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Source Code
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#include <bits/stdc++.h>
#include <string.h>
/* Cesar
P = C = K = Zn
ek(x) = (x+k) mod n
dk(y) = (y-k) mod n
Ex:
plaintext: ATTACK
key: 17
ciphertext: RKKRTB
*/
char *encryptCesar(char *plaintext, int key)
{
int length = strlen(plaintext);
char* cipher = (char*)malloc(sizeof(char)*(length-1));
for(int i=0; i < length; i++)
cipher[i] = 65 + (plaintext[i] + key -65)%26;
cipher[length] = '\0';
return cipher;
}
char *decryptCesar(char *ciphertext, int key)
{
int length = strlen(ciphertext);
char* plain = (char*)malloc(sizeof(char)*(length-1));
for(int i=0; i < length; i++)
plain[i] = 65 + (ciphertext[i] - key - 65 + 26)%26;
plain[length] = '\0';
return plain;
}
int main()
{
char plain[100] = "ATTACK";
char cipher[100] = "RKKRTB";
printf("Encrypt: %s => %s", plain,encryptCesar(plain, 17));
printf("\nDecrypt: %s <= %s\n\n", cipher,decryptCesar(cipher, 17));
char xaumh[100], xaugm[100]; int k;
printf("Nhap ban ro: ");
fflush(stdin);
gets(xaumh);
printf("Nhap key: ");
fflush(stdin);
scanf("%d", &k);
printf("Encrypt: %s => %s", xaumh,encryptCesar(xaumh, k));
printf("\n\nNhap ban ma: ");
fflush(stdin);
gets(xaugm);
printf("Nhap key: ");
fflush(stdin);
scanf("%d", &k);
printf("Decrypt: %s => %s", xaumh,decryptCesar(xaugm, k));
getchar();
return 0;
}
Compiling Program...
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Text
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