Reading and writing rsa keys to a pem file in C

RSA keys are symmetrical, you can use any of them as a private key or public key, it is just a matter of your choice (but DSA keys are NOT symmetrical). The program below generates two 2048 bits long RSA keys, then it saves them to files and reads them back into memory. That should give you the idea how to do it.

#include <stdio.h>#include <string.h>#include <sys/types.h>#include <sys/stat.h>#include <openssl/bio.h>#include <openssl/err.h>#include <openssl/rsa.h>#include <openssl/evp.h>#include <openssl/pem.h>const char* pcszPassphrase = "open sezamee";static void gen_callback(int iWhat, int inPrime, void* pParam);static void init_openssl(void);static void cleanup_openssl(void);static int passwd_callback(char *pcszBuff,int size,int rwflag, void *pPass);static EVP_PKEY* create_rsa_key(void);static void handle_openssl_error(void);int main(int argc, char **argv){    int iRet = EXIT_SUCCESS;    EVP_PKEY* pPrivKey = NULL;    EVP_PKEY* pPubKey  = NULL;    FILE*     pFile    = NULL;    const EVP_CIPHER* pCipher = NULL;    init_openssl();    pPrivKey = create_rsa_key();    pPubKey  = create_rsa_key();    if(pPrivKey && pPubKey)    {/* Save the keys */        if((pFile = fopen("privkey.pem","wt")) && (pCipher = EVP_aes_256_cbc()))        {            if(!PEM_write_PrivateKey(pFile,pPrivKey,pCipher,                                    (unsigned char*)pcszPassphrase,                                    (int)strlen(pcszPassphrase),NULL,NULL))            {                fprintf(stderr,"PEM_write_PrivateKey failed.\n");                handle_openssl_error();                iRet = EXIT_FAILURE;            }            fclose(pFile);            pFile = NULL;            if(iRet == EXIT_SUCCESS)            {                if((pFile = fopen("pubkey.pem","wt")) && PEM_write_PUBKEY(pFile,pPubKey))                    fprintf(stderr,"Both keys saved.\n");                else                {                    handle_openssl_error();                    iRet = EXIT_FAILURE;                }                if(pFile)                {                    fclose(pFile);                    pFile = NULL;                }            }        }        else        {            fprintf(stderr,"Cannot create \"privkey.pem\".\n");            handle_openssl_error();            iRet = EXIT_FAILURE;            if(pFile)            {                fclose(pFile);                pFile = NULL;            }        }        if(iRet == EXIT_SUCCESS)        {/* Read the keys */            EVP_PKEY_free(pPrivKey);            pPrivKey = NULL;            EVP_PKEY_free(pPubKey);            pPubKey = NULL;            if((pFile = fopen("privkey.pem","rt")) &&                (pPrivKey = PEM_read_PrivateKey(pFile,NULL,passwd_callback,(void*)pcszPassphrase)))            {                fprintf(stderr,"Private key read.\n");            }            else            {                fprintf(stderr,"Cannot read \"privkey.pem\".\n");                handle_openssl_error();                iRet = EXIT_FAILURE;            }            if(pFile)            {                fclose(pFile);                pFile = NULL;            }            if((pFile = fopen("pubkey.pem","rt")) &&                (pPubKey = PEM_read_PUBKEY(pFile,NULL,NULL,NULL)))            {                fprintf(stderr,"Public key read.\n");            }            else            {                fprintf(stderr,"Cannot read \"pubkey.pem\".\n");                handle_openssl_error();                iRet = EXIT_FAILURE;            }        }    }    if(pPrivKey)    {        EVP_PKEY_free(pPrivKey);        pPrivKey = NULL;    }    if(pPubKey)    {        EVP_PKEY_free(pPubKey);        pPubKey = NULL;    }    cleanup_openssl();    return iRet;}EVP_PKEY* create_rsa_key(void){    RSA *pRSA      = NULL;    EVP_PKEY* pKey = NULL;    pRSA = RSA_generate_key(2048,RSA_3,gen_callback,NULL);    pKey = EVP_PKEY_new();    if(pRSA && pKey && EVP_PKEY_assign_RSA(pKey,pRSA))    {        /* pKey owns pRSA from now */        if(RSA_check_key(pRSA) <= 0)        {            fprintf(stderr,"RSA_check_key failed.\n");            handle_openssl_error();            EVP_PKEY_free(pKey);            pKey = NULL;        }    }    else    {        handle_openssl_error();        if(pRSA)        {            RSA_free(pRSA);            pRSA = NULL;        }        if(pKey)        {            EVP_PKEY_free(pKey);            pKey = NULL;        }    }    return pKey;}void gen_callback(int iWhat, int inPrime, void* pParam){    char c='*';    switch(iWhat)    {        case 0: c = '.';  break;        case 1: c = '+';  break;        case 2: c = '*';  break;        case 3: c = '\n'; break;    }    fprintf(stderr,"%c",c);}int passwd_callback(char *pcszBuff,int size,int rwflag, void *pPass){    size_t unPass = strlen((char*)pPass);    if(unPass > (size_t)size)        unPass = (size_t)size;    memcpy(pcszBuff, pPass, unPass);    return (int)unPass;}void init_openssl(void){    if(SSL_library_init())    {        SSL_load_error_strings();        OpenSSL_add_all_algorithms();        RAND_load_file("/dev/urandom", 1024);    }    else        exit(EXIT_FAILURE);}void cleanup_openssl(void){    CRYPTO_cleanup_all_ex_data();    ERR_free_strings();    ERR_remove_thread_state(0);    EVP_cleanup();}void handle_openssl_error(void){    ERR_print_errors_fp(stderr);}

The answer given by sirgeorge has a mistake in the Code. The create_rsa_key should not be called twice. If it is called twice then private key does not have matching public key. This results in problems during decryption.Modifications needed in main method

RSA *pRSA      = NULL;pRSA = RSA_generate_key(2048,RSA_3,gen_callback,NULL);    pPrivKey = create_rsa_key(pRSA);pPubKey  = create_rsa_key(pRSA);

Modifications needed in create_rsa_key

EVP_PKEY* create_rsa_key(RSA *pRSA){    EVP_PKEY* pKey = NULL;    pKey = EVP_PKEY_new();    if(pRSA && pKey && EVP_PKEY_assign_RSA(pKey,pRSA))    {        /* pKey owns pRSA from now */        if(RSA_check_key(pRSA) <= 0)        {            fprintf(stderr,"RSA_check_key failed.\n");            handle_openssl_error();            EVP_PKEY_free(pKey);            pKey = NULL;        }    }    else    {        handle_openssl_error();        if(pRSA)        {            RSA_free(pRSA);            pRSA = NULL;        }        if(pKey)        {            EVP_PKEY_free(pKey);            pKey = NULL;        }    }    return pKey;}