=pod

=head1 NAME

OSSL_PARAM - a structure to pass or request object parameters

=head1 SYNOPSIS

 #include <openssl/core.h>

 typedef struct ossl_param_st OSSL_PARAM;
 struct ossl_param_st {
     const char *key;             /* the name of the parameter */
     unsigned char data_type;     /* declare what kind of content is in data */
     void *data;                  /* value being passed in or out */
     size_t data_size;            /* data size */
     size_t *return_size;         /* OPTIONAL: address to content size */
 };

=head1 DESCRIPTION

C<OSSL_PARAM> is a type that allows passing arbitrary data for some
object between two parties that have no or very little shared
knowledge about their respective internal structures for that object.

A typical usage example could be an application that wants to set some
parameters for an object, or wants to find out some parameters of an
object.

Arrays of this type can be used for two purposes:

=over 4

=item *

Setting parameters for some object.
The caller sets up the C<OSSL_PARAM> array and calls some function
(the I<setter>) that has intimate knowledge about the object that can
take the data from the C<OSSL_PARAM> array and assign them in a
suitable form for the internal structure of the object.

=item *

Request parameters of some object.
The caller (the I<requestor>) sets up the C<OSSL_PARAM> array and
calls some function (the I<responder>) that has intimate knowledge
about the object, which can take the internal data of the object and
copy (possibly convert) that to the memory prepared by the
I<requestor> and pointed at with the C<OSSL_PARAM> C<data>.

=back

=head2 C<OSSL_PARAM> fields

=over 4

=item C<key>

The identity of the parameter in the form of a string.

=item C<data_type>

=for comment It's still debated if this field should be present, or if
the type should always be implied by how it's used.
Either way, these data types will have to be passed together with the
names as an array of OSSL_ITEM, for discovery purposes.

The C<data_type> is a value that describes the type and organization of
the data.
See L</Supported types> below for a description of the types.

=item C<data>

=item C<data_size>

C<data> is a pointer to the memory where the parameter data is (when
setting parameters) or shall (when requesting parameters) be stored,
and C<data_size> is its size in bytes.
The organization of the data depends on the parameter type and flag.

=item C<return_size>

When an array of C<OSSL_PARAM> is used to request data, the
I<responder> must set this field to indicate the actual size of the
parameter data.
In case the C<data_size> is too small for the data, the I<responder>
must still set this field to indicate the minimum data size required.

=back

B<NOTE:>

The key names and associated types are defined by the entity that
offers these parameters, i.e. names for parameters provided by the
OpenSSL libraries are defined by the libraries, and names for
parameters provided by providers are defined by those providers,
except for the pointer form of strings (see data type descriptions
below).
Entities that want to set or request parameters need to know what
those keys are and of what type, any functionality between those two
entities should remain oblivious and just pass the C<OSSL_PARAM> array
along.

=head2 Supported types

The C<data_type> field can be one of the following types:

=over 4

=item C<OSSL_PARAM_INTEGER>

=item C<OSSL_PARAM_UNSIGNED_INTEGER>

The parameter data is an integer (signed or unsigned) of arbitrary
length, organized in native form, i.e. most significant byte first on
Big-Endian systems, and least significant byte first on Little-Endian
systems.

=item C<OSSL_PARAM_REAL>

The parameter data is a floating point value in native form.

=item C<OSSL_PARAM_UTF8_STRING>

The parameter data is a printable string.

=item C<OSSL_PARAM_OCTET_STRING>

The parameter data is an arbitrary string of bytes.

=item C<OSSL_PARAM_UTF8_PTR>

The parameter data is a pointer to a printable string.

The difference between this and C<OSSL_PARAM_UTF8_STRING> is that C<data>
doesn't point directly at the data, but to a pointer that points to the data.

This is used to indicate that constant data is or will be passed,
and there is therefore no need to copy the data that is passed, just
the pointer to it.

C<data_size> must be set to the size of the data, not the size of the
pointer to the data.
If this is used in a parameter request,
C<data_size> is not relevant.  However, the I<responder> will set
C<*return_size> to the size of the data.

Note that the use of this type is B<fragile> and can only be safely
used for data that remains constant and in a constant location for a
long enough duration (such as the life-time of the entity that
offers these parameters).

=item C<OSSL_PARAM_OCTET_PTR>

The parameter data is a pointer to an arbitrary string of bytes.

The difference between this and C<OSSL_PARAM_OCTET_STRING> is that
C<data> doesn't point directly at the data, but to a pointer that
points to the data.

This is used to indicate that constant data is or will be passed, and
there is therefore no need to copy the data that is passed, just the
pointer to it.

C<data_size> must be set to the size of the data, not the size of the
pointer to the data.
If this is used in a parameter request,
C<data_size> is not relevant.  However, the I<responder> will set
C<*return_size> to the size of the data.

Note that the use of this type is B<fragile> and can only be safely
used for data that remains constant and in a constant location for a
long enough duration (such as the life-time of the entity that
offers these parameters).

=back

=head1 NOTES

Both when setting and requesting parameters, the functions that are
called will have to decide what is and what is not an error.
The recommended behaviour is:

=over 4

=item *

Keys that a I<setter> or I<responder> doesn't recognise should simply
be ignored.
That in itself isn't an error.

=item *

If the keys that a called I<setter> recognises form a consistent
enough set of data, that call should succeed.

=item *

A I<responder> must never change the fields of an C<OSSL_PARAM>, it
may only change the contents of the memory that C<data> and
C<return_size> point at.

=item *

If the data type for a key that it's associated with is incorrect,
the called function may return an error.

The called function may also try to convert the data to a suitable
form (for example, it's plausible to pass a large number as an octet
string, so even though a given key is defined as an
C<OSSL_PARAM_UNSIGNED_INTEGER>, is plausible to pass the value as an
C<OSSL_PARAM_OCTET_STRING>), but this is in no way mandatory.

=item *

If a I<responder> finds that some data sizes are too small for the
requested data, it must set C<*return_size> for each such
C<OSSL_PARAM> item to the required size, and eventually return an
error.

=back

=begin comment RETURN VALUES doesn't make sense for a manual that only
describes a type, but document checkers still want that section, and
to have more than just the section title.

=head1 RETURN VALUES

txt

=end comment

=head1 EXAMPLES

A couple of examples to just show how C<OSSL_PARAM> arrays could be
set up.

=head3 Example 1

This example is for setting parameters on some object:

    #include <openssl/core.h>

    const char *foo = "some string";
    size_t foo_l = strlen(foo) + 1;
    const char bar[] = "some other string";
    const OSSL_PARAM set[] = {
        { "foo", OSSL_PARAM_UTF8_STRING_PTR, &foo, foo_l, NULL },
        { "bar", OSSL_PARAM_UTF8_STRING, &bar, sizeof(bar), NULL },
        { NULL, 0, NULL, 0, NULL }
    };

=head3 Example 2

This example is for requesting parameters on some object:

    const char *foo = NULL;
    size_t foo_l;
    char bar[1024];
    size_t bar_l;
    const OSSL_PARAM request[] = {
        { "foo", OSSL_PARAM_UTF8_STRING_PTR, &foo, 0 /*irrelevant*/, &foo_l },
        { "bar", OSSL_PARAM_UTF8_STRING, &bar, sizeof(bar), &bar_l },
        { NULL, 0, NULL, 0, NULL }
    };

A I<responder> that receives this array (as C<params> in this example)
could fill in the parameters like this:

    /* const OSSL_PARAM *params */

    int i;

    for (i = 0; params[i].key != NULL; i++) {
        if (strcmp(params[i].key, "foo") == 0) {
            *(char **)params[i].data = "foo value";
            *params[i].return_size = 10; /* size of "foo value" */
        } else if (strcmp(params[i].key, "bar") == 0) {
            memcpy(params[i].data, "bar value", 10);
            *params[i].return_size = 10; /* size of "bar value" */
        }
        /* Ignore stuff we don't know */
    }

=head1 SEE ALSO

L<openssl-core.h(7)>, L<OSSL_PARAM_get_int32_t(3)>

=head1 HISTORY

C<OSSL_PARAM> was added 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
