leoferlopes / secret Goto Github PK

#include <stdio.h>

/*
 * Argumentos:
 *  - b: base da exponenciação.
 *  - e: expoente.
 *  - n: módulo (n > 1).
 * Retorna: b^e (mod n).
 */
unsigned short binExp(unsigned short b, int e, unsigned short n) {

    unsigned int res = b;
    unsigned int y = 1;
    
    /* Caso base. */
    if (e == 0) return(1);

    while(e > 1) {
    
        if (e & 1) {
        
            /* 
             * Caso especial: expoente é ímpar. 
             * Acumular uma potência de 'res' em 'y'.
             */
            y = (y * res) % (unsigned int) n;
            e = e - 1;
        }

        /*
         * Elevamos 'res' ao quadrado, dividimos expoente por 2.
         */
        res = (res * res) % (unsigned int) n;
        e = e / 2;
    }
    
    return((unsigned short) ((res * y) % (unsigned int) n));
}

int main(){
    unsigned short b = 123;

    unsigned short e = 29;
    unsigned short d = 1625;

    unsigned short n = 2281;

    unsigned short encrypted = binExp(b, e, n);
    unsigned short decrypted = binExp(encrypted, d, n);

    printf("encrypted %d\n", encrypted);
    printf("decrypted %d\n", decrypted);
}