OSSL_ALGORITHM(3ssl) | OpenSSL | OSSL_ALGORITHM(3ssl) |

# NAME

OSSL_ALGORITHM - OpenSSL Core type to define a fetchable algorithm

# SYNOPSIS

#include <openssl/core.h> typedef struct ossl_algorithm_st OSSL_ALGORITHM; struct ossl_algorithm_st { const char *algorithm_names; /* key */ const char *property_definition; /* key */ const OSSL_DISPATCH *implementation; const char *algorithm_description; };

# DESCRIPTION

The **OSSL_ALGORITHM** type is a *public structure* that
describes an algorithm that a provider(7) provides. Arrays of this
type are returned by providers on demand from the OpenSSL libraries to
describe what algorithms the providers provide implementations of, and with
what properties.

Arrays of this type must be terminated with a tuple where
*algorithm_names* is NULL.

This type of array is typically returned by the provider's operation querying function, further described in "Provider Functions" in provider-base(7).

**OSSL_ALGORITHM** fields

*algorithm_names*- This string is a colon separated set of names / identities, and is used by
the appropriate fetching functionality (such as
EVP_CIPHER_fetch(3), EVP_MD_fetch(3), etc) to find the
desired algorithm.
Multiple names / identities allow a specific algorithm implementation to be fetched multiple ways. For example, the RSA algorithm has the following known identities:

- "RSA"
- "rsaEncryption"
This is the name of the algorithm's OBJECT IDENTIFIER (OID), as given by the PKCS#1 RFC's ASN.1 module https://www.rfc-editor.org/rfc/rfc8017#appendix-C

- 1.2.840.113549.1.1.1
This is the OID itself for "rsaEncryption", in canonical decimal text form.

The resulting *algorithm_names* string would look like
this:

"RSA:rsaEncryption:1.2.840.113549.1.1.1"

The OpenSSL libraries use the first of the algorithm names as the main or canonical name, on a per algorithm implementation basis.

See the notes "On the subject of algorithm names" below
for a more in depth discussion on *algorithm_names* and how that may
interact with applications and libraries, including OpenSSL's.

*property_definition*- This string defines a set of properties associated with a particular
algorithm implementation, and is used by the appropriate fetching
functionality (such as EVP_CIPHER_fetch(3), EVP_MD_fetch(3),
etc) for a finer grained lookup of an algorithm implementation, which is
useful in case multiple implementations of the same algorithm are
available.
See property(7) for a further description of the contents of this string.

*implementation*- Pointer to an OSSL_DISPATCH(3) array, containing pointers to the functions of a particular algorithm implementation.
*algorithm_description*- A string with a short human-readable description of the algorithm.

# NOTES

## On the subject of algorithm names

Providers may find the need to register ASN.1 OIDs for algorithms
using OBJ_create(3) (via the **core_obj_create** upcall described
in provider-base(7), because some application or library -- possibly
still the OpenSSL libraries, even -- use NIDs to look up algorithms.

In that scenario, you must make sure that the corresponding
**OSSL_ALGORITHM**'s *algorithm_names* includes both the short and
the long name.

Most of the time, registering ASN.1 OIDs like this shouldn't be
necessary, and applications and libraries are encouraged to use
OBJ_obj2txt(3) to get a text representation of the OID, which may be
a long or short name for OIDs that are registered, or the OID itself in
canonical decimal text form if not (or if OBJ_obj2txt(3) is called
with *no_name* = 1).

It's recommended to make sure that the corresponding
**OSSL_ALGORITHM**'s *algorithm_names* include known names as well
as the OID itself in canonical decimal text form. That should cover all
scenarios.

# SEE ALSO

crypto(7), provider-base(7), openssl-core.h(7), openssl-core_dispatch.h(7), OSSL_DISPATCH(3)

# HISTORY

**OSSL_ALGORITHM** was added in OpenSSL 3.0

# COPYRIGHT

Copyright 2022 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 https://www.openssl.org/source/license.html.

2023-09-20 | 3.1.3 |