docs/SSLCERTS.md - platform/external/curl - Git at Google

 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 `Schannel` 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_CAINFO, cacert);`

     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 be 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 file 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.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.
	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 `Schannel` 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_CAINFO, cacert);`

	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 be 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 file 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.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.