3단계

#include <stdio.h>

#include <stdlib.h>

#include <ctype.h>

#pragma warning(disable: 4326 4996 6031)

void main()

{

char sMsg[80];

int nKey = 3, nMore = true;

while (nMore) {

printf("? ");

gets_s(sMsg);

if (sMsg[0] == 0)

break;

else if (sMsg[0] == '@') {

printf("Key ? ");

gets_s(sMsg);

nKey = atoi(sMsg);

}

else {

printf("  ");

char Encrypt(char chr, int nKey);

for (int i = 0; sMsg[i]; i++)

putchar(Encrypt(sMsg[i], nKey));

putchar('\n');

}

putchar('\n');

}

printf("Bye, ....\n");

}

char Encrypt(char chr, int nKey)

{

// 대소문자만 암호화

if (isalpha(chr)) {

/**

if (isupper(chr))

chr = (chr - 'A' + nKey + 26) % 26 + 'A';

else

chr = (chr - 'a' + nKey + 26) % 26 + 'a';

*/

char cBgn = isupper(chr) ? 'A' : 'a';

chr = (chr - cBgn + nKey + 26) % 26 + cBgn;

}

return chr;

}

/*****

? I Love You

  L Oryh Brx

? @

Key ? -3

? L Oryh Brx

  I Love You

?

Bye, ....

*****/