How do I decrypt a private key file and sign some text using openssl calls in C?

I have 2 separate programs (spliced together below). The first generates开发者_开发百科 the key pair and saves to files (works fine). The second opens the private key, decrypting with a pass phrase and then I need it to sign a string of text. The code below fails on the PEM_read_PrivateKey() (last) call (can't see why). Can anyone point me at what I am doing wrong and then what openssl calls I should make to use the private key to sign some text?

int main (int argc, char *argv[])
{
    char *priv_pem = "priv.pem";
    char *pub_pem  = "pub.pem";
    char *pass     = "Password";
    FILE *fp;   
    int bits = 4096;
    unsigned long exp = RSA_F4;

    RSA *rsa;
    EVP_PKEY *pkey;

    //  GENERATE KEY
    rsa=RSA_generate_key(bits,exp,NULL,NULL);
    if (RSA_check_key(rsa)!=1)
        Exit(1,"Error whilst checking key","");
    pkey = EVP_PKEY_new();
    EVP_PKEY_assign_RSA(pkey, rsa);

    //  WRITE ENCRYPTED PRIVATE KEY
    if (!(fp = fopen(priv_pem, "w")))
        Exit(2,"Error opening PEM file",priv_pem);
    if (!PEM_write_PrivateKey(fp,pkey,EVP_aes_256_cbc(),NULL,0,NULL,pass))
        Exit(3,"Error writing PEM file",priv_pem);
    fclose(fp);

    //  WRITE PUBLIC KEY
    if (!(fp = fopen(pub_pem, "w")))
        Exit(4,"Error opening PEM file",pub_pem);
    if (!PEM_write_PUBKEY(fp, pkey))
        Exit(5,"Error writing PEM file",pub_pem);
    fclose(fp);
    // ------- End of key generation program -------

    // ------- Start of text signing program -------
    //  READ IN ENCRYPTED PRIVATE KEY
    if (!(fp = fopen(priv_pem, "r")))
        Exit(6,"Error reading encrypted private key file",priv_pem);
    if (!PEM_read_PrivateKey(fp,&pkey,NULL,pass))
        Exit(7,"Error decrypting private key file",priv_pem);
    fclose(fp);

    // Sign some text using the private key....

    //  FREE
    RSA_free(rsa);  
    return 0;
}

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Have you initialised pkey to NULL before you pass &pkey to PEM_read_PrivateKey()? If not, it will attempt to re-use the EVP_PKEY structure that pkey points to - and if pkey is uninitialised, it will be looking at a random spot in memory.

You can use ERR_print_errors_fp(stderr); to dump the OpenSSL error stack to stderr when an error occurs - this is often helpful in finding the problem.

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Thanks @caf for your help. By trial and error I fixed PEM_read_PrivateKey() error by adding the following to the start:

if (EVP_get_cipherbyname("aes-256-cbc") == NULL)
    OpenSSL_add_all_algorithms();

However, I'm still looking for the best (practice) way of generating the keys and then using the private key for signing. From my limited understanding, I am looking for openssl methods that adhere to RSA's "PKCS #1 v2.0: RSA Cryptography Standard"