OpenSSL i2o_ECPublicKey not working

Question

I have this code:

#include <stdio.h>
#include <openssl/sha.h>
#include <openssl/ssl.h>

int main(){
    printf("OpenSSL version: %s\n",OPENSSL_VERSION_TEXT);
    EC_KEY * key = EC_KEY_new_by_curve_name(NID_secp256k1);
    if(!EC_KEY_generate_key(key)){
        printf("GENERATE KEY FAIL\n"); 
        return 1;
    }
    u_int8_t pubSize = i2o_ECPublicKey(key, NULL);
    if(!pubSize){
        printf("PUB KEY TO DATA ZERO\n"); 
        return 1;
    }
    u_int8_t * pubKey = malloc(pubSize);
    if(i2o_ECPublicKey(key, &pubKey) != pubSize){
        printf("PUB KEY TO DATA FAIL\n"); 
        return 1;
    }
    u_int8_t * hash = malloc(SHA256_DIGEST_LENGTH);
    SHA256(pubKey, pubSize, hash);
    for (int x = 0; x < 32; x++) {
        printf("%.2x",hash[x]);
    }
    EC_KEY_free(key);
    free(pubKey);
    free(hash);
    return 0;
}

As you see I'm trying to hash a public key and print it. The SHA hash fails sha256_block_data_order. Here's more information...

The version is given as: OpenSSL 1.0.1c 10 May 2012 pubSize is set to 65

After the second i2o_ECPublicKey, the pubKey data is somehow invalidated:

(gdb) p/x *pubKey @ 65
Cannot access memory at address 0x4d0ff1

However before the second i2o_ECPublicKey, the allocated pubKey data gives:

(gdb) p/x *pubKey @ 65
$1 = {0x0 <repeats 65 times>}

So the malloc allocation is fine. The second i2o_ECPublicKey call doesn't work as expected. How do I read a EC public key into bytes?

Thank you.

Answer 1

i2o_ECPublicKey moves your pointer by the number of bytes written to the buffer, so that it is at the end of what was written. You will need to pass in a copy of the pointer.

The following change fixes it for me:

            u_int8_t * pubKey = malloc(pubSize);
    +       u_int8_t * pubKey2 = pubKey;
    -       if(i2o_ECPublicKey(key, &pubKey) != pubSize){
    +       if(i2o_ECPublicKey(key, &pubKey2) != pubSize){
                printf("PUB KEY TO DATA FAIL\n");