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Add a skeleton README-PROVIDERS file

The current content of this README file are just meant to be a
starting point and an incentive to add more. Most of the text
was borrowed from the [OpenSSL 3.0 Wiki], which is the reason
why a added Matt as co-author. To be continued...

[OpenSSL 3.0 Wiki]: https://wiki.openssl.org/index.php/OpenSSL_3.0

Co-authored-by: Matt Caswell <matt@openssl.org>

Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14042)
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Dr. Matthias St. Pierre 1 year ago
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Providers
=========
- [Standard Providers](#standard-providers)
- [The Default Provider](#the-default-provider)
- [The Legacy Provider](#the-legacy-provider)
- [The FIPS Provider](#the-fips-provider)
- [The Base Provider](#the-base-provider)
- [The Null Provider](#the-null-provider)
- [Loading Providers](#loading-providers)
Standard Providers
==================
Providers are containers for algorithm implementations. Whenever a cryptographic
algorithm is used via the high level APIs a provider is selected. It is that
provider implementation that actually does the required work. There are five
providers distributed with OpenSSL. In the future we expect third parties to
distribute their own providers which can be added to OpenSSL dynamically.
Documentation about writing providers is available on the [provider(7)]
manual page.
[provider(7)]: https://www.openssl.org/docs/manmaster/man7/provider.html
The Default Provider
--------------------
The default provider collects together all of the standard built-in OpenSSL
algorithm implementations. If an application doesn't specify anything else
explicitly (e.g. in the application or via config), then this is the provider
that will be used. It is loaded automatically the first time that we try to
get an algorithm from a provider if no other provider has been loaded yet.
If another provider has already been loaded then it won't be loaded
automatically. Therefore if you want to use it in conjunction with other
providers then you must load it explicitly.
This is a "built-in" provider which means that it is compiled and linked
into the libcrypto library and does not exist as a separate standalone module.
The Legacy Provider
-------------------
The legacy provider is a collection of legacy algorithms that are either no
longer in common use or considered insecure and strongly discouraged from use.
However, some applications may need to use these algorithms for backwards
compatibility reasons. This provider is **not** loaded by default.
This may mean that some applications upgrading from earlier versions of OpenSSL
may find that some algorithms are no longer available unless they load the
legacy provider explicitly.
Algorithms in the legacy provider include MD2, MD4, MDC2, RMD160, CAST5,
BF (Blowfish), IDEA, SEED, RC2, RC4, RC5 and DES (but not 3DES).
The FIPS Provider
-----------------
The FIPS provider contains a sub-set of the algorithm implementations available
from the default provider, consisting of algorithms conforming to FIPS standards.
It is intended that this provider will be FIPS140-2 validated.
In some cases there may be minor behavioural differences between algorithm
implementations in this provider compared to the equivalent algorithm in the
default provider. This is typically in order to conform to FIPS standards.
The Base Provider
-----------------
The base provider contains a small sub-set of non-cryptographic algorithms
available in the default provider. For example, it contains algorithms to
serialize and deserialize keys to files. If you do not load the default
provider then you should always load this one instead (in particular, if
you are using the FIPS provider).
The Null Provider
-----------------
The null provider is "built-in" to libcrypto and contains no algorithm
implementations. In order to guarantee that the default provider is not
automatically loaded, the null provider can be loaded instead.
This can be useful if you are using non-default library contexts and want
to ensure that the default library context is never used unintentionally.
Loading Providers
=================
Providers to be loaded can be specified in the OpenSSL config file.
See the [config(5)] manual page for information about how to configure
providers via the config file, and how to automatically activate them.
[config(5)]: https://www.openssl.org/docs/manmaster/man5/config.html
The following is a minimal config file example to load and activate both
the legacy and the default provider in the default library context.
openssl_conf = openssl_init
[openssl_init]
providers = provider_sect
[provider_sect]
default = default_sect
legacy = legacy_sect
[default_sect]
activate = 1
[legacy_sect]
activate = 1
It is also possible to load providers programmatically. For example you can
load the legacy provider into the default library context as shown below.
Note that once you have explicitly loaded a provider into the library context
the default provider will no longer be automatically loaded. Therefore you will
often also want to explicitly load the default provider, as is done here:
#include <stdio.h>
#include <stdlib.h>
#include <openssl/provider.h>
int main(void)
{
OSSL_PROVIDER *legacy;
OSSL_PROVIDER *deflt;
/* Load Multiple providers into the default (NULL) library context */
legacy = OSSL_PROVIDER_load(NULL, "legacy");
if (legacy == NULL) {
printf("Failed to load Legacy provider\n");
exit(EXIT_FAILURE);
}
deflt = OSSL_PROVIDER_load(NULL, "default");
if (deflt == NULL) {
printf("Failed to load Default provider\n");
OSSL_PROVIDER_unload(legacy);
exit(EXIT_FAILURE);
}
/* Rest of application */
OSSL_PROVIDER_unload(legacy);
OSSL_PROVIDER_unload(deflt);
exit(EXIT_SUCCESS);
}

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