trouble converting from void pointer to char pointer - c

Question

I'm using the paillier library for cryptography.
There is a function namely paillier_ciphertext_to_bytes which converts to bytes. Upon checking the paillier.h, it has the return type void*.
I would like to take the string this generates which I believe is hex, and convert it to a decimal number using strtol.
However, when I cast the output of paillier_ciphertext_to_bytes to a char*, it doesn't work as expected.
This is my code
#include<stdio.h>
#include<stdlib.h>
#include<gmp.h>
//#include<paillier.h>
#include"paillier.h"
#include <string.h>
#include<malloc.h>
int main()
{
int n=4; //degree of polynomial
int i=0;
char* str;
paillier_pubkey_t *pub_key=(paillier_pubkey_t *)malloc(sizeof(paillier_pubkey_t));
paillier_prvkey_t *prv_key=(paillier_prvkey_t *)malloc(sizeof(paillier_prvkey_t));
paillier_keygen(4,&pub_key,&prv_key,paillier_get_rand_devrandom);
for(i=0;i<n;i++)
{
unsigned int p=rand()%20;
paillier_plaintext_t *ptext = paillier_plaintext_from_ui(p);
paillier_ciphertext_t *ctext;
ctext = paillier_enc(0, pub_key, ptext, paillier_get_rand_devrandom);
str = (char*)paillier_ciphertext_to_bytes(n,ctext);
printf("str==[%s]\n",str);
printf("number_str==[%d]\n",(int)strtol(str,NULL,16));
}
return 0;
}
This is the output I get
str==[]
number_str==[0]
str==[]
number_str==[0]
str==[]
number_str==[0]
str==[]
number_str==[0]
This is the paillier.h library code, where I looked up the signature of paillier_ciphertext_to_bytes
Side note,
I only want to convert the encryption to a number. Actually there is no reason for me to believe that paillier_ciphertext_to_bytes can be cast to a char*. So, I've also tried converting it to an int* and then print out it's value as follows
int* myintp;
myintp = (int*)paillier_ciphertext_to_bytes(n,ctext);
printf("number==[%d]\n",*myintp);
This always give me an integer number, but I'm not sure if this is correct(the decimal representation of the encrypted string)
EDIT :
As per Felix's comment, I've tried this
#include<stdio.h>
#include<stdlib.h>
#include<gmp.h>
//#include<paillier.h>
#include"paillier.h"
#include <string.h>
#include<malloc.h>
int main()
{
int n=4; //degree of polynomial
int i=0;
void* myvoid;
FILE* fp;
fp = fopen("file.txt","a");
paillier_pubkey_t *pub_key=(paillier_pubkey_t *)malloc(sizeof(paillier_pubkey_t));
paillier_prvkey_t *prv_key=(paillier_prvkey_t *)malloc(sizeof(paillier_prvkey_t));
paillier_keygen(4,&pub_key,&prv_key,paillier_get_rand_devrandom);
for(i=0;i<n;i++)
{
unsigned int p=rand()%20;
paillier_plaintext_t *ptext = paillier_plaintext_from_ui(p);
paillier_ciphertext_t *ctext;
ctext = paillier_enc(0, pub_key, ptext, paillier_get_rand_devrandom);
myvoid=paillier_ciphertext_to_bytes(PAILLIER_BITS_TO_BYTES(pub_key->bits)*2, ctext);
fwrite(myvoid, 1, PAILLIER_BITS_TO_BYTES(pub_key->bits)*2, fp);
printf("as int : %d\n",*(int*)myvoid);
printf("as str : %d\n",(int)strtol((char*)myvoid,NULL,16));
}
fclose(fp);
return 0;
}
But I get unprintable characters in my output file when I try to print myvoid, and when I cast to integer and string, I get this output
as int : 31136768
as str : 0
as int : 493106432
as str : 0
as int : 493111552
as str : 0
as int : 493092864
as str : 0

Answer 1

Without knowing the library in question, it's pretty clear from the name of the function what it does: It gives you the actual encrypted bytes contained in some library-specific container type. Of course you can just take it as char * (there's no need to cast and you shouldn't, but that's another question).
Typically encrypted data contains any possible byte value, which includes 0 as well as non-printable characters and so on. So, having a char * pointer to it doesn't make it a C string. What you would do now is encode it in some ascii encoding, often base64 is used. This would give you a string representation suitable for writing to a file or sending in an email.
About the part of interpreting it as an int: This is a very bad idea. As encrypted data can contain any possible byte values, it really should only be treated as this: bytes. With int you could have for example two different representations of 0, so you'd lose information. And that's just one example of what could go wrong.
char * or void * is the correct type for a sequence of arbitrary bytes. Of course, to do anything useful with it, you need to know how many bytes there are in the encrypted form. Look in your library's documentation for how to know.