Question

Language: C

Write an encoder and a decoder for a modified "book cipher." A book cipher uses a document or book as the cipher key, and the cipher itself uses numbers that reference the words within the text. For example, one of the Beale ciphers used an edition of The Declaration of Independence as the cipher key. The cipher you will write will use a pair of numbers corresponding to each letter in the text. The first number denotes the position of a word in the key text (starting at 0), and the second number denotes the position of the letter in the word (also starting at 0). For instance, given the following key text (the numbers correspond to the index of the first word in the line):

[0] 'Twas brillig, and the slithy toves Did gyre and gimble in the wabe;
[13] All mimsy were the borogoves, And the mome raths outgrabe.
[23] "Beware the Jabberwock, my son! The jaws that bite, the claws that catch!
[36] Beware the Jubjub bird, and shun The frumious Bandersnatch!"
[45] He took his vorpal sword in hand: Long time the manxome foe he sought—

The word "computer" can be encoded with the following pairs of numbers:
35,0 catch
5,1 toves
42,3 frumious
48,3 vorpal
22,1 outgrabe
34,3 that
23,5 Beware
7,2 gyre
Placing these pairs into a cipher text, we get the following:
35,0,5,1,42,3,48,3,22,1,34,3,23,5,7,2

If you are encoding a phrase, rather than just a single word, spaces in the original english phrase will also appear in the ciphered text. So, the phrase "all done" (using the above Jabberwocky poem) might appear as:
0,3,1,4,13,1 6,0,46,2,44,2,3,2

Only spaces in the key text should be considered delimiters. All other punctuation in the key text are to be considered part of a key word. Thus the first word in the Jabberwocky poem, 'Twas, will have the following characters and positions for key word 0:
Position 0: '
Position 1: T
Position 2: w
Position 3: a
Position 4: s

Developing the Program
You should approach this assignment in several parts. The first part will be to write a menu driven program that prompts the user for the following actions:
1) Read in the name of a text file to use as a cipher key
2) Create a cipher using the input text file (and save the result to a file)
3) Decode an existing cipher (prompt user for a file to read containing the cipher text)
4) Exit the program

Answer 1

// code to intercept any of the .cry files but without knowledge of the books used

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

#define N_CAPITAL_LETTERS 26
#define LTRTOIDX(x) (x-'A')

int main(int argc, char *argv[])
{
char plain[_MAX_PATH], codfile[_MAX_PATH];
char cipher[_MAX_PATH], positions[_MAX_PATH];
char drive[_MAX_DRIVE];
char dir[_MAX_DIR];
char fname[_MAX_FNAME];
char ext[_MAX_EXT];

FILE fp_plain, fp_cod, fp_cipher, fp_pos;
long pos[N_CAPITAL_LETTERS];
int ch_plain, ch_cod;

// Argument processing
if (argc != 3) goto error1;
_splitpath(argv[1], drive, dir, fname, ext );
if (strlen(ext) == 0) strcpy(ext, "cod");
_makepath(codfile, drive, dir, fname, ext);
_makepath(positions, drive, dir, fname, "pos");
_splitpath(argv[2], drive, dir, fname, ext );
if (strlen(ext) == 0) strcpy(ext, "txt");
_makepath(plain, drive, dir, fname, ext);
_makepath(cipher, drive, dir, fname, "cry");

// File opening
if ((fp_plain = fopen(plain, "r")) == NULL) goto error3;
if ((fp_cod = fopen(codfile, "r")) == NULL) goto error3;
if ((fp_cipher = fopen(cipher, "w")) == NULL) goto error3;
fp_pos = fopen(positions, "rb+");

// Position array intialization
if (fp_pos == NULL) memset(pos, 0, sizeof(pos));
else {
fread(pos, sizeof(long), N_CAPITAL_LETTERS, fp_pos);
fclose(fp_pos);
}
// Main loop
do {
ch_plain = toupper(getc(fp_plain));
if (isalpha(ch_plain))
{
fseek(fp_cod, pos[LTRTOIDX(ch_plain)], SEEK_SET);
do {
ch_cod = getc(fp_cod);
} while (!(ch_cod == ch_plain || ch_cod == EOF));
if (ch_cod == ch_plain) {
pos[LTRTOIDX(ch_plain)] = ftell(fp_cod);
fprintf (fp_cipher, "%1ld ", pos[LTRTOIDX(ch_plain)]);
} else goto error2;
}
} while (ch_plain != EOF);
// Termination
fp_pos = fopen(positions, "wb");
fwrite (pos, sizeof(long), N_CAPITAL_LETTERS, fp_pos);
fclose(fp_plain); fclose(fp_cod); fclose(fp_cipher); fclose(fp_pos);
return 0;
// Error handling
e1: printf ("USAGE: bkcode codfile message\n"); return 1;
e2: printf ("Run out of letters %c!\n", ch_plain);
fclose(fp_plain); fclose(fp_cod);
fclose(fp_cipher); remove(cipher); return 1;
e3: printf ("Can not open files\n"); return 1

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