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;

}

I get this error after second i2o_ECPublicKey:

(gdb) p/x *pubKey @ 65

Cannot access memory at address 0x4d0ff1

Please help.

Answer 1

When your right something in the buffer, the pointer is moved to point to the location at the end of what you have written. So, i2o_ECPublicKey moves the pointer by the number of bytes you have written into the buffer.

Try this code and it’ll work:

           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");