Secure Programming Cookbook for C and C++

4.3. Representing Binary Keys (or Other Raw Data) as Hexadecimal

Problem

You want to print out keys in hexadecimal format, either for debugging or for easy communication.

Solution

The easiest way is to use the “%X” specifier in the printf() family of functions. In C++, you can set the ios::hex flag on a stream object before outputting a value, then clear the flag afterward.

Discussion

Here is a function called spc_print_hex() that prints arbitrary data of a specified length in formatted hexadecimal:

#include <stdio.h>
#include <string.h>
   
#define BYTES_PER_GROUP 4 
#define GROUPS_PER_LINE 4
   
/* Don't change these */
#define BYTES_PER_LINE (BYTES_PER_GROUP * GROUPS_PER_LINE)
   
void spc_print_hex(char *prefix, unsigned char *str, int len) {
  unsigned long i, j, preflen = 0;
   
  if (prefix) {
    printf("%s", prefix);
    preflen = strlen(prefix);
  }
   
  for (i = 0;  i < len;  i++) {
    printf("%02X ", str[i]);
    if (((i % BYTES_PER_LINE) =  = (BYTES_PER_LINE - 1)) && ((i + 1) != len)) {
      putchar('\n');
      for (j = 0;  j < preflen;  j++) putchar(' ');
    }
    else if ((i % BYTES_PER_GROUP) =  = (BYTES_PER_GROUP - 1)) putchar(' ');
  }
  putchar('\n');
}

This function takes the following arguments:

prefix

String to be printed in front of the hexadecimal output. Subsequent lines of output are indented appropriately.

str

String to be printed, in binary. It is represented as an unsigned char * to make the code simpler. The caller will probably want to cast, or it can be easily rewritten to be a void *, which would require this code to ...