genrate_ecc_keys.c

// gcc -Wall ecdsapubkey.c -o ecdsapubkey -lcrypto
#include <stdio.h>
#include <stdlib.h>
#include <openssl/ec.h>
#include <openssl/obj_mac.h>
#include <openssl/bn.h>

int main()
{
     EC_KEY *eckey = NULL;
     EC_POINT *pub_key = NULL;
     const EC_GROUP *group = NULL;
     BIGNUM *start;
     BIGNUM *res;
     BN_CTX *ctx;

     start = BN_new();
     ctx = BN_CTX_new(); // ctx is an optional buffer to save time from allocating and deallocating memory whenever required

     res = start;
     BN_hex2bn(&res,"8A9A35145C4EA5260DF9972C804FE2D3F9F3D7A2AC01A6BEB21C82BB30957B3952273AC9166B90C1207347A925780F84A1D2359E7AA05201C674D2B9746FCA07");
     eckey = EC_KEY_new_by_curve_name(NID_secp256k1);
    
     group = EC_KEY_get0_group(eckey);
     pub_key = EC_POINT_new(group);


	  printf("private key : "); BN_print_fp(stdout, res); printf("\n");
 	  EC_KEY_set_private_key(eckey, res);
     /* pub_key is a new uninitialized `EC_POINT*`.  priv_key res is a `BIGNUM*`. */
     if (!EC_POINT_mul(group, pub_key, res, NULL, NULL, ctx))
       printf("Error at EC_POINT_mul.\n");

//     assert(EC_POINT_bn2point(group, &res, pub_key, ctx)); // Null here

     EC_KEY_set_public_key(eckey, pub_key);

     char *cc = EC_POINT_point2hex(group, pub_key, 4, ctx);

     char *c=cc;

     int i;
	
printf("public key : ");
     for (i=0; i<130; i++) // 1 byte 0x42, 32 bytes for X coordinate, 32 bytes for Y coordinate
     {
       printf("%c", *c++);
     }

     printf("\n");

     BN_CTX_free(ctx);

     free(cc);

     return 0;
}