openssl/doc/internal/man3/evp_generic_fetch.pod

=pod

=head1 NAME

evp_generic_fetch - generic algorithm fetcher and method creator for EVP

=head1 SYNOPSIS

 /* Only for EVP source */
 #include "evp_locl.h"

 void *evp_generic_fetch(OPENSSL_CTX *libctx, int operation_id,
                         const char *name, const char *properties,
                         void *(*new_method)(const OSSL_DISPATCH *fns,
                                             OSSL_PROVIDER *prov),
                         int (*upref_method)(void *),
                         void (*free_method)(void *));

=head1 DESCRIPTION

evp_generic_fetch() calls ossl_method_construct() with the given
C<libctx>, C<operation_id>, C<name>, and C<properties> and uses
it to create an EVP method with the help of the functions
C<new_method>, C<upref_method>, and C<free_method>.

The three functions are supposed to:

=over 4

=item new_method()

creates an internal method from function pointers found in the
dispatch table C<fns>.

=item upref_method()

increments the reference counter for the given method, if there is
one.

=item free_method()

frees the given method.

=back

=head1 RETURN VALUES

evp_generic_fetch() returns a method on success, or B<NULL> on error.

=head1 EXAMPLES

This is a short example of the fictitious EVP API and operation called
C<EVP_FOO>.

To begin with, let's assume something like this in
C<include/openssl/core_numbers.h>:

    #define OSSL_OP_FOO                         100

    #define OSSL_OP_FOO_NEWCTX_FUNC            2001
    #define OSSL_OP_FOO_INIT                   2002
    #define OSSL_OP_FOO_OPERATE                2003
    #define OSSL_OP_FOO_CLEANCTX_FUNC          2004
    #define OSSL_OP_FOO_FREECTX_FUNC           2005
    OSSL_CORE_MAKE_FUNC(void *,OP_foo_newctx,(void))
    OSSL_CORE_MAKE_FUNC(int,OP_foo_init,(void *vctx))
    OSSL_CORE_MAKE_FUNC(int,OP_foo_operate,(void *vctx,
                                            unsigned char *out, size_t *out_l,
                                            unsigned char *in, size_t in_l))
    OSSL_CORE_MAKE_FUNC(void,OP_foo_cleanctx,(void *vctx))
    OSSL_CORE_MAKE_FUNC(void,OP_foo_freectx,(void *vctx))

And here's the implementation of the FOO method fetcher:

    /* typedef struct evp_foo_st EVP_FOO */
    struct evp_foo_st {
        OSSL_PROVIDER *prov;
	CRYPTO_REF_COUNT refcnt;
        OSSL_OP_foo_newctx_fn *newctx;
        OSSL_OP_foo_init_fn *init;
        OSSL_OP_foo_operate_fn *operate;
        OSSL_OP_foo_cleanctx_fn *cleanctx;
        OSSL_OP_foo_freectx_fn *freectx;
    };

    /*
     * In this example, we have a public method creator and destructor.
     * It's not absolutely necessary, but is in the spirit of OpenSSL.
     */
    EVP_FOO *EVP_FOO_meth_from_dispatch(const OSSL_DISPATCH *fns,
                                        OSSL_PROVIDER *prov)
    {
        EVP_FOO *foo = NULL;

        if ((foo = OPENSSL_zalloc(sizeof(*foo))) == NULL)
            return NULL;

        for (; fns->function_id != 0; fns++) {
            switch (fns->function_id) {
            case OSSL_OP_FOO_NEWCTX_FUNC:
                foo->newctx = OSSL_get_OP_foo_newctx(fns);
                break;
            case OSSL_OP_FOO_INIT:
                foo->init = OSSL_get_OP_foo_init(fns);
                break;
            case OSSL_OP_FOO_OPERATE:
                foo->operate = OSSL_get_OP_foo_operate(fns);
                break;
            case OSSL_OP_FOO_CLEANCTX_FUNC:
                foo->cleanctx = OSSL_get_OP_foo_cleanctx(fns);
                break;
            case OSSL_OP_FOO_FREECTX_FUNC:
                foo->freectx = OSSL_get_OP_foo_freectx(fns);
                break;
            }
        }
        foo->prov = prov;
        if (prov)
            ossl_provider_upref(prov);

        return foo;
    }

    EVP_FOO_meth_free(EVP_FOO *foo)
    {
        if (foo != NULL) {
            OSSL_PROVIDER *prov = foo->prov;

            OPENSSL_free(foo);
            ossl_provider_free(prov);
        }
    }

    static void *foo_from_dispatch(const OSSL_DISPATCH *fns,
                                   OSSL_PROVIDER *prov)
    {
        return EVP_FOO_meth_from_dispatch(fns, prov);
    }

    static int foo_upref(void *vfoo)
    {
        EVP_FOO *foo = vfoo;
        int ref = 0;

        CRYPTO_UP_REF(&foo->refcnt, &ref, foo_lock);
        return 1;
    }

    static void foo_free(void *vfoo)
    {
        EVP_FOO_meth_free(vfoo);
    }

    EVP_FOO *EVP_FOO_fetch(OPENSSL_CTX *ctx,
                           const char *name,
                           const char *properties)
    {
        EVP_FOO *foo =
            evp_generic_fetch(ctx, OSSL_OP_FOO, name, properties,
                              foo_from_dispatch, foo_upref, foo_free);

        /*
         * If this method exists in legacy form, with a constant NID for the
         * given |name|, this is the spot to find that NID and set it in
         * the newly constructed EVP_FOO instance.
         */

        return foo;

    }

And finally, the library functions:

    /* typedef struct evp_foo_st EVP_FOO_CTX */
    struct evp_foo_ctx_st {
        const EVP_FOO *foo;
        void *provctx;		/* corresponding provider context */
    };

    int EVP_FOO_CTX_reset(EVP_FOO_CTX *c)
    {
        if (c == NULL)
            return 1;
        if (c->foo != NULL && c->foo->cleanctx != NULL)
            c->foo->cleanctx(c->provctx);
        return 1;
    }

    EVP_FOO_CTX *EVP_FOO_CTX_new(void)
    {
        return OPENSSL_zalloc(sizeof(EVP_FOO_CTX));
    }

    void EVP_FOO_CTX_free(EVP_FOO_CTX *c)
    {
        EVP_FOO_CTX_reset(c);
        c->foo->freectx(c->provctx);
        OPENSSL_free(c);
    }

    int EVP_FooInit(EVP_FOO_CTX *c, const EVP_FOO *foo)
    {
        int ok = 1;

        c->foo = foo;
        if (c->provctx == NULL)
            c->provctx = c->foo->newctx();

        ok = c->foo->init(c->provctx);

        return ok;
    }

    int EVP_FooOperate(EVP_FOO_CTX *c, unsigned char *out, size_t *outl,
                       const unsigned char *in, size_t inl)
    {
        int ok = 1;

        ok = c->foo->update(c->provctx, out, inl, &outl, in, inl);
        return ok;
    }

=head1 SEE ALSO

L<ossl_method_construct>

=head1 HISTORY

The functions described here were all 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