Why is my MD5 value printing with extra "f" characters?

Question

I have strange problem when using at() method of std::string. I'd like to calculate md5 hash for given string using this library: http://sourceforge.net/projects/libmd5-rfc/files/ Hash is calculated correctly, but there is a problem with printing it human way. The output is:

af04084897ebbf299b04082d105ab724
ffffffaf040848ffffff97ffffffebffffffbf29ffffff9b04082d105affffffb724

Code is:

  #include <stdio.h>
  #include<string>
  #include<iostream>

  extern "C" {
      #include "md5.h"
  }

  int main()
  {
      md5_state_t state;
      md5_byte_t digest[16];

      std::string callid("f83bc385-26da-df11-95d5-0800275903dd@pc-archdev");

      md5_init(&state);
      md5_append(&state, (const md5_byte_t*)callid.c_str(), callid.length());

      std::string callid_digest((const char*)digest, 16);

      for(int i = 0; i < 16; ++i) {
          printf("%02x", digest[i]);
      }

      printf("\n");

      for(int i = 0; i < 16; ++i) {
          const char c = callid_digest.at(i);
          printf("%02x", c);
      }

      printf("\n");
  }

Where do the "f" characters come from ?

Answer 1

Your byte values are being sign-extended.

That happens when you promote the (signed) char to a wider type and the top bit is set, because it tries to preserve the sign (which is why you're seeing the extra f characters only for values greater than 0x7f). Using an unsigned char should fix the problem:

const unsigned char c = callid_digest.at(i); // may need to cast.