=pod

=head1 NAME

provider - OpenSSL operation implementation providers

=head1 SYNOPSIS

=for openssl generic

#include <openssl/provider.h>

=head1 DESCRIPTION

=head2 General

A I<provider>, in OpenSSL terms, is a unit of code that provides one
or more implementations for various operations for diverse algorithms
that one might want to perform.

An I<operation> is something one wants to do, such as encryption and
decryption, key derivation, MAC calculation, signing and verification,
etc.

An I<algorithm> is a named method to perform an operation.
Very often, the algorithms revolve around cryptographic operations,
but may also revolve around other types of operation, such as managing
certain types of objects.

=head2 Provider

I<NOTE: This section is mostly interesting for provider authors.>

A I<provider> offers an initialization function, as a set of base
functions in the form of an B<OSSL_DISPATCH> array, and by extension,
a set of B<OSSL_ALGORITHM>s (see L<openssl-core.h(7)>).
It may be a dynamically loadable module, or may be built-in, in
OpenSSL libraries or in the application.
If it's a dynamically loadable module, the initialization function
must be named C<OSSL_provider_init> and must be exported.
If it's built-in, the initialization function may have any name.

The initialization function must have the following signature:

 int NAME(const OSSL_PROVIDER *provider,
          const OSSL_DISPATCH *in, const OSSL_DISPATCH **out,
          void **provctx);

I<provider> is the OpenSSL library object for the provider, and works
as a handle for everything the OpenSSL libraries need to know about
the provider.
For the provider itself, it may hold some interesting information,
and is also passed to some of the functions given in the dispatch
array I<in>.

I<in> is a dispatch array of base functions offered by the OpenSSL
libraries, and the available functions are further described in
L<provider-base(7)>.

I<*out> must be assigned a dispatch array of base functions that the
provider offers to the OpenSSL libraries.
The functions that may be offered are further described in
L<provider-base(7)>, and they are the central means of communication
between the OpenSSL libraries and the provider.

I<*provctx> should be assigned a provider specific context to allow
the provider multiple simultaneous uses.
This pointer will be passed to various operation functions offered by
the provider.

One of the functions the provider offers to the OpenSSL libraries is
the central mechanism for the OpenSSL libraries to get access to
operation implementations for diverse algorithms.
Its referred to with the number B<OSSL_FUNC_PROVIDER_QUERY_OPERATION>
and has the following signature:

 const OSSL_ALGORITHM *provider_query_operation(void *provctx,
                                                int operation_id,
                                                const int *no_store);

I<provctx> is the provider specific context that was passed back by
the initialization function.

I<operation_id> is an operation identity (see L</Operations> below).

I<no_store> is a flag back to the OpenSSL libraries which, when
nonzero, signifies that the OpenSSL libraries will not store a
reference to the returned data in their internal store of
implementations.

The returned B<OSSL_ALGORITHM> is the foundation of any OpenSSL
library API that uses providers for their implementation, most
commonly in the I<fetching> type of functions
(see L</Fetching algorithms> below).

=head2 Operations

I<NOTE: This section is mostly interesting for provider authors.>

Operations are referred to with numbers, via macros with names
starting with C<OSSL_OP_>.

With each operation comes a set of defined function types that a
provider may or may not offer, depending on its needs.

Currently available operations are:

=over 4

=item Digests

In the OpenSSL libraries, the corresponding method object is
B<EVP_MD>.
The number for this operation is B<OSSL_OP_DIGEST>.
The functions the provider can offer are described in
L<provider-digest(7)>

=item Symmetric ciphers

In the OpenSSL libraries, the corresponding method object is
B<EVP_CIPHER>.
The number for this operation is B<OSSL_OP_CIPHER>.
The functions the provider can offer are described in
L<provider-cipher(7)>

=item Message Authentication Code (MAC)

In the OpenSSL libraries, the corresponding method object is
B<EVP_MAC>.
The number for this operation is B<OSSL_OP_MAC>.
The functions the provider can offer are described in
L<provider-mac(7)>

=item Key Derivation Function (KDF)

In the OpenSSL libraries, the corresponding method object is
B<EVP_KDF>.
The number for this operation is B<OSSL_OP_KDF>.
The functions the provider can offer are described in
L<provider-kdf(7)>

=item Key Exchange

In the OpenSSL libraries, the corresponding method object is
B<EVP_KEYEXCH>.
The number for this operation is B<OSSL_OP_KEYEXCH>.
The functions the provider can offer are described in
L<provider-keyexch(7)>

=item Serialization

In the OpenSSL libraries, the corresponding method object is
B<OSSL_SERIALIZER>.
The number for this operation is B<OSSL_OP_SERIALIZER>.
The functions the provider can offer are described in
L<provider-serializer(7)>

=back

=head2 Fetching algorithms

=head3 Explicit fetch

I<NOTE: This section is mostly interesting to OpenSSL users.>

Users of the OpenSSL libraries never query the provider directly for
its diverse implementations and dispatch tables.
Instead, the diverse OpenSSL APIs often have fetching functions that
do the work, and they return an appropriate method object back to the
user.
These functions usually have the name C<APINAME_fetch>, where
C<APINAME> is the name of the API, for example L<EVP_MD_fetch(3)>.

These fetching functions follow a fairly common pattern, where three
arguments are passed:

=over 4

=item The library context

See L<OPENSSL_CTX(3)> for a more detailed description.
This may be NULL to signify the default (global) library context, or a
context created by the user.
Only providers loaded in this library context (see
L<OSSL_PROVIDER_load(3)>) will be considered by the fetching
function.

=item An identifier

This is most commonly an algorithm name (this is the case for all EVP
methods), but may also be called something else.

=for comment For example, an OSSL_STORE implementation would use the
URI scheme as an identifier.

=item A property query string

See L<property(7)> for a more detailed description.
This is used to select more exactly which providers will get to offer
an implementation.

=back

The method object that is fetched can then be used with diverse other
functions that use them, for example L<EVP_DigestInit_ex(3)>.

=head3 Implicit fetch

I<NOTE: This section is mostly interesting to OpenSSL users.>

OpenSSL has a number of functions that return a method object with no
associated implementation, such as L<EVP_sha256(3)>,
L<EVP_blake2b512(3)> or L<EVP_aes_128_cbc(3)>, which are present for
compatibility with OpenSSL before version 3.0.

When they are used with functions like L<EVP_DigestInit_ex(3)> or
L<EVP_CipherInit_ex(3)>, the actual implementation to be used is
fetched implicitly using default search criteria.

Implicit fetching can also occur with functions such as
L<EVP_PKEY_derive_init_ex(3)> where a NULL algorithm parameter is
supplied.
In this case an algorithm implementation is implicitly fetched using
default search criteria and an algorithm name that is consistent with
the type of EVP_PKEY being used.

=head3 Algorithm naming

Algorithm names are case insensitive. Any particular algorithm can have multiple
aliases associated with it. The canonical OpenSSL naming scheme follows this
format:

ALGNAME[VERSION?][-SUBNAME[VERSION?]?][-SIZE?][-MODE?]

VERSION is only present if there are multiple versions of an algorithm (e.g.
MD2, MD4, MD5).  It may be omitted if there is only one version.

SUBNAME may be present where multiple algorithms are combined together,
e.g. MD5-SHA1.

SIZE is only present if multiple versions of an algorithm exist with different
sizes (e.g. AES-128-CBC, AES-256-CBC)

MODE is only present where applicable.

Other aliases may exist for example where standards bodies or common practice
use alternative names or names that OpenSSL has used historically.

=head1 OPENSSL PROVIDERS

OpenSSL comes with a set of providers.

The algorithms available in each of these providers may vary due to build time
configuration options. The L<openssl-list(1)> command can be used to list the
currently available algorithms.

The names of the algorithms shown from L<openssl-list(1)> can be used as an
algorithm identifier to the appropriate fetching function.

=head2 Default provider

The default provider is built in as part of the F<libcrypto> library.
Should it be needed (if other providers are loaded and offer
implementations of the same algorithms), the property "default=yes"
can be used as a search criterion for these implementations.

=head2 FIPS provider

The FIPS provider is a dynamically loadable module, and must therefore
be loaded explicitly, either in code or through OpenSSL configuration
(see L<config(5)>).
Should it be needed (if other providers are loaded and offer
implementations of the same algorithms), the property "fips=yes" can
be used as a search criterion for these implementations.

=head2 Legacy provider

The legacy provider is a dynamically loadable module, and must therefore
be loaded explicitly, either in code or through OpenSSL configuration
(see L<config(5)>).
Should it be needed (if other providers are loaded and offer
implementations of the same algorithms), the property "legacy=yes" can be
used as a search criterion for these implementations.

=head1 EXAMPLES

=head2 Fetching

Fetch any available implementation of SHA2-256 in the default context:

 EVP_MD *md = EVP_MD_fetch(NULL, "SHA2-256", NULL);
 ...
 EVP_MD_meth_free(md);

Fetch any available implementation of AES-128-CBC in the default context:

 EVP_CIPHER *cipher = EVP_CIPHER_fetch(NULL, "AES-128-CBC", NULL);
 ...
 EVP_CIPHER_meth_free(cipher);

Fetch an implementation of SHA2-256 from the default provider in the default
context:

 EVP_MD *md = EVP_MD_fetch(NULL, "SHA2-256", "default=yes");
 ...
 EVP_MD_meth_free(md);

Fetch an implementation of SHA2-256 that is not from the default provider in the
default context:

 EVP_MD *md = EVP_MD_fetch(NULL, "SHA2-256", "default=no");
 ...
 EVP_MD_meth_free(md);

Fetch an implementation of SHA2-256 from the default provider in the specified
context:

 EVP_MD *md = EVP_MD_fetch(ctx, "SHA2-256", "default=yes");
 ...
 EVP_MD_meth_free(md);

Load the legacy provider into the default context and then fetch an
implementation of WHIRLPOOL from it:

 /* This only needs to be done once - usually at application start up */
 OSSL_PROVIDER *legacy = OSSL_PROVIDER_load(NULL, "legacy");

 EVP_MD *md = EVP_MD_fetch(NULL, "WHIRLPOOL", "legacy=yes");
 ...
 EVP_MD_meth_free(md);

Note that in the above example the property string "legacy=yes" is optional
since, assuming no other providers have been loaded, the only implementation of
the "whirlpool" algorithm is in the "legacy" provider. Also note that the
default provider should be explicitly loaded if it is required in addition to
other providers:

 /* This only needs to be done once - usually at application start up */
 OSSL_PROVIDER *legacy = OSSL_PROVIDER_load(NULL, "legacy");
 OSSL_PROVIDER *default = OSSL_PROVIDER_load(NULL, "default");

 EVP_MD *md_whirlpool = EVP_MD_fetch(NULL, "whirlpool", NULL);
 EVP_MD *md_sha256 = EVP_MD_fetch(NULL, "SHA2-256", NULL);
 ...
 EVP_MD_meth_free(md_whirlpool);
 EVP_MD_meth_free(md_sha256);


=head1 SEE ALSO

L<EVP_DigestInit_ex(3)>, L<EVP_EncryptInit_ex(3)>,
L<EVP_PKEY_derive_init_ex(3)>, 
L<OPENSSL_CTX(3)>,
L<EVP_set_default_properties(3)>,
L<EVP_MD_fetch(3)>,
L<EVP_CIPHER_fetch(3)>,
L<EVP_KEYMGMT_fetch(3)>,
L<openssl-core.h(7)>,
L<provider-base(7)>,
L<provider-digest(7)>,
L<provider-cipher(7)>,
L<provider-keyexch(7)>

=head1 HISTORY

The concept of providers and everything surrounding them was
introduced in OpenSSL 3.0.

=head1 COPYRIGHT

Copyright 2019 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License").  You may not use
this file except in compliance with the License.  You can obtain a copy
in the file LICENSE in the source distribution or at
L<https://www.openssl.org/source/license.html>.

=cut
