EVP_PKEY_KEYGEN(3)

📦 version 4.4.0, in alpine 3.24, arch rolling

💖 NAME 💖

hey bestie, welcome to EVP_PKEY_KEYGEN(3) — straight from the Library Functions Manual. this page is the whole key-and-parameter generation squad: EVP_PKEY_keygen_init, EVP_PKEY_keygen, EVP_PKEY_paramgen_init, EVP_PKEY_paramgen, EVP_PKEY_gen_cb, EVP_PKEY_CTX_set_cb, EVP_PKEY_CTX_get_cb, EVP_PKEY_CTX_set0_keygen_info, EVP_PKEY_CTX_get_keygen_info, EVP_PKEY_CTX_set_app_data, EVP_PKEY_CTX_get_app_data, EVP_PKEY_CTX_set_data, EVP_PKEY_CTX_get_data — key and parameter generation functions. periodt. 💅

✨ SYNOPSIS ✨

you're in library “libcrypto” land, and the include is #include <openssl/evp.h>.

int
EVP_PKEY_keygen_init(EVP_PKEY_CTX *ctx);

int
EVP_PKEY_keygen(EVP_PKEY_CTX *ctx, EVP_PKEY **ppkey);

int
EVP_PKEY_paramgen_init(EVP_PKEY_CTX *ctx);

int
EVP_PKEY_paramgen(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);

void
EVP_PKEY_CTX_set0_keygen_info(EVP_PKEY_CTX *ctx, int *dat, int datlen);

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 *app_data);

void *
EVP_PKEY_CTX_get_app_data(EVP_PKEY_CTX *ctx);

void
EVP_PKEY_CTX_set_data(EVP_PKEY_CTX *ctx, void *data);

void *
EVP_PKEY_CTX_get_data(EVP_PKEY_CTX *ctx);

💅 DESCRIPTION 💅

okay so EVP_PKEY_keygen_init() is the setup girlie: it initializes a public key algorithm context using key ctx->pkey for a key generation operation.

EVP_PKEY_keygen() is the actual main character move — it performs a key generation operation, and the generated key gets written to ppkey. slay.

the functions EVP_PKEY_paramgen_init() and EVP_PKEY_paramgen() are basically the same vibe, except parameters are generated instead of keys.

EVP_PKEY_CTX_set_cb() and EVP_PKEY_CTX_get_cb() set and retrieve the key or parameter generation callback, respectively. callback management, but make it cute.

the function EVP_PKEY_CTX_set0_keygen_info() sets the parameters associated with the generation operation to the array dat containing datlen integer parameters. important bestie: the caller retains ownership of the dat array; it will never be freed by the library. so do not come for that memory, because it is not yours. 🫶

the function EVP_PKEY_CTX_get_keygen_info() returns parameters associated with the generation operation. if idx is -1, the total number of parameters available is returned. any non-negative value returns the value of that parameter. and EVP_PKEY_CTX_get_keygen_info() with a non-negative value for idx should only be called within the generation callback. iykyk.

if the callback returns 0, then the key generation operation is aborted and an error occurs. this might occur during a time consuming operation where a user clicks on a "cancel" button. very real user behavior, honestly.

EVP_PKEY_CTX_set_app_data() and EVP_PKEY_CTX_get_app_data() set and retrieve an opaque pointer. this can be used to set some application defined value which can be retrieved in the callback: for example a handle which is used to update a "progress dialog". progress bar lore. ✨

the deprecated functions EVP_PKEY_CTX_set_data() and EVP_PKEY_CTX_get_data() set and retrieve a different opaque pointer that is ignored by the library. deprecated means deprecated, bestie — do not build your whole personality on these. 💀

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 the context, it's giving efficiency.

the meaning of the parameters passed to the callback will depend 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. so if your callback is not getting called, that can just be the algorithm's whole attitude — no shade.

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. not every "generate" is out here generating from scratch, okay?

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. so yes, the private key is lowkey carrying the whole ensemble. 👑

🎀 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. in particular, a return value of -2 indicates the operation is not supported by the public key algorithm. rude, but noted.

callback functions of the type EVP_PKEY_gen_cb() are supposed to return 1 on success or 0 on error.

EVP_PKEY_CTX_get_cb() returns a function pointer to the currently installed callback function or NULL if no callback function is installed. no callback? NULL. iconic absence.

EVP_PKEY_CTX_get_keygen_info() returns the number of available parameters if idx is -1, one of these parameters if idx is greater than or equal to zero but less than the number of available parameters, or 0 otherwise.

EVP_PKEY_CTX_get_app_data() and EVP_PKEY_CTX_get_data() return the pointer that was last passed to the corresponding set function, or NULL if the corresponding set function was never called on ctx. if you never set it, you get NULL — no magical mystery pointer, bestie. 🦋

🧚 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;
EVP_PKEY *pkey = NULL, *param;

/* Assumes that param is already set up. */
ctx = EVP_PKEY_CTX_new(param, NULL);
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;

        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;
}

🦋 SEE ALSO 🦋

⭐ HISTORY ⭐

these functions first appeared in OpenSSL 1.0.0 and have been available since OpenBSD 4.9. vintage-ish, but still relevant. 🌈

xoxo, Debian 💖 this page is dated June 8, 2025, and it's EVP_PKEY_KEYGEN(3) — byeee 🫶

📦 where this page comes from