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EVP_PKEY_KEYGEN(3) Library Functions Manual EVP_PKEY_KEYGEN(3)

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 — key and parameter generation functions

#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 *data);

void *
EVP_PKEY_CTX_get_app_data(EVP_PKEY_CTX *ctx);

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

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

The functions () and () are similar except parameters are generated.

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

The function () 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 () 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 () and () 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".

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

The functions () and () 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.

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.

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;
}

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

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

July 21, 2024 OpenBSD-current