Secure Programming Cookbook for C and C++

4.8. Converting Text Keys to Binary Keys

Problem

A user enters a textual representation of a key or other binary data (see Recipe 4.7). You need to convert it to binary.

Solution

Parse out the words, then look them up in the dictionary to reconstruct the actual bits, as shown in the code included in the next section.

Discussion

This function spc_words2bin( ) uses the wordlist.h file provided on the book’s web site, and it can be changed as described in Recipe 4.7.

#include <stdlib.h> #include <string.h> #include <ctype.h> #include "wordlist.h" #define BITS_IN_LIST 11 #define MAX_WORDLEN 4 unsigned char *spc_words2bin(unsigned char *str, size_t *outlen) { int cmp, i; size_t bitsinword, curbits, needed, reslen; unsigned int ix, min, max; unsigned char *p = str, *r, *res, word[MAX_WORDLEN + 1]; curbits = reslen = *outlen = 0; if(!(r = res = (unsigned char *)malloc((strlen(str) + 1) / 2)) return 0; memset(res, 0, (strlen(str) + 1) / 2); for (;;) { while (isspace(*p)) p++; if (!*p) break; /* The +1 is because we expect to see a space or a NULL after each and every * word; otherwise, there's a syntax error. */ for (i = 0; i < MAX_WORDLEN + 1; i++) { if (!*p || isspace(*p)) break; if (islower(*p)) word[i] = *p++ - ' '; else if (isupper(*p)) word[i] = *p++; else { free(res); return 0; } } if (i = = MAX_WORDLEN + 1) { free(res); return 0; } word[i] = 0; min = 0; max = (1 << BITS_IN_LIST) - 1; do { if (max < min) { free(res); return 0; /* Word not in list! */ } ix = (max + min) / 2; cmp = strcmp(word, ...