openssl hash producing partially different output - c++ ubuntu - c++

Question

Below is my code for creating a hash in openssl.
I was wondering why is my output always partially diffrent.
int main()
{
char alg[] = "MD5";
char buf[EVP_MAX_KEY_LENGTH] = "5";
unsigned char *testKey;
unsigned int olen;
testKey = simple_digest(alg, buf, EVP_MAX_KEY_LENGTH,&olen);
std::cout << "Printing key : >>";
print_hex(testKey,EVP_MAX_KEY_LENGTH);
std::cout << "<<" << std::endl;
}
unsigned char *simple_digest(char *alg, char *buf, unsigned int len,unsigned int *olen)
{
const EVP_MD *m;
EVP_MD_CTX ctx;
unsigned char *ret;
OpenSSL_add_all_digests();
if (!(m = EVP_get_digestbyname(alg)))
return NULL;
if (!(ret = (unsigned char *)malloc(EVP_MAX_MD_SIZE)))
return NULL;
EVP_DigestInit(&ctx, m);
EVP_DigestUpdate(&ctx, buf, len);
EVP_DigestFinal(&ctx, ret, olen);
// std::cout << "computed key" << ret << std::endl;
return ret;
}
void print_hex(unsigned char *bs, unsigned int n)
{
int i;
for (i = 0; i < n; i++)
printf("%02x", bs[i]);
printf("\n");
}
Output :
3afb8ebc9c93bf6d40285736f210b7856af8bab4d040ca090043ca09f840ca09
3afb8ebc9c93bf6d40285736f210b7856af8bab490a5f909c0a7f909b8a5f909
As you can see only the back few characters differ.
Thanks in advance! :D
Update (correct working version) :
int main()
{
char alg[] = "MD5";
char buf[EVP_MAX_KEY_LENGTH] = "5";
unsigned char *testKey;
unsigned int olen;
testKey = simple_digest(alg, buf,strlen(buf),&olen);
std::cout << "Printing key : >>";
print_hex(testKey,olen);
std::cout << "<<" << std::endl;
}
unsigned char *simple_digest(char *alg, char *buf, unsigned int len,unsigned int *olen)
{
const EVP_MD *m;
EVP_MD_CTX ctx;
unsigned char *ret;
OpenSSL_add_all_digests();
if (!(m = EVP_get_digestbyname(alg)))
return NULL;
if (!(ret = (unsigned char *)malloc(EVP_MAX_KEY_LENGTH)))
return NULL;
EVP_DigestInit(&ctx, m);
EVP_DigestUpdate(&ctx, buf, len);
EVP_DigestFinal(&ctx, ret, olen);
return ret;
}
void print_hex(unsigned char *bs, unsigned int n)
{
int i;
for (i = 0; i < n; i++)
printf("%02x", bs[i]);
}
output :
Printing key : >>e4da3b7fbbce2345d7772b0674a318d5<<

Answer 1

You are trying to hash a one-character string, "5", but you are telling EVP_Digest_Update that your buffer length is EVP_MAX_KEY_LENGTH. You need to pass the actual length of your buffer, not the maximum length it could be.
For completeness, here is what your main function should look like:
int main()
{
char alg[] = "MD5";
char buf[EVP_MAX_KEY_LENGTH] = "5";
unsigned char *testKey;
unsigned int olen;
# Pass the true length of but
testKey = simple_digest(alg, buf, strlen(buf), &olen);
std::cout << "Printing key : >>";
# Pass the true length of testKey
print_hex(testKey,olen);
std::cout << "<<" << std::endl;
}
One other caveat: you probably don't want to use EVP_MAX_KEY_LENGTH for your buffer, as that would limit the size of the message you could hash.