Secure Programming Cookbook for C and C++

4.7. Representing Keys (or Other Binary Data) as English Text

Problem

You want to use an easy-to-read format for displaying keys (or fingerprints or some other interesting binary data). English would work better than a hexadecimal representation because people’s ability to recognize the key as correct by sight will be better.

Solution

Map a particular number of bits to a dictionary of words. The dictionary should be of such a size that an exact mapping to a number of bits is possible. That is, the dictionary should have a number of entries that is a power of two.

Discussion

The spc_bin2words() function shown here converts a binary string of the specified number of bytes into a string of English words. This function takes two arguments: str is the binary string to convert, and len is the number of bytes to be converted.

#include <string.h> #include <stdlib.h> #include "wordlist.h" #define BITS_IN_LIST 11 #define MAX_WORDLEN 4 /* len parameter is measured in bytes. Remaining bits padded with 0. */ unsigned char *spc_bin2words(const unsigned char *str, size_t len) { short add_space = 0; size_t i, leftbits, leftovers, scratch = 0, scratch_bits = 0; unsigned char *p, *res; res = (unsigned char *)malloc((len * 8 / BITS_IN_LIST + 1) * (MAX_WORDLEN + 1)); if (!res) abort( ); res[0] = 0; for (i = 0; i < len; i++) { leftovers = str[i]; leftbits = 8; while (leftbits) { if (scratch_bits + leftbits <= BITS_IN_LIST) { scratch |= (leftovers << (BITS_IN_LIST - leftbits - scratch_bits)); scratch_bits ...