passing both --without-ca-bundle and
+    --without-ca-path to the configure script.
+
+    If you use Internet Explorer, this is one way to get extract the CA cert
+    for a particular server:
+
+     - View the certificate by double-clicking the padlock
+     - Find out where the CA certificate is kept (Certificate>
+       Authority Information Access>URL)
+     - Get a copy of the crt file using curl
+     - Convert it from crt to PEM using the openssl tool:
+       openssl x509 -inform DES -in yourdownloaded.crt \
+       -out outcert.pem -text
+     - Add the 'outcert.pem' to the CA certificate store or use it stand-alone
+       as described below.
+
+    If you use the 'openssl' tool, this is one way to get extract the CA cert
+    for a particular server:
+
+     - `openssl s_client -showcerts -servername server -connect server:443 > cacert.pem`
+     - type "quit", followed by the "ENTER" key
+     - The certificate will have "BEGIN CERTIFICATE" and "END CERTIFICATE"
+       markers.
+     - If you want to see the data in the certificate, you can do: "openssl
+       x509 -inform PEM -in certfile -text -out certdata" where certfile is
+       the cert you extracted from logfile. Look in certdata.
+     - If you want to trust the certificate, you can add it to your CA
+       certificate store or use it stand-alone as described. Just remember that
+       the security is no better than the way you obtained the certificate.
+
+ 4. If you're using the curl command line tool, you can specify your own CA
+    cert path by setting the environment variable `CURL_CA_BUNDLE` to the path
+    of your choice.
+
+    If you're using the curl command line tool on Windows, curl will search
+    for a CA cert file named "curl-ca-bundle.crt" in these directories and in
+    this order:
+      1. application's directory
+      2. current working directory
+      3. Windows System directory (e.g. C:\windows\system32)
+      4. Windows Directory (e.g. C:\windows)
+      5. all directories along %PATH%
+
+ 5. Get a better/different/newer CA cert bundle! One option is to extract the
+    one a recent Firefox browser uses by running 'make ca-bundle' in the curl
+    build tree root, or possibly download a version that was generated this
+    way for you: [CA Extract](https://curl.haxx.se/docs/caextract.html)
+
+Neglecting to use one of the above methods when dealing with a server using a
+certificate that isn't signed by one of the certificates in the installed CA
+certificate store, will cause SSL to report an error ("certificate verify
+failed") during the handshake and SSL will then refuse further communication
+with that server.
+
+Certificate Verification with NSS
+---------------------------------
+
+If libcurl was built with NSS support, then depending on the OS distribution,
+it is probably required to take some additional steps to use the system-wide
+CA cert db. RedHat ships with an additional module, libnsspem.so, which
+enables NSS to read the OpenSSL PEM CA bundle. On openSUSE you can install
+p11-kit-nss-trust which makes NSS use the system wide CA certificate store. NSS
+also has a new [database format](https://wiki.mozilla.org/NSS_Shared_DB).
+
+Starting with version 7.19.7, libcurl automatically adds the 'sql:' prefix to
+the certdb directory (either the hardcoded default /etc/pki/nssdb or the
+directory configured with SSL_DIR environment variable). To check which certdb
+format your distribution provides, examine the default certdb location:
+/etc/pki/nssdb; the new certdb format can be identified by the filenames
+cert9.db, key4.db, pkcs11.txt; filenames of older versions are cert8.db,
+key3.db, secmod.db.
+
+Certificate Verification with Schannel and Secure Transport
+-----------------------------------------------------------
+
+If libcurl was built with Schannel (Microsoft's native TLS engine) or Secure
+Transport (Apple's native TLS engine) support, then libcurl will still perform
+peer certificate verification, but instead of using a CA cert bundle, it will
+use the certificates that are built into the OS. These are the same
+certificates that appear in the Internet Options control panel (under Windows)
+or Keychain Access application (under OS X). Any custom security rules for
+certificates will be honored.
+
+Schannel will run CRL checks on certificates unless peer verification is
+disabled. Secure Transport on iOS will run OCSP checks on certificates unless
+peer verification is disabled. Secure Transport on OS X will run either OCSP
+or CRL checks on certificates if those features are enabled, and this behavior
+can be adjusted in the preferences of Keychain Access.
+
+HTTPS proxy
+-----------
+
+Since version 7.52.0, curl can do HTTPS to the proxy separately from the
+connection to the server. This TLS connection is handled separately from the
+server connection so instead of `--insecure` and `--cacert` to control the
+certificate verification, you use `--proxy-insecure` and `--proxy-cacert`.
+With these options, you make sure that the TLS connection and the trust of the
+proxy can be kept totally separate from the TLS connection to the server.