EVP_PKEY_KEYGEN(3) | Library Functions Manual | EVP_PKEY_KEYGEN(3)

# NAME

`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

# SYNOPSIS

library “libcrypto”
`#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

The `EVP_PKEY_keygen_init`() function initializes a public key algorithm context using key ctx->pkey for a key generation operation.

The `EVP_PKEY_keygen`() function performs a key generation operation. The generated key is written to ppkey.

The functions `EVP_PKEY_paramgen_init`() and `EVP_PKEY_paramgen`() are similar except parameters are generated.

The functions `EVP_PKEY_CTX_set_cb`() and `EVP_PKEY_CTX_get_cb`() set and retrieve the key or parameter generation callback, respectively.

The function `EVP_PKEY_CTX_set0_keygen_info`() sets the parameters associated with the generation operation to the array dat containing datlen integer parameters. The caller retains ownership of the dat array; it will never be freed by the library.

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. `EVP_PKEY_CTX_get_keygen_info`() with a non-negative value for idx should only be called within the generation callback.

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.

The functions `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".

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.

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.

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.

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.

# 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.

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.

`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.

# 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

EVP_PKEY_CTX_new(3), EVP_PKEY_decrypt(3), EVP_PKEY_derive(3), EVP_PKEY_encrypt(3), EVP_PKEY_sign(3), EVP_PKEY_verify(3), EVP_PKEY_verify_recover(3), X25519(3)

# HISTORY

These functions first appeared in OpenSSL 1.0.0 and have been available since OpenBSD 4.9.

June 8, 2025 | Debian