Selections

The following constants can be used for selection:

EVP_PKEY_KEY_PARAMETERS

Only key parameters will be selected.

EVP_PKEY_PUBLIC_KEY

Only public key components will be selected. This includes optional key parameters.

EVP_PKEY_KEYPAIR

Any keypair components will be selected. This includes the private key, public key and key parameters.

Creating an RSA keypair using raw key data

#include <openssl/evp.h>
/*
 * These are extremely small to make this example simple.  A real
 * and secure application will not use such small numbers.  A real
 * and secure application is expected to use BIGNUMs, and to build
 * this array dynamically.
 */
unsigned long rsa_n = 0xbc747fc5;
unsigned long rsa_e = 0x10001;
unsigned long rsa_d = 0x7b133399;
OSSL_PARAM params[] = {
    OSSL_PARAM_ulong("n", &rsa_n),
    OSSL_PARAM_ulong("e", &rsa_e),
    OSSL_PARAM_ulong("d", &rsa_d),
    OSSL_PARAM_END
};
int main()
{
    EVP_PKEY_CTX *ctx = EVP_PKEY_CTX_new_from_name(NULL, "RSA", NULL);
    EVP_PKEY *pkey = NULL;
    if (ctx == NULL
        || EVP_PKEY_fromdata_init(ctx) <= 0
        || EVP_PKEY_fromdata(ctx, &pkey, EVP_PKEY_KEYPAIR, params) <= 0)
        exit(1);
    /* Do what you want with |pkey| */
}

Creating an ECC keypair using raw key data

#include <openssl/evp.h>
#include <openssl/param_build.h>
#include <openssl/ec.h>
/*
 * Fixed data to represent the private and public key.
 */
const unsigned char priv_data[] = {
    0xb9, 0x2f, 0x3c, 0xe6, 0x2f, 0xfb, 0x45, 0x68,
    0x39, 0x96, 0xf0, 0x2a, 0xaf, 0x6c, 0xda, 0xf2,
    0x89, 0x8a, 0x27, 0xbf, 0x39, 0x9b, 0x7e, 0x54,
    0x21, 0xc2, 0xa1, 0xe5, 0x36, 0x12, 0x48, 0x5d
};
/* UNCOMPRESSED FORMAT */
const unsigned char pub_data[] = {
    POINT_CONVERSION_UNCOMPRESSED,
    0xcf, 0x20, 0xfb, 0x9a, 0x1d, 0x11, 0x6c, 0x5e,
    0x9f, 0xec, 0x38, 0x87, 0x6c, 0x1d, 0x2f, 0x58,
    0x47, 0xab, 0xa3, 0x9b, 0x79, 0x23, 0xe6, 0xeb,
    0x94, 0x6f, 0x97, 0xdb, 0xa3, 0x7d, 0xbd, 0xe5,
    0x26, 0xca, 0x07, 0x17, 0x8d, 0x26, 0x75, 0xff,
    0xcb, 0x8e, 0xb6, 0x84, 0xd0, 0x24, 0x02, 0x25,
    0x8f, 0xb9, 0x33, 0x6e, 0xcf, 0x12, 0x16, 0x2f,
    0x5c, 0xcd, 0x86, 0x71, 0xa8, 0xbf, 0x1a, 0x47
};
int main()
{
    EVP_PKEY_CTX *ctx;
    EVP_PKEY *pkey = NULL;
    BIGNUM *priv;
    OSSL_PARAM_BLD *param_bld;
    OSSL_PARAM *params = NULL;
    int exitcode = 0;
    priv = BN_bin2bn(priv_data, sizeof(priv_data), NULL);
    param_bld = OSSL_PARAM_BLD_new();
    if (priv != NULL && param_bld != NULL
        && OSSL_PARAM_BLD_push_utf8_string(param_bld, "group",
                                           "prime256v1", 0)
        && OSSL_PARAM_BLD_push_BN(param_bld, "priv", priv)
        && OSSL_PARAM_BLD_push_octet_string(param_bld, "pub",
                                            pub_data, sizeof(pub_data)))
        params = OSSL_PARAM_BLD_to_param(param_bld);
    ctx = EVP_PKEY_CTX_new_from_name(NULL, "EC", NULL);
    if (ctx == NULL
        || params == NULL
        || EVP_PKEY_fromdata_init(ctx) <= 0
        || EVP_PKEY_fromdata(ctx, &pkey, EVP_PKEY_KEYPAIR, params) <= 0) {
        exitcode = 1;
    } else {
        /* Do what you want with |pkey| */
    }
    EVP_PKEY_free(pkey);
    EVP_PKEY_CTX_free(ctx);
    OSSL_PARAM_free(params);
    OSSL_PARAM_BLD_free(param_bld);
    BN_free(priv);
    exit(exitcode);
}

Finding out params for an unknown key type

#include <openssl/evp.h>
#include <openssl/core.h>
/* Program expects a key type as first argument */
int main(int argc, char *argv[])
{
    EVP_PKEY_CTX *ctx = EVP_PKEY_CTX_new_from_name(NULL, argv[1], NULL);
    const OSSL_PARAM *settable_params = NULL;
    if (ctx == NULL)
       exit(1);
   settable_params = EVP_PKEY_fromdata_settable(ctx, EVP_PKEY_KEYPAIR);
   if (settable_params == NULL)
        exit(1);
    for (; settable_params->key != NULL; settable_params++) {
        const char *datatype = NULL;
        switch (settable_params->data_type) {
        case OSSL_PARAM_INTEGER:
            datatype = "integer";
            break;
        case OSSL_PARAM_UNSIGNED_INTEGER:
            datatype = "unsigned integer";
            break;
        case OSSL_PARAM_UTF8_STRING:
            datatype = "printable string (utf-8 encoding expected)";
            break;
        case OSSL_PARAM_UTF8_PTR:
            datatype = "printable string pointer (utf-8 encoding expected)";
            break;
        case OSSL_PARAM_OCTET_STRING:
            datatype = "octet string";
            break;
        case OSSL_PARAM_OCTET_PTR:
            datatype = "octet string pointer";
            break;
        }
        printf("%s : %s ", settable_params->key, datatype);
        if (settable_params->data_size == 0)
            printf("(unlimited size)\n");
        else
            printf("(maximum size %zu)\n", settable_params->data_size);
    }
}

The descriptor OSSL_PARAM(3) returned by EVP_PKEY_fromdata_settable() may also be used programmatically, for example with OSSL_PARAM_allocate_from_text(3).