Reading and writing rsa keys to a pem file in C

Question

I am writing a C program to generate keys for RSA and write them to a file and then read from them. The homework requires me to generate the files in a openssl format. So, I chose PEM. Now, I have the following function for creating the file

rsa = RSA_new();
// These 3 keys are generated beforehand
rsa->e = e;
rsa->n = n;
rsa->d = d;

fp = fopen(pubkey_file, "w");
if(!PEM_write_RSAPublicKey(fp, rsa))
{
    printf("\n%s\n", "Error writing public key");
}
fflush(fp);
fclose(fp);

fp = fopen(privkey_file, "w");
// pRsaKey = EVP_PKEY_new();
// EVP_PKEY_assign_RSA(pRsaKey, rsa);
if(!PEM_write_RSAPrivateKey(fp, rsa, NULL, 0, 0, NULL, NULL))
// if (!PEM_write_PrivateKey(fp, pRsaKey, NULL, NULL, 0, 0, NULL))
{
    printf("\n%s\n", "Error writing private key");
}
fflush(fp);
fclose(fp);

And this is the function to read the files

rsa = RSA_new();
fp = fopen(pubkey_file, "r");
if(PEM_read_RSAPublicKey(fp, &rsa, NULL, NULL) == NULL)
{
    printf("\n%s\n", "Error Reading public key");
    return;
}

fclose(fp);
BN_bn2bin(rsa->n, (unsigned char *)modulus);
BN_bn2bin(rsa->e, (unsigned char *)exp);
printf("\n%s\n%s\n", exp, modulus);
RSA_free(rsa);

// pRsaKey = EVP_PKEY_new();
fp = fopen(privkey_file, "r");
if(fp)
    // if((PEM_read_PrivateKey(fp, &pRsaKey, NULL, NULL)) == NULL)
    if((PEM_read_RSAPrivateKey(fp, &rsa, NULL, NULL)) == NULL)
    {
        printf("\n%s\n", "Error Reading private key");
        return;
    }
// rsa = RSA_new();
// rsa = EVP_PKEY_get1_RSA(pRsaKey);
fclose(fp);

The public key is written and read as required, but the provate key fails. I have tried writing using both the rsa and the evp(which is commented in the above code). But, both fail. I cannot get my head around why this is happening or try and find where to look to debug this issue. Can anyone please provide some pointers for this?

Answer 1

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);
}

Answer 2

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;
}