Random characters when retrieving private/public KEY (OpenSSL) from file

Question

I am trying to make a program that after it generates a public/private key pair using OppenSSL EC function EC_KEY_generate_key store them in separate files and retrieve them to generate an ECDH KEY.

My problem is that although I store them correctly(without any additional character), when I read the file and try to convert the hex characters to BIGNUM a character '04' or '00' randomly appears (or even sometimes not). So when I try to set the public/private keys and check the whole key, it fails. Could anyone help me with this issue? Could the key checking failure be caused by these characters or they are normal?

Here is my code that generates/stores the private key(the public one is the same):

    EC_KEY *b = NULL;
const BIGNUM *ppriv_b;
FILE *claveprivb;
const EC_GROUP *group;

b = EC_KEY_new_by_curve_name(NID_X9_62_prime192v1);
group = EC_KEY_get0_group(b);

EC_KEY_generate_key(b);
    claveprivb = fopen("/tmp/mnt/claveprivb", "w+");
    ppriv_b = EC_KEY_get0_private_key(b);
if ((ppriv_b != NULL)) 
    BN_print_fp(claveprivb,ppriv_b);
    fclose(claveprivb);

    //Afterwards do the same with the public key

And here is my code for retrieving the Private key:

    int i, s, blen, bout, ret = 0;
unsigned char *bbuf;
FILE *clavepriv, *clavetotalb;
const char cpriv_string[PRIVATE_KEY_SIZE];
BIGNUM *priv;
EC_KEY *b = NULL;
const EC_GROUP *groupb;

    b = EC_KEY_new_by_curve_name(NID_X9_62_prime192v1);
groupb = EC_KEY_get0_group(b);
    //Open the file with the hexadecimals (PRIVATE KEY)
    clavepriv = fopen("/tmp/mnt/claveprivb", "r");
kk2 = fread(&cpriv_string, sizeof(char), PRIVATE_KEY_SIZE, clavepriv);

priv = BN_new();
    //THIS FUNCTION (HEX2BN) GENERATES THE RANDOM CHARACTER: 
kk2 = BN_hex2bn(&priv, cpriv_string);
ret = EC_KEY_set_private_key(b, priv);

    //HERE I retrieve the public key by the same way and set it into EC_KEY b,
    //the same random character appears in the public key

    if (!EC_KEY_check_key(b)) {
    printf("EC_KEY_check_key failed\n");
} else {
    printf("Key verified OK\n");
}
    //It fails when try to check it.

int k;
clavetotalb = fopen("/tmp/mnt/clavetotalb", "w+");
k = EC_KEY_print_fp(clavetotalb, b, 0);

bout = ECDH_compute_key(bbuf, blen, EC_KEY_get0_public_key(b), b,
        KDF1_SHA1);

Any advise would be very appreciated!!!!Thanks!!!

After I read the answering post, I have tried to use these methods to decode and encode the public key, but by the time I try to compute the ECDH key I get a segmentation fault. The target of my program is to generate two EC keys, write them into several files and then, retrieve them and compute an ECDH key with them. This is the list of the things I change from my original program in the first thread, please tell me if something is wrong:

* Generate EC key (public & private)
* Decode the private key with i2d_ECPrivatekey()
* Decode the public key with i2o_ECPublickey()
* Write them into several files.
* Read the file with the public key.
* Encode it with o2i_ECPublickey()
* Read the file with the private key.
* Encode it with d2i_ECPrivatekey().
* Compute the ECDH key.(Here is where I get the segmentation fault)

I am quite fed up with this OpenSSL library...it is so inaccessible for first-time users...