#include <stdio.h>
#include <stdlib.h>
#include <math.h>
// Funcao para calcular o MDC usando o Algoritmo de Euclides
int gcd(int a, int b) {
return b == 0 ? a : gcd(b, a % b);
}
// Funcao para calcular o inverso modular de a modulo m usando o Algoritmo de Euclides Estendido
int mod_inverse(int a, int m) {
int m0 = m, t, q;
int x0 = 0, x1 = 1;
if (m == 1) return 0;
while (a > 1) {
q = a / m;
t = m;
m = a % m;
a = t;
t = x0;
x0 = x1 - q * x0;
x1 = t;
}
if (x1 < 0) x1 += m0;
return x1;
}
// Funcao para calcular a potencia modular (base^exp % mod)
int mod_pow(int base, int exp, int mod) {
int result = 1;
base = base % mod;
while (exp > 0) {
if (exp % 2 == 1) {
result = (result * base) % mod;
}
exp = exp >> 1;
base = (base * base) % mod;
}
return result;
}
// Funcao para encriptar a mensagem usando a chave publica (e, n)
int encrypt(int message, int e, int n) {
return mod_pow(message, e, n);
}
// Funcao para desencriptar a mensagem usando a chave privada (d, n)
int decrypt(int ciphertext, int d, int n) {
return mod_pow(ciphertext, d, n);
}
// Funcao para converter uma letra em numero (A=11, B=12, ..., Z=36)
int char_to_num(char c) {
return c - 'A' + 11; // Convertendo A=11, B=12, ..., Z=36
}
// Funcao para converter um numero de volta para letra
char num_to_char(int num) {
return num + 'A' - 11;
}
int main() {
int p, q, e, n, phi, d;
// Solicitar os numeros primos p e q
printf("Digite o primeiro numero primo (p): ");
scanf("%d", &p);
printf("Digite o segundo numero primo (q): ");
scanf("%d", &q);
// Calcular n = p * q
n = p * q;
// Calcular phi(n) = (p - 1) * (q - 1)
phi = (p - 1) * (q - 1);
// Solicitar o valor de e, tal que 1 < e < phi(n) e e seja coprimo com phi(n)
printf("Digite o numero e (exponencial de encriptacao): ");
scanf("%d", &e);
// Verificar se e e phi(n) sao coprimos
if (gcd(e, phi) != 1) {
printf("Erro: e nao e coprimo com phi(n). Escolha outro valor de e.\n");
return 1;
}
// Calcular d, o inverso modular de e modulo phi(n)
d = mod_inverse(e, phi);
// Exibir as chaves
printf("Chave publica: (e = %d, n = %d)\n", e, n);
printf("Chave privada: (d = %d, n = %d)\n", d, n);
// Solicitar o tamanho do bloco
int block_size;
printf("Digite o tamanho do bloco (em numero de caracteres): ");
scanf("%d", &block_size);
// Entrada da mensagem
char message[block_size + 1];
printf("Digite a mensagem (com %d caracteres): ", block_size);
scanf("%s", message);
// Converter a mensagem em numeros e encriptar
printf("Mensagem original: %s\n", message);
int encrypted[block_size];
for (int i = 0; i < block_size; i++) {
int num = char_to_num(message[i]);
encrypted[i] = encrypt(num, e, n);
printf("Mensagem encriptada (letra %c): %d\n", message[i], encrypted[i]);
}
// Desencriptar a mensagem
printf("Mensagem desencriptada: ");
for (int i = 0; i < block_size; i++) {
int decrypted = decrypt(encrypted[i], d, n);
printf("%c", num_to_char(decrypted));
}
printf("\n");
return 0;
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OneCompiler's C online editor supports stdin and users can give inputs to programs using the STDIN textbox under the I/O tab. Following is a sample C program which takes name as input and print your name with hello.
#include <stdio.h>
int main()
{
char name[50];
printf("Enter name:");
scanf("%s", name);
printf("Hello %s \n" , name );
return 0;
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C language is one of the most popular general-purpose programming language developed by Dennis Ritchie at Bell laboratories for UNIX operating system. The initial release of C Language was in the year 1972. Most of the desktop operating systems are written in C Language.
When ever you want to perform a set of operations based on a condition if-else is used.
if(conditional-expression) {
// code
} else {
// code
}
You can also use if-else for nested Ifs and if-else-if ladder when multiple conditions are to be performed on a single variable.
Switch is an alternative to if-else-if ladder.
switch(conditional-expression) {
case value1:
// code
break; // optional
case value2:
// code
break; // optional
...
default:
// code to be executed when all the above cases are not matched;
}
For loop is used to iterate a set of statements based on a condition.
for(Initialization; Condition; Increment/decrement){
// code
}
While is also used to iterate a set of statements based on a condition. Usually while is preferred when number of iterations are not known in advance.
while(condition) {
// code
}
Do-while is also used to iterate a set of statements based on a condition. It is mostly used when you need to execute the statements atleast once.
do {
// code
} while (condition);
Array is a collection of similar data which is stored in continuous memory addresses. Array values can be fetched using index. Index starts from 0 to size-1.
data-type array-name[size];
data-type array-name[size][size];
Function is a sub-routine which contains set of statements. Usually functions are written when multiple calls are required to same set of statements which increases re-usuability and modularity.
Two types of functions are present in C
Library functions are the in-built functions which are declared in header files like printf(),scanf(),puts(),gets() etc.,
User defined functions are the ones which are written by the programmer based on the requirement.
return_type function_name(parameters);
function_name (parameters)
return_type function_name(parameters) {
//code
}