Deciphering based on lexical distributions

cipher.c

#include <stdio.h>
#include <stdlib.h>

// THIS ASSUMES ASCII-- not portable to EBCDIC systems

static int shiftletter(int c, int r);

int main(int argc, char *argv[])
{
    if (argc <= 1) return 1;
    
    int shift = atoi(argv[1]);
    int c;
    
    while ((c = getchar()) != EOF)
        putchar(shiftletter(c, shift));
}

/* shiftletter: shift char right one in the alphabet */
static int shiftletter(int c, int r)
{
    if ('a' <= c && c <= 'z') return (c - 'a' + r) % ('z' - 'a' + 1) + 'a';
    if ('A' <= c && c <= 'Z') return (c - 'A' + r) % ('Z' - 'A' + 1) + 'A';
    return c;
}

decipher.c

#include <stdio.h>
#include <ctype.h>
#include <string.h>
#include <float.h>

#define BUFSIZE 10000
#define NLETTER 26

// THIS ASSUMES ASCII-- not portable to EBCDIC systems

char s[BUFSIZE];

/* contains freq of letters A-Z in the english language */
/* analyzed lexical frequencies in words_alpha.txt */
float normFreq[] = {
    0.084643,
    0.018296,
    0.043775,
    0.032389,
    0.107721,
    0.011228,
    0.023643,
    0.026432,
    0.089566,
    0.001561,
    0.007673,
    0.055774,
    0.030105,
    0.071948,
    0.071993,
    0.032524,
    0.001683,
    0.070433,
    0.071618,
    0.066070,
    0.037627,
    0.009464,
    0.006412,
    0.003003,
    0.020196,
    0.004223,
};

static int shiftletter(int c, int r);
static int alphapos(int c);

int main(void)
{
    char *x = s;
    int c, n, len;
    
    int bestShift = 0;
    float minDev = FLT_MAX;

    while ((c = getchar()) != EOF)      /* copy */
    {
        *x++ = c;
        if (isalpha(c)) ++n;
    }
    len = x - s;
    
    for (int i = 0; i < NLETTER; ++i)    
    {
        int freq[NLETTER] = {0};
        for (int j = 0; j < len; ++j)   /* shift text right 1 */
        {
            if (isalpha(s[j]))
            {
                freq[alphapos(s[j])]++;
                s[j] = shiftletter(s[j], 1);
            }
        }
        
        float stdDevSqr = 0;
        for (int j = 0; j < NLETTER; ++j)    /* get percentage & calc square of std dev */
        {
            float x = (float)freq[j] / n - normFreq[j];
            stdDevSqr += x * x;
        }
        
        if (stdDevSqr < minDev)         /* get min */
        {
            minDev = stdDevSqr;
            bestShift = i;
        }
    }
    
    for (int i = 0; i < len; ++i)       /* get shifted result */
        putchar(shiftletter(s[i], bestShift));
    putchar('\n');
}

/* shiftletter: shift char right one in the alphabet */
static int shiftletter(int c, int r)
{
    if ('a' <= c && c <= 'z') return (c - 'a' + r) % ('z' - 'a' + 1) + 'a';
    if ('A' <= c && c <= 'Z') return (c - 'A' + r) % ('Z' - 'A' + 1) + 'A';
    return c;
}

/* alphapos: returns position of character in the alphabet, -1 otherwise */
static int alphapos(int c)
{
    if ('a' <= c && c <= 'z') return c - 'a';
    if ('A' <= c && c <= 'Z') return c - 'A';
    return -1;
}