char* giving different results when condition changes - c

Question

I am creating a simple md5 bruteforce-like program using C. The only issue I am having is that the Found String: output completely changes if I were to replace a part of the if statement.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdlib.h>
#include <assert.h>
#include <time.h>
#include <stdbool.h>
#if defined(__APPLE__)
# define COMMON_DIGEST_FOR_OPENSSL
# include <CommonCrypto/CommonDigest.h>
# define SHA1 CC_SHA1
#else
# include <openssl/md5.h>
#endif
char *str2md5(const char *str, int length) {
int n;
MD5_CTX c;
unsigned char digest[16];
char *out = (char*)malloc(33);
MD5_Init(&c);
while (length > 0) {
if (length > 512) {
MD5_Update(&c, str, 512);
} else {
MD5_Update(&c, str, length);
}
length -= 512;
str += 512;
}
MD5_Final(digest, &c);
for (n = 0; n < 16; ++n) {
snprintf(&(out[n*2]), 16*2, "%02x", (unsigned int)digest[n]);
}
return out;
}
typedef struct md5data {
char* output;
char* strin;
} md5data;
md5data getrand() {
for (int i = 0; i < 10; ++i)
{
rand();
srand(rand());
}
unsigned char strin[50];
for (int i = 0; i < 50; i++)
{
strin[i] = (rand()%94)+32;
}
strin[49] = '\0';
char* string = &strin;
char *output = str2md5(string, strlen(string));
md5data out;
out.output = output;
out.strin = string;
return out;
}
bool starts_with(const char* a, const char* b)
{
if(strncmp(a, b, strlen(b)) == 0) return 1;
return 0;
}
int main() {
char input;
printf("%s","Enter Search String: ");
scanf("%s",&input);
srand(time(NULL));
while(1 == 1) {
md5data md5 = getrand();
if(starts_with(md5.output,&input)) {
printf("Found String: %s\nMD5: %s\n",md5.strin,md5.output);
break;
}
}
return 0;
}
Whenever I compile and execute, the first line of output is usually something like Found String: 0????
However, if I change starts_with(md5.output,&input) to something like 1==1 or anything like that, the output is something like Found String: qM39$dcX_ZqFM9]?>jKhxSl#m2xrAxaL*
What is causing the output to change and why is it happening?

Answer 1

The problem is on lines:
char input;
printf("%s","Enter Search String: ");
scanf("%s",&input);
input should be a char array (a buffer), and not a single char.
For example:
char input[256];
printf("%s","Enter Search String: ");
scanf("%s",&input);
In the current state, the scanf results in a buffer overflow on your stack, which causes undefined results.

Answer 2

I ended up fixing this issue by adding a global variable, removing md5data, and using strcpy to copy the value form getrand into the global variable.
Code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdlib.h>
#include <assert.h>
#include <time.h>
#include <stdbool.h>
#if defined(__APPLE__)
# define COMMON_DIGEST_FOR_OPENSSL
# include <CommonCrypto/CommonDigest.h>
# define SHA1 CC_SHA1
#else
# include <openssl/md5.h>
#endif
char *str2md5(const char *str, int length) {
int n;
MD5_CTX c;
unsigned char digest[16];
char *out = (char*)malloc(33);
MD5_Init(&c);
while (length > 0) {
if (length > 512) {
MD5_Update(&c, str, 512);
} else {
MD5_Update(&c, str, length);
}
length -= 512;
str += 512;
}
MD5_Final(digest, &c);
for (n = 0; n < 16; ++n) {
snprintf(&(out[n*2]), 16*2, "%02x", (unsigned int)digest[n]);
}
return out;
}
unsigned char stringglobal[50];
char* outputglobal;
void getrand() {
for (int i = 0; i < 10; ++i)
{
rand();
srand(rand());
}
unsigned char strin[50] = {0};
for (int i = 0; i < 50; i++)
{
strin[i] = (rand()%94)+32;
}
strin[49] = '\0';
char* string = &strin[0];
char *output = str2md5(string, strlen(string));
outputglobal = output;
strcpy(stringglobal,string);
}
bool starts_with(const char* a, const char* b)
{
if(strncmp(a, b, strlen(b)) == 0) return 1;
return 0;
}
int main() {
char input[256];
printf("%s","Enter Search String: ");
scanf("%s",&input);
srand(time(NULL));
while(1 == 1) {
getrand();
if(starts_with(outputglobal,&input)) {
printf("Found! String: %s\nMD5: %s\n",stringglobal,outputglobal);
break;
}
}
return 0;
}