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#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#define CTOI(c) ((c) - 'A')
#define ITOC(c) ((c) + 'A')
void preprocess(char *str);
int main(int argc, char **argv)
{
char *key, *ciphertext, *decodepos, *decodeneg;
long offset;
size_t keylength, textlength;
if (argc != 4) {
fprintf(stderr, "Usage: %s <key> <offset> <ciphertext>\n", argv[0]);
exit(EXIT_FAILURE);
}
key = argv[1];
offset = strtol(argv[2], NULL, 10);
ciphertext = argv[3];
preprocess(key);
preprocess(ciphertext);
keylength = strlen(key);
textlength = strlen(ciphertext);
decodepos = malloc(textlength + 1);
decodeneg = malloc(textlength + 1);
for (size_t i = 0; i < textlength; i++) {
decodepos[i] = ITOC((CTOI(ciphertext[i]) + offset + CTOI(key[i % keylength])) % 26);
decodeneg[i] = ITOC((CTOI(ciphertext[i]) + 52 - offset - CTOI(key[i % keylength])) % 26);
}
decodepos[textlength] = '\0';
decodeneg[textlength] = '\0';
printf("+ %s\n- %s\n", decodepos, decodeneg);
free(decodepos);
free(decodeneg);
return EXIT_SUCCESS;
}
void preprocess(char *str)
{
char *src = str, *dest = str;
while (*src != '\0') {
if (isalpha(*src))
*(dest++) = toupper(*src);
src++;
}
*dest = '\0';
}
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