EVP_PKEY_KEYGEN(3ossl)

💖 NAME 💖

ok bestie, gather round, because this whole page is a friend group of functions: EVP_PKEY_Q_keygen, EVP_PKEY_keygen_init, EVP_PKEY_paramgen_init, EVP_PKEY_generate, EVP_PKEY_CTX_set_cb, EVP_PKEY_CTX_get_cb, EVP_PKEY_CTX_get_keygen_info, EVP_PKEY_CTX_set_app_data, EVP_PKEY_CTX_get_app_data, EVP_PKEY_gen_cb, EVP_PKEY_paramgen, and EVP_PKEY_keygen — aka key and parameter generation and check functions ✨ (yes that's the actual manual name, we're in the 3ossl OpenSSL manual, hi hello 🫶)

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🌸 SYNOPSIS 🌸

#include <openssl/evp.h>

EVP_PKEY *EVP_PKEY_Q_keygen(OSSL_LIB_CTX *libctx, const char *propq,
                            const char *type, ...);

int EVP_PKEY_keygen_init(EVP_PKEY_CTX *ctx);
int EVP_PKEY_paramgen_init(EVP_PKEY_CTX *ctx);
int EVP_PKEY_generate(EVP_PKEY_CTX *ctx, EVP_PKEY **ppkey);
int EVP_PKEY_paramgen(EVP_PKEY_CTX *ctx, EVP_PKEY **ppkey);
int EVP_PKEY_keygen(EVP_PKEY_CTX *ctx, EVP_PKEY **ppkey);

typedef int EVP_PKEY_gen_cb(EVP_PKEY_CTX *ctx);

void EVP_PKEY_CTX_set_cb(EVP_PKEY_CTX *ctx, EVP_PKEY_gen_cb *cb);
EVP_PKEY_gen_cb *EVP_PKEY_CTX_get_cb(EVP_PKEY_CTX *ctx);

int EVP_PKEY_CTX_get_keygen_info(EVP_PKEY_CTX *ctx, int idx);

void EVP_PKEY_CTX_set_app_data(EVP_PKEY_CTX *ctx, void *data);
void *EVP_PKEY_CTX_get_app_data(EVP_PKEY_CTX *ctx);

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🦋 DESCRIPTION 🦋

so. generating keys. sometimes it's giving effortless, you just generate the key's numbers and you're done, no thoughts, head empty, slay. 💅 BUT — and this is the plot twist — some key types need key parameters too (people call them domain parameters, but like, not exclusively that, don't box them in). And on top of all that, the caller might wanna set their own user-provided generation parameters that further affect the key parameter or key generation, like "hey I want the key to be this big." 🎀

So to flexibly allow everything I just described (iconic of them), the whole thing is split into three acts:

1. initialize a key algorithm context
2. set user-provided parameters
3. the actual key parameter / key generation function itself

First things first, the key algorithm context has to be created with EVP_PKEY_CTX_new(3) or its variants — go read that manual for the deets, bestie. 🫶

• EVP_PKEY_keygen_init() — initializes a public key algorithm context ctx for a key generation operation.
• EVP_PKEY_paramgen_init() — literally the same vibe as EVP_PKEY_keygen_init(), except it's key parameters that get generated.

Once you've initialized, you can hand over your generation parameters with EVP_PKEY_CTX_ctrl(3) or EVP_PKEY_CTX_set_params(3), or literally any other function described in those manuals. no gatekeeping here 💜

• EVP_PKEY_generate() — does the actual generation operation. The resulting key parameters or key get written to *ppkey. And listen: if *ppkey is NULL when you call this, it gets allocated for you, and you (the caller) should free it with EVP_PKEY_free(3) when you're done with it. be a good citizen, free your keys 🧚
• EVP_PKEY_paramgen() and EVP_PKEY_keygen() — these do exactly the same thing as EVP_PKEY_generate(), but first they check that the matching EVP_PKEY_paramgen_init() or EVP_PKEY_keygen_init() was used to initialize ctx. They're the older functions kept around for backward compatibility. It is safe to use EVP_PKEY_generate() instead. periodt.

The function EVP_PKEY_set_cb() sets the key or parameter generation callback to cb. And EVP_PKEY_CTX_get_cb() returns the key or parameter generation callback. 🔁

EVP_PKEY_CTX_get_keygen_info() returns parameters associated with the generation operation. If idx is -1, the total number of available parameters is returned. Any non-negative value returns the value of that parameter. Heads up tho: EVP_PKEY_CTX_gen_keygen_info() with a nonnegative value for idx should only be called from within the generation callback. 👀

If the callback returns 0, then the key generation operation is aborted and an error occurs. This might happen during a time-consuming operation where a user clicks a "cancel" button. (the cancel button is the main character, honestly 💀)

EVP_PKEY_CTX_set_app_data() and EVP_PKEY_CTX_get_app_data() associate an opaque, application-defined pointer with an EVP_PKEY_CTX object. ✨

This pointer is not interpreted by the library at all — it is reserved entirely for use by the application. You can use it to store arbitrary context or state that needs to be reachable wherever the corresponding EVP_PKEY_CTX is available. it's your little secret pocket 🎀

EVP_PKEY_Q_keygen() abstracts away the whole explicit EVP_PKEY_CTX situation and gives you a 'quick' but limited way of generating a new asymmetric key pair. It's the shorthand for simple and common key generation cases. As usual, the library context libctx and property query propq can be given for fetching algorithms from providers. Then:

• if type is "RSA", a size_t parameter must be given to specify the size of the RSA key.
• if type is "EC", a string parameter must be given to specify the name of the EC curve. The list of built-in curves can be obtained programmatically using EC_get_builtin_curves(3), and their names can be retrieved using OSSL_EC_curve_nid2name(3).
• if type is: "ED25519", "ED448", "SM2", "X25519", "X448", "ML-DSA-44", "ML-DSA-65", "ML-DSA-87", "ML-KEM-512", "ML-KEM-768", or "ML-KEM-1024" — no further parameters are needed. 💅

Other key types may be possible if they're supplied by the loaded providers. EVP_PKEY_Q_keygen() may be usable with such key types as long as they do not require further parameters.

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⭐ RETURN VALUES ⭐

EVP_PKEY_keygen_init(), EVP_PKEY_paramgen_init(), EVP_PKEY_keygen() and EVP_PKEY_paramgen() return 1 for success and 0 or a negative value for failure. And specifically: a return value of -2 means the operation is not supported by the public key algorithm. 😭

EVP_PKEY_Q_keygen() returns an EVP_PKEY, or NULL on failure.

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🌈 NOTES 🌈

After the call to EVP_PKEY_keygen_init() or EVP_PKEY_paramgen_init(), algorithm-specific control operations can be performed to set any appropriate parameters for the operation. 🎛️

The functions EVP_PKEY_keygen() and EVP_PKEY_paramgen() can be called more than once on the same context if several operations are performed using the same parameters. reuse, recycle, iconic ♻️

The meaning of the parameters passed to the callback depends on the algorithm and the specific implementation of the algorithm. Some might not give any useful information at all during key or parameter generation. Others might not even call the callback. (ghosting, but make it cryptography 💀)

The operation performed by key or parameter generation depends on the algorithm used. In some cases (e.g. EC with a supplied named curve) the "generation" option merely sets the appropriate fields in an EVP_PKEY structure.

In OpenSSL, an EVP_PKEY structure containing a private key also contains the public key components and parameters (if any). An OpenSSL private key is equivalent to what some libraries call a "key pair". A private key can be used in functions which require the use of a public key or parameters. 👑

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🎀 EXAMPLES 🎀

Generate a 2048 bit RSA key:

#include <openssl/evp.h>
#include <openssl/rsa.h>

EVP_PKEY_CTX *ctx;
EVP_PKEY *pkey = NULL;

ctx = EVP_PKEY_CTX_new_id(EVP_PKEY_RSA, NULL);
if (!ctx)
    /* Error occurred */
if (EVP_PKEY_keygen_init(ctx) <= 0)
    /* Error */
if (EVP_PKEY_CTX_set_rsa_keygen_bits(ctx, 2048) <= 0)
    /* Error */

/* Generate key */
if (EVP_PKEY_keygen(ctx, &pkey) <= 0)
    /* Error */

Generate a key from a set of parameters:

#include <openssl/evp.h>
#include <openssl/rsa.h>

EVP_PKEY_CTX *ctx;
ENGINE *eng;
EVP_PKEY *pkey = NULL, *param;

/* Assumed param, eng are set up already */
ctx = EVP_PKEY_CTX_new(param, eng);
if (!ctx)
    /* Error occurred */
if (EVP_PKEY_keygen_init(ctx) <= 0)
    /* Error */

/* Generate key */
if (EVP_PKEY_keygen(ctx, &pkey) <= 0)
    /* Error */

Example of generation callback for OpenSSL public key implementations:

/* Application data is a BIO to output status to */

EVP_PKEY_CTX_set_app_data(ctx, status_bio);

static int genpkey_cb(EVP_PKEY_CTX *ctx)
{
    char c = '*';
    BIO *b = EVP_PKEY_CTX_get_app_data(ctx);
    int p = EVP_PKEY_CTX_get_keygen_info(ctx, 0);

    if (p == 0)
        c = '.';
    if (p == 1)
        c = '+';
    if (p == 2)
        c = '*';
    if (p == 3)
        c = '\n';
    BIO_write(b, &c, 1);
    (void)BIO_flush(b);
    return 1;
}

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👑 SEE ALSO 👑

EVP_RSA_gen(3), EVP_EC_gen(3), EVP_PKEY_CTX_new(3), EVP_PKEY_encrypt(3), EVP_PKEY_decrypt(3), EVP_PKEY_sign(3), EVP_PKEY_verify(3), EVP_PKEY_verify_recover(3), EVP_PKEY_derive(3) 🦋

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💀 HISTORY 💀

EVP_PKEY_keygen_init(), int EVP_PKEY_paramgen_init(), EVP_PKEY_keygen(), EVP_PKEY_paramgen(), EVP_PKEY_gen_cb(), EVP_PKEY_CTX_set_cb(), EVP_PKEY_CTX_get_cb(), EVP_PKEY_CTX_get_keygen_info(), EVP_PKEY_CTX_set_app_data() and EVP_PKEY_CTX_get_app_data() were added in OpenSSL 1.0.0.

EVP_PKEY_Q_keygen() and EVP_PKEY_generate() were added in OpenSSL 3.0. ✨

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📜 COPYRIGHT 📜

Copyright 2006-2026 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, we ate and left absolutely zero crumbs 🍓

sending you off with love from the OpenSSL 3.5.9 manual, dated 2026-09-29, page EVP_PKEY_KEYGEN(3ossl) — now go forth and generate some keys, you've got this 💖✨🫶