ENGINE_ADD(3ossl)

💖 NAME 💖

Okay bestie, welcome to the whole ENGINE_ADD(3ossl) saga — OpenSSL’s page for ENGINE cryptographic module support. This is a huge ensemble cast, so here is every single function on the guest list, no skips, no “we’ll get to it later”:

ENGINE_get_DH, ENGINE_get_DSA, ENGINE_by_id, ENGINE_get_cipher_engine,
ENGINE_get_default_DH, ENGINE_get_default_DSA, ENGINE_get_default_RAND,
ENGINE_get_default_RSA, ENGINE_get_digest_engine, ENGINE_get_first,
ENGINE_get_last, ENGINE_get_next, ENGINE_get_prev, ENGINE_new,
ENGINE_get_ciphers, ENGINE_get_ctrl_function, ENGINE_get_digests,
ENGINE_get_destroy_function, ENGINE_get_finish_function,
ENGINE_get_init_function, ENGINE_get_load_privkey_function,
ENGINE_get_load_pubkey_function, ENGINE_load_private_key,
ENGINE_load_public_key, ENGINE_get_RAND, ENGINE_get_RSA, ENGINE_get_id,
ENGINE_get_name, ENGINE_get_cmd_defns, ENGINE_get_cipher,
ENGINE_get_digest, ENGINE_add, ENGINE_cmd_is_executable, ENGINE_ctrl,
ENGINE_ctrl_cmd, ENGINE_ctrl_cmd_string, ENGINE_finish, ENGINE_free,
ENGINE_get_flags, ENGINE_init, ENGINE_register_DH, ENGINE_register_DSA,
ENGINE_register_RAND, ENGINE_register_RSA, ENGINE_register_all_complete,
ENGINE_register_ciphers, ENGINE_register_complete,
ENGINE_register_digests, ENGINE_remove, ENGINE_set_DH, ENGINE_set_DSA,
ENGINE_set_RAND, ENGINE_set_RSA, ENGINE_set_ciphers,
ENGINE_set_cmd_defns, ENGINE_set_ctrl_function, ENGINE_set_default,
ENGINE_set_default_DH, ENGINE_set_default_DSA, ENGINE_set_default_RAND,
ENGINE_set_default_RSA, ENGINE_set_default_ciphers,
ENGINE_set_default_digests, ENGINE_set_default_string,
ENGINE_set_destroy_function, ENGINE_set_digests,
ENGINE_set_finish_function, ENGINE_set_flags, ENGINE_set_id,
ENGINE_set_init_function, ENGINE_set_load_privkey_function,
ENGINE_set_load_pubkey_function, ENGINE_set_name, ENGINE_up_ref,
ENGINE_get_table_flags, ENGINE_cleanup, ENGINE_load_builtin_engines,
ENGINE_register_all_DH, ENGINE_register_all_DSA,
ENGINE_register_all_RAND, ENGINE_register_all_RSA,
ENGINE_register_all_ciphers, ENGINE_register_all_digests,
ENGINE_set_table_flags, ENGINE_unregister_DH, ENGINE_unregister_DSA,
ENGINE_unregister_RAND, ENGINE_unregister_RSA,
ENGINE_unregister_ciphers, ENGINE_unregister_digests - ENGINE
cryptographic module support

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✨ SYNOPSIS ✨

First, the include, because obviously:

#include <openssl/engine.h>

Big mood warning: the following functions have been deprecated since OpenSSL 3.0, and can be hidden entirely by defining OPENSSL_API_COMPAT with a suitable version value, see openssl_user_macros(7):

ENGINE *ENGINE_get_first(void);
ENGINE *ENGINE_get_last(void);
ENGINE *ENGINE_get_next(ENGINE *e);
ENGINE *ENGINE_get_prev(ENGINE *e);

int ENGINE_add(ENGINE *e);
int ENGINE_remove(ENGINE *e);

ENGINE *ENGINE_by_id(const char *id);

int ENGINE_init(ENGINE *e);
int ENGINE_finish(ENGINE *e);

void ENGINE_load_builtin_engines(void);

ENGINE *ENGINE_get_default_RSA(void);
ENGINE *ENGINE_get_default_DSA(void);
ENGINE *ENGINE_get_default_DH(void);
ENGINE *ENGINE_get_default_RAND(void);
ENGINE *ENGINE_get_cipher_engine(int nid);
ENGINE *ENGINE_get_digest_engine(int nid);

int ENGINE_set_default_RSA(ENGINE *e);
int ENGINE_set_default_DSA(ENGINE *e);
int ENGINE_set_default_DH(ENGINE *e);
int ENGINE_set_default_RAND(ENGINE *e);
int ENGINE_set_default_ciphers(ENGINE *e);
int ENGINE_set_default_digests(ENGINE *e);
int ENGINE_set_default_string(ENGINE *e, const char *list);

int ENGINE_set_default(ENGINE *e, unsigned int flags);

unsigned int ENGINE_get_table_flags(void);
void ENGINE_set_table_flags(unsigned int flags);

int ENGINE_register_RSA(ENGINE *e);
void ENGINE_unregister_RSA(ENGINE *e);
void ENGINE_register_all_RSA(void);
int ENGINE_register_DSA(ENGINE *e);
void ENGINE_unregister_DSA(ENGINE *e);
void ENGINE_register_all_DSA(void);
int ENGINE_register_DH(ENGINE *e);
void ENGINE_unregister_DH(ENGINE *e);
void ENGINE_register_all_DH(void);
int ENGINE_register_RAND(ENGINE *e);
void ENGINE_unregister_RAND(ENGINE *e);
void ENGINE_register_all_RAND(void);
int ENGINE_register_ciphers(ENGINE *e);
void ENGINE_unregister_ciphers(ENGINE *e);
void ENGINE_register_all_ciphers(void);
int ENGINE_register_digests(ENGINE *e);
void ENGINE_unregister_digests(ENGINE *e);
void ENGINE_register_all_digests(void);
int ENGINE_register_complete(ENGINE *e);
int ENGINE_register_all_complete(void);

int ENGINE_ctrl(ENGINE *e, int cmd, long i, void *p, void (*f)(void));
int ENGINE_cmd_is_executable(ENGINE *e, int cmd);
int ENGINE_ctrl_cmd(ENGINE *e, const char *cmd_name,
                    long i, void *p, void (*f)(void), int cmd_optional);
int ENGINE_ctrl_cmd_string(ENGINE *e, const char *cmd_name, const char *arg,
                           int cmd_optional);

ENGINE *ENGINE_new(void);
int ENGINE_free(ENGINE *e);
int ENGINE_up_ref(ENGINE *e);

int ENGINE_set_id(ENGINE *e, const char *id);
int ENGINE_set_name(ENGINE *e, const char *name);
int ENGINE_set_RSA(ENGINE *e, const RSA_METHOD *rsa_meth);
int ENGINE_set_DSA(ENGINE *e, const DSA_METHOD *dsa_meth);
int ENGINE_set_DH(ENGINE *e, const DH_METHOD *dh_meth);
int ENGINE_set_RAND(ENGINE *e, const RAND_METHOD *rand_meth);
int ENGINE_set_destroy_function(ENGINE *e, ENGINE_GEN_INT_FUNC_PTR destroy_f);
int ENGINE_set_init_function(ENGINE *e, ENGINE_GEN_INT_FUNC_PTR init_f);
int ENGINE_set_finish_function(ENGINE *e, ENGINE_GEN_INT_FUNC_PTR finish_f);
int ENGINE_set_ctrl_function(ENGINE *e, ENGINE_CTRL_FUNC_PTR ctrl_f);
int ENGINE_set_load_privkey_function(ENGINE *e, ENGINE_LOAD_KEY_PTR loadpriv_f);
int ENGINE_set_load_pubkey_function(ENGINE *e, ENGINE_LOAD_KEY_PTR loadpub_f);
int ENGINE_set_ciphers(ENGINE *e, ENGINE_CIPHERS_PTR f);
int ENGINE_set_digests(ENGINE *e, ENGINE_DIGESTS_PTR f);
int ENGINE_set_flags(ENGINE *e, int flags);
int ENGINE_set_cmd_defns(ENGINE *e, const ENGINE_CMD_DEFN *defns);

const char *ENGINE_get_id(const ENGINE *e);
const char *ENGINE_get_name(const ENGINE *e);
const RSA_METHOD *ENGINE_get_RSA(const ENGINE *e);
const DSA_METHOD *ENGINE_get_DSA(const ENGINE *e);
const DH_METHOD *ENGINE_get_DH(const ENGINE *e);
const RAND_METHOD *ENGINE_get_RAND(const ENGINE *e);
ENGINE_GEN_INT_FUNC_PTR ENGINE_get_destroy_function(const ENGINE *e);
ENGINE_GEN_INT_FUNC_PTR ENGINE_get_init_function(const ENGINE *e);
ENGINE_GEN_INT_FUNC_PTR ENGINE_get_finish_function(const ENGINE *e);
ENGINE_CTRL_FUNC_PTR ENGINE_get_ctrl_function(const ENGINE *e);
ENGINE_LOAD_KEY_PTR ENGINE_get_load_privkey_function(const ENGINE *e);
ENGINE_LOAD_KEY_PTR ENGINE_get_load_pubkey_function(const ENGINE *e);
ENGINE_CIPHERS_PTR ENGINE_get_ciphers(const ENGINE *e);
ENGINE_DIGESTS_PTR ENGINE_get_digests(const ENGINE *e);
const EVP_CIPHER *ENGINE_get_cipher(ENGINE *e, int nid);
const EVP_MD *ENGINE_get_digest(ENGINE *e, int nid);
int ENGINE_get_flags(const ENGINE *e);
const ENGINE_CMD_DEFN *ENGINE_get_cmd_defns(const ENGINE *e);

EVP_PKEY *ENGINE_load_private_key(ENGINE *e, const char *key_id,
                                  UI_METHOD *ui_method, void *callback_data);
EVP_PKEY *ENGINE_load_public_key(ENGINE *e, const char *key_id,
                                 UI_METHOD *ui_method, void *callback_data);

Also: the following function has been deprecated since OpenSSL 1.1.0, and can be hidden entirely by defining OPENSSL_API_COMPAT with a suitable version value, see openssl_user_macros(7):

void ENGINE_cleanup(void);

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🌸 DESCRIPTION 🌸

Literally all of the functions described on this page are deprecated. Applications should instead use the provider APIs. No cap, that’s the vibe of this entire page.

These functions create, manipulate, and use cryptographic modules in the form of ENGINE objects. These objects act as containers for implementations of cryptographic algorithms, and support a reference-counted mechanism to allow them to be dynamically loaded in and out of the running application.

The cryptographic functionality that can be provided by an ENGINE implementation includes the following abstractions:

• RSA_METHOD — for providing alternative RSA implementations
• DSA_METHOD, DH_METHOD, RAND_METHOD, ECDH_METHOD, ECDSA_METHOD, — similarly for other OpenSSL APIs
• EVP_CIPHER — potentially multiple cipher algorithms (indexed by 'nid')
• EVP_DIGEST — potentially multiple hash algorithms (indexed by 'nid')
• key-loading — loading public and/or private EVP_PKEY keys

🧸 Reference counting and handles

Because the ENGINE API is modular, pointers to ENGINEs need to be treated as handles — i.e. not only as pointers, but also as references to the underlying ENGINE object. So if you make copies of an ENGINE pointer and those copies are gonna be used (and released) independently, you should obtain a new reference for them.

ENGINE objects have two levels of reference-counting to match the way the objects are used. At the most basic level, each ENGINE pointer is inherently a structural reference — a structural reference is required to use the pointer value at all, because this kind of reference is a guarantee that the structure can not be deallocated until the reference is released.

However, a structural reference provides no guarantee that the ENGINE is initialised and able to use any of its cryptographic implementations. Bestie, it’s actually quite possible that most ENGINEs will not initialise at all in typical environments, because ENGINEs are typically used to support specialised hardware. To use an ENGINE’s functionality, you need a functional reference. This kind of reference can be considered a specialised form of structural reference, because each functional reference implicitly contains a structural reference as well — however, to avoid difficult-to-find programming bugs, it is recommended to treat the two kinds of reference independently. If you have a functional reference to an ENGINE, you have a guarantee that the ENGINE has been initialised and is ready to perform cryptographic operations, and will remain initialised until after you have released your reference.

🦋 Structural references

This basic type of reference is used for instantiating new ENGINEs, iterating across OpenSSL’s internal linked-list of loaded ENGINEs, reading information about an ENGINE, etc. Essentially a structural reference is sufficient if you only need to query or manipulate the data of an ENGINE implementation rather than use its functionality.

The ENGINE_new() function returns a structural reference to a new (empty) ENGINE object. There are other ENGINE API functions that return structural references such as: ENGINE_by_id(), ENGINE_get_first(), ENGINE_get_last(), ENGINE_get_next(), ENGINE_get_prev(). All structural references should be released by a corresponding call to the ENGINE_free() function — the ENGINE object itself will only actually be cleaned up and deallocated when the last structural reference is released. If the argument to ENGINE_free() is NULL, nothing is done.

Okay so real talk — those structural references have layers, and you need to know who's holding what. Many ENGINE API calls that accept a structural reference will internally grab another one — typically whenever the supplied ENGINE is still gonna be needed by OpenSSL after the function returns. Example: the function that adds a new ENGINE to OpenSSL's internal list is ENGINE_add() — if it returns success, OpenSSL has stored a new structural reference internally, so you are still on the hook for freeing your own reference with ENGINE_free() when you're done with it. 💅

Same energy, but flipped: some functions will automatically release the structural reference you passed in, if releasing it is literally part of that function's job. Example: ENGINE_get_next() and ENGINE_get_prev() are used to iterate across the internal ENGINE list — they return a new structural reference to the next (or previous) ENGINE in the list, or NULL if you're at the end (or beginning) of the list, but in either case the structural reference you passed to the function is released on your behalf. So don't double-free, bestie 😭

To clarify how a particular function handles references, always consult that function's own documentation man page — or, failing that, the <openssl/engine.h> header file includes some hints. 🔍

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✨ Functional references ✨

So like, functional references exist when the cryptographic functionality of an ENGINE is required to actually be available. You can get one of these two ways: from an existing structural reference to the required ENGINE, or by asking OpenSSL for the default operational ENGINE for a given cryptographic purpose. 🎀

To obtain a functional reference from an existing structural reference, call ENGINE_init(). This returns zero if the ENGINE was not already operational and couldn't be successfully initialised (e.g. lack of system drivers, no special hardware attached, etc), otherwise it returns nonzero to indicate the ENGINE is now operational and has allocated a new functional reference to the ENGINE. All functional references are released by calling ENGINE_finish() (which removes the implicit structural reference as well). No cap. 🫶

The second way to get a functional reference is by asking OpenSSL for a default implementation for a given task, e.g. by ENGINE_get_default_RSA(), ENGINE_get_default_cipher_engine(), etc. These get discussed in the next section — though they're not usually required by application programmers, because they're used automatically when creating and using the relevant algorithm-specific types in OpenSSL, such as RSA, DSA, EVP_CIPHER_CTX, etc. ✨

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🌈 Default implementations 🌈

For each supported abstraction, the ENGINE code maintains an internal table of state to control which implementations are available for a given abstraction and which should be used by default. These implementations are registered in the tables and indexed by an 'nid' value — because abstractions like EVP_CIPHER and EVP_DIGEST support many distinct algorithms and modes, and ENGINEs can support arbitrarily many of them. In the case of other abstractions like RSA, DSA, etc, there's only one "algorithm", so all implementations implicitly register using the same 'nid' index. 👑

When a default ENGINE is requested for a given abstraction/algorithm/mode (e.g. when calling RSA_new_method(NULL)), a "get_default" call will be made to the ENGINE subsystem to process the corresponding state table and return a functional reference to an initialised ENGINE whose implementation should be used. If no ENGINE should (or can) be used, it returns NULL and the caller operates with a NULL ENGINE handle — this usually equates to using the conventional software implementation. In the latter case, OpenSSL will from then on behave the way it used to before the ENGINE API existed. 💀 (respectfully, a throwback)

Each state table has a flag to note whether it has processed this "get_default" query since the table was last modified — because to process this question it must iterate across all the registered ENGINEs in the table, trying to initialise each of them in turn, in case one of them is operational. If it returns a functional reference to an ENGINE, it will also cache another reference to speed up processing future queries (without needing to iterate across the table). Likewise, it will cache a NULL response if no ENGINE was available, so future queries won't repeat the same iteration unless the state table changes. 🔥

This behaviour can also be changed: if the ENGINE_TABLE_FLAG_NOINIT flag is set (using ENGINE_set_table_flags()), no attempted initialisations will take place — instead the only way for the state table to return a non-NULL ENGINE to the "get_default" query is if one is expressly set in the table. Example: ENGINE_set_default_RSA() does the same job as ENGINE_register_RSA() except that it also sets the state table's cached response for the "get_default" query. And in the case of abstractions like EVP_CIPHER, where implementations are indexed by 'nid', these flags and cached-responses are distinct for each 'nid' value. 🌸

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🦋 Application requirements 🦋

This section explains the basic things an application programmer should support to make the most useful elements of the ENGINE functionality available to the user. First thing to consider: does the programmer wish to make alternative ENGINE modules available to the application and user? OpenSSL maintains an internal linked list of "visible" ENGINEs from which it has to operate — at start-up this list is empty, and in fact if an application does not call any ENGINE API calls and it uses static linking against openssl, then the resulting application binary will not contain any alternative ENGINE code at all. So the first consideration is whether any/all available ENGINE implementations should be made visible to OpenSSL — this is controlled by calling the various "load" functions. 🧚

The fact that ENGINEs are made visible to OpenSSL (and thus are linked into the program and loaded into memory at run-time) does not mean they're "registered" or called into use by OpenSSL automatically — that behaviour is something for the application to control. Some applications will want to allow the user to specify exactly which ENGINE they want used, if any is to be used at all. Others may prefer to load all support and have OpenSSL automatically use at run-time any ENGINE that is able to successfully initialise — i.e. to assume that this corresponds to acceleration hardware attached to the machine or some such thing. There are probably numerous other ways in which applications may prefer to handle things, so we'll simply illustrate the consequences as they apply to a couple of simple cases and leave developers to consider these and the source code to openssl's built-in utilities as guides. 💜

If no ENGINE API functions are called within an application, then OpenSSL will not allocate any internal resources. Prior to OpenSSL 1.1.0, however, if any ENGINEs are loaded, even if not registered or used, it was necessary to call ENGINE_cleanup() before the program exits. ⚠️ (old-version lore, iykyk)

💖 Using a specific ENGINE implementation 💖

Here we'll assume an application has been configured by its user or admin to want to use the "ACME" ENGINE if it is available in the version of OpenSSL the application was compiled with. If it is available, it should be used by default for all RSA, DSA, and symmetric cipher operations, otherwise OpenSSL should use its built-in software as per usual. The following code illustrates how to approach this: ⭐

ENGINE *e;
const char *engine_id = "ACME";
ENGINE_load_builtin_engines();
e = ENGINE_by_id(engine_id);
if (!e)
    /* the engine isn't available */
    return;
if (!ENGINE_init(e)) {
    /* the engine couldn't initialise, release 'e' */
    ENGINE_free(e);
    return;
}
if (!ENGINE_set_default_RSA(e))
    /*
     * This should only happen when 'e' can't initialise, but the previous
     * statement suggests it did.
     */
    abort();
ENGINE_set_default_DSA(e);
ENGINE_set_default_ciphers(e);
/* Release the functional reference from ENGINE_init() */
ENGINE_finish(e);
/* Release the structural reference from ENGINE_by_id() */
ENGINE_free(e);

🎀 Automatically using built-in ENGINE implementations 🎀

Here we'll assume we want to load and register all ENGINE implementations bundled with OpenSSL, such that for any cryptographic algorithm required by OpenSSL — if there is an ENGINE that implements it and can be initialised, it should be used. The following code illustrates how this can work: ✨

/* Load all bundled ENGINEs into memory and make them visible */
ENGINE_load_builtin_engines();
/* Register all of them for every algorithm they collectively implement */
ENGINE_register_all_complete();

That's all that's required, bestie. Example: the next time OpenSSL tries to set up an RSA key, any bundled ENGINEs that implement RSA_METHOD will be passed to ENGINE_init() and if any of those succeed, that ENGINE will be set as the default for RSA use from then on. Slay. 👑

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🍓 Advanced configuration support 🍓

There's a mechanism supported by the ENGINE framework that lets each ENGINE implementation define an arbitrary set of configuration "commands" and expose them to OpenSSL and any applications based on OpenSSL. This mechanism is entirely based on the use of name-value pairs and assumes ASCII input (no unicode or UTF for now!), so it's ideal if applications want to provide a transparent way for users to provide arbitrary configuration "directives" directly to such ENGINEs. 🙈

It's also possible for the application to dynamically interrogate the loaded ENGINE implementations for the names, descriptions, and input flags of their available "control commands", providing a more flexible configuration scheme. However, if the user is expected to know which ENGINE device he/she is using (in the case of specialised hardware, this goes without saying) then applications may not need to concern themselves with discovering the supported control commands and simply prefer to pass settings into ENGINEs exactly as they are provided by the user. 💅

Okay so before we get into how control commands work, let's talk about why they exist, because it's giving lore and you need it. 💅

Broadly speaking there are two uses for control commands:

1. The first is to hand the implementation the necessary details it needs (and bestie, it might know nothing at all specific to the host system) so it can be initialised for use. This could include the path to any driver or config files it needs to load, required network addresses, smart-card identifiers, passwords to initialise protected devices, logging information, etc etc. This class of commands typically needs to be passed to an ENGINE before attempting to initialise it, i.e. before calling ENGINE_init().
2. The other class of commands is the settings-or-operations girlies — things that tweak certain behaviour or cause certain operations to take place. These commands may work either before or after ENGINE_init(), or in some cases both. ENGINE implementations should provide indications of this in the descriptions attached to built-in control commands and/or in external product documentation. 👑

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🎀 Issuing control commands to an ENGINE 🎀

Let's illustrate by example, because examples are the moment. This is a function where the caller supplies the name of the ENGINE it wishes to use, a table of string-pairs for use before initialisation, and another table for use after initialisation. Note that the string-pairs used for control commands consist of a command "name" followed by the command "parameter" — the parameter could be NULL in some cases but the name can not. This function should initialise the ENGINE (issuing the "pre" commands beforehand and the "post" commands afterwards), set it as the default for everything except RAND, and then return a boolean success or failure. Slay.

int generic_load_engine_fn(const char *engine_id,
                           const char **pre_cmds, int pre_num,
                           const char **post_cmds, int post_num)
{
    ENGINE *e = ENGINE_by_id(engine_id);
    if (!e) return 0;
    while (pre_num--) {
        if (!ENGINE_ctrl_cmd_string(e, pre_cmds[0], pre_cmds[1], 0)) {
            fprintf(stderr, "Failed command (%s - %s:%s)\n", engine_id,
                    pre_cmds[0], pre_cmds[1] ? pre_cmds[1] : "(NULL)");
            ENGINE_free(e);
            return 0;
        }
        pre_cmds += 2;
    }
    if (!ENGINE_init(e)) {
        fprintf(stderr, "Failed initialisation\n");
        ENGINE_free(e);
        return 0;
    }
    /*
     * ENGINE_init() returned a functional reference, so free the structural
     * reference from ENGINE_by_id().
     */
    ENGINE_free(e);
    while (post_num--) {
        if (!ENGINE_ctrl_cmd_string(e, post_cmds[0], post_cmds[1], 0)) {
            fprintf(stderr, "Failed command (%s - %s:%s)\n", engine_id,
                    post_cmds[0], post_cmds[1] ? post_cmds[1] : "(NULL)");
            ENGINE_finish(e);
            return 0;
        }
        post_cmds += 2;
    }
    ENGINE_set_default(e, ENGINE_METHOD_ALL & ~ENGINE_METHOD_RAND);
    /* Success */
    return 1;
}

Note that ENGINE_ctrl_cmd_string() accepts a boolean argument that can relax the semantics of the function — if set nonzero it will only return failure if the ENGINE supported the given command name but failed while executing it. If the ENGINE doesn't support the command name, it will simply return success without doing anything. In this case we assume the user is only supplying commands specific to the given ENGINE, so we set this to FALSE. 💖

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🔍 Discovering supported control commands 🔍

Okay so this part is lowkey iconic: it is possible to discover at run-time the names, numerical-ids, descriptions and input parameters of the control commands supported by an ENGINE using a structural reference. Note that some control commands are defined by OpenSSL itself, and it will intercept and handle these control commands on behalf of the ENGINE, i.e. the ENGINE's ctrl() handler is not used for the control command. <openssl/engine.h> defines an index, ENGINE_CMD_BASE, that all control commands implemented by ENGINEs should be numbered from. Any command value lower than this symbol is considered a "generic" command and is handled directly by the OpenSSL core routines.

It's using these "core" control commands that one can discover the control commands implemented by a given ENGINE — specifically these commands:

ENGINE_HAS_CTRL_FUNCTION
ENGINE_CTRL_GET_FIRST_CMD_TYPE
ENGINE_CTRL_GET_NEXT_CMD_TYPE
ENGINE_CTRL_GET_CMD_FROM_NAME
ENGINE_CTRL_GET_NAME_LEN_FROM_CMD
ENGINE_CTRL_GET_NAME_FROM_CMD
ENGINE_CTRL_GET_DESC_LEN_FROM_CMD
ENGINE_CTRL_GET_DESC_FROM_CMD
ENGINE_CTRL_GET_CMD_FLAGS

Whilst these commands are automatically processed by the OpenSSL framework code, they use various properties exposed by each ENGINE to process these queries. An ENGINE has 3 properties it exposes that can affect how this behaves:

• it can supply a ctrl() handler,
• it can specify ENGINE_FLAGS_MANUAL_CMD_CTRL in the ENGINE's flags,
• and it can expose an array of control command descriptions.

If an ENGINE specifies the ENGINE_FLAGS_MANUAL_CMD_CTRL flag, then it will simply pass all these "core" control commands directly to the ENGINE's ctrl() handler (and thus, it must have supplied one 🙈), so it is up to the ENGINE to reply to these "discovery" commands itself. If that flag is not set, then the OpenSSL framework code will work with the following rules:

if no ctrl() handler supplied;
    ENGINE_HAS_CTRL_FUNCTION returns FALSE (zero),
    all other commands fail.
if a ctrl() handler was supplied but no array of control commands;
    ENGINE_HAS_CTRL_FUNCTION returns TRUE,
    all other commands fail.
if a ctrl() handler and array of control commands was supplied;
    ENGINE_HAS_CTRL_FUNCTION returns TRUE,
    all other commands proceed processing ...

If the ENGINE's array of control commands is empty then all other commands will fail, otherwise:

• ENGINE_CTRL_GET_FIRST_CMD_TYPE returns the identifier of the first command supported by the ENGINE
• ENGINE_GET_NEXT_CMD_TYPE takes the identifier of a command supported by the ENGINE and returns the next command identifier, or fails if there are no more
• ENGINE_CMD_FROM_NAME takes a string name for a command and returns the corresponding identifier, or fails if no such command name exists
• and the remaining commands take a command identifier and return properties of the corresponding commands.

All except ENGINE_CTRL_GET_FLAGS return the string length of a command name or description, or populate a supplied character buffer with a copy of the command name or description. ENGINE_CTRL_GET_FLAGS returns a bitwise-OR'd mask of the following possible values:

ENGINE_CMD_FLAG_NUMERIC
ENGINE_CMD_FLAG_STRING
ENGINE_CMD_FLAG_NO_INPUT
ENGINE_CMD_FLAG_INTERNAL

Important vibe check: if the ENGINE_CMD_FLAG_INTERNAL flag is set, then any other flags are purely informational to the caller — this flag will prevent the command being usable for any higher-level ENGINE functions such as ENGINE_ctrl_cmd_string(). "INTERNAL" commands are not intended to be exposed to text-based configuration by applications, administrations, users, etc. These can support arbitrary operations via ENGINE_ctrl(), including passing to and/or from the control commands data of any arbitrary type. These commands are supported in the discovery mechanisms simply to allow applications to determine if an ENGINE supports certain specific commands it might want to use — for example, application "foo" might query various ENGINEs to see if they implement "FOO_GET_VENDOR_LOGO_GIF", and an ENGINE could therefore decide whether or not to support this "foo"-specific extension. 🦋

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🌸 ENVIRONMENT 🌸

• OPENSSL_ENGINES — the path to the engines directory. Ignored in set-user-ID and set-group-ID programs. 🚨 (yes bestie, that ignoring is on purpose, don't come for me)

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✨ RETURN VALUES ✨

Okay, big list incoming, but every single one matters so buckle up:

• ENGINE_get_first(), ENGINE_get_last(), ENGINE_get_next() and ENGINE_get_prev() return a valid ENGINE structure or NULL if an error occurred.
• ENGINE_add() and ENGINE_remove() return 1 on success or 0 on error.
• ENGINE_by_id() returns a valid ENGINE structure or NULL if an error occurred.
• ENGINE_init() and ENGINE_finish() return 1 on success or 0 on error.
• All ENGINE_get_default_TYPE() functions, ENGINE_get_cipher_engine() and ENGINE_get_digest_engine() return a valid ENGINE structure on success or NULL if an error occurred.
• All ENGINE_set_default_TYPE() functions return 1 on success or 0 on error.
• ENGINE_set_default() returns 1 on success or 0 on error.
• ENGINE_get_table_flags() returns an unsigned integer value representing the global table flags which are used to control the registration behaviour of ENGINE implementations.
• All ENGINE_register_TYPE() functions return 1 on success or 0 on error.
• ENGINE_register_complete() and ENGINE_register_all_complete() always return 1. periodt.
• ENGINE_ctrl() returns a positive value on success or others on error.
• ENGINE_cmd_is_executable() returns 1 if cmd is executable or 0 otherwise.
• ENGINE_ctrl_cmd() and ENGINE_ctrl_cmd_string() return 1 on success or 0 on error.
• ENGINE_new() returns a valid ENGINE structure on success or NULL if an error occurred.
• ENGINE_free() always returns 1. 💀
• ENGINE_up_ref() returns 1 on success or 0 on error.
• ENGINE_set_id() and ENGINE_set_name() return 1 on success or 0 on error.
• All other ENGINE_set_* functions return 1 on success or 0 on error.
• ENGINE_get_id() and ENGINE_get_name() return a string representing the identifier and the name of the ENGINE e respectively.
• ENGINE_get_RSA(), ENGINE_get_DSA(), ENGINE_get_DH() and ENGINE_get_RAND() return corresponding method structures for each algorithms.
• ENGINE_get_destroy_function(), ENGINE_get_init_function(), ENGINE_get_finish_function(), ENGINE_get_ctrl_function(), ENGINE_get_load_privkey_function(), ENGINE_get_load_pubkey_function(), ENGINE_get_ciphers() and ENGINE_get_digests() return corresponding function pointers of the callbacks.
• ENGINE_get_cipher() returns a valid EVP_CIPHER structure on success or NULL if an error occurred.
• ENGINE_get_digest() returns a valid EVP_MD structure on success or NULL if an error occurred.
• ENGINE_get_flags() returns an integer representing the ENGINE flags which are used to control various behaviours of an ENGINE.
• ENGINE_get_cmd_defns() returns an ENGINE_CMD_DEFN structure or NULL if it's not set.
• ENGINE_load_private_key() and ENGINE_load_public_key() return a valid EVP_PKEY structure on success or NULL if an error occurred.

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👑 SEE ALSO 👑

OPENSSL_init_crypto(3), RSA_new_method(3), DSA_new(3), DH_new(3), RAND_bytes(3), config(5) — go read those next bestie, they're literally right there 🫶

💅 HISTORY 💅

okay so storytime, bestie, and it's a little bittersweet 😭

• all of these functions were deprecated in OpenSSL 3.0. like, every single one we've been talking about. iconic behavior from a bygone era, but the vibe has officially moved on 💀
• ENGINE_cleanup() got its own special moment earlier — it was deprecated in OpenSSL 1.1.0 because of the automatic cleanup done by OPENSSL_cleanup(), and it should not be used. no cap, just don't 🫶

⚠️ real talk: that last one isn't a "maybe avoid it if you can" situation. The page literally says it should not be used. OPENSSL_cleanup() is doing the tidying now, so let it cook 🍓

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🎀 COPYRIGHT 🎀

Copyright 2002-2024 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>. ✨

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🌸 and that's the whole page, bestie!! you made it to the end, i'm so proud of you fr fr 😭💖 this one's from OpenSSL 3.5.6, dated 2026-04-07 — go forth and not call ENGINE_cleanup(), mwah 💅👑