From bcde52891e80500700d2a66e96fbdf504f771056 Mon Sep 17 00:00:00 2001
From: Akshay <nerdy@peppe.rs>
Date: Wed, 30 Dec 2020 14:31:44 +0530
Subject: add dh & rsa

---
 q8/dh.c    | 38 +++++++++++++++++++++++++++++
 q8/rsa.cpp | 81 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 119 insertions(+)
 create mode 100644 q8/dh.c
 create mode 100644 q8/rsa.cpp

diff --git a/q8/dh.c b/q8/dh.c
new file mode 100644
index 0000000..074dfc7
--- /dev/null
+++ b/q8/dh.c
@@ -0,0 +1,38 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <time.h>
+
+// Function to compute a^m mod n
+int compute(int a, int m, int n) {
+  int r;
+  int y = 1;
+
+  while (m > 0) {
+    r = m % 2;
+    if (r == 1)
+      y = (y * a) % n;
+    a = a * a % n;
+
+    m = m / 2;
+  }
+
+  return y;
+}
+
+// C program to demonstrate Diffie-Hellman algorithm
+int main() {
+  int p = 23; // modulus
+  int g = 5;  // base
+  int a, b; // a - Alice's Secret Key, b - Bob's Secret Key.
+  int A, B; // A - Alice's Public Key, B - Bob's Public Key
+  srand(time(0));
+  a = rand(); // or use rand()
+  A = compute(g, a, p);
+  srand(time(0));
+  b = rand(); // or use rand()
+  B = compute(g, b, p);
+  int keyA = compute(B, a, p);
+  int keyB = compute(A, b, p);
+  printf("\nAlice's Secret Key is %d\nBob's Secret Key is %d\n\n", keyA, keyB);
+  return 0;
+}
diff --git a/q8/rsa.cpp b/q8/rsa.cpp
new file mode 100644
index 0000000..c38e7d6
--- /dev/null
+++ b/q8/rsa.cpp
@@ -0,0 +1,81 @@
+#include <iostream>
+#include <math.h>
+#include <stdlib.h>
+#include <string.h>
+
+using namespace std;
+
+long int gcd(long int a, long int b) {
+  if (a == 0)
+    return b;
+  if (b == 0)
+    return a;
+  return gcd(b, a % b);
+}
+
+long int isprime(long int a) {
+  int i;
+  for (i = 2; i < a; i++) {
+    if ((a % i) == 0)
+      return 0;
+  }
+  return 1;
+}
+
+long int encrypt(char ch, long int n, long int e) {
+  int i;
+  long int temp = ch;
+  for (i = 1; i < e; i++)
+    temp = (temp * ch) % n;
+  return temp;
+}
+
+char decrypt(long int ch, long int n, long int d) {
+  int i;
+  long int temp = ch;
+  for (i = 1; i < d; i++)
+    ch = (temp * ch) % n;
+  return ch;
+}
+
+int main() {
+  long int i, len;
+  long int p, q, n, phi, e, d, cipher[50];
+  char text[50];
+
+  cout << "Enter the text to be encrypted: ";
+  cin.getline(text, sizeof(text));
+  len = strlen(text);
+  do {
+    p = rand() % 30;
+  } while (!isprime(p));
+  do {
+    q = rand() % 30;
+  } while (!isprime(q));
+  n = p * q;
+  phi = (p - 1) * (q - 1);
+  do {
+    e = rand() % phi;
+  } while (gcd(phi, e) != 1);
+  do {
+    d = rand() % phi;
+  } while (((d * e) % phi) != 1);
+  cout << "Two prime numbers (p and q) are: " << p << " and " << q << endl;
+  cout << "n(p * q) = " << p << " * " << q << " = " << p * q << endl;
+  cout << "(p - 1) * (q - 1) = " << phi << endl;
+  cout << "Public key (n,  e): (" << n << ", " << e << ")\n";
+  cout << "Private key (n, d): (" << n << ", " << d << ")\n";
+  for (i = 0; i < len; i++)
+    cipher[i] = encrypt(text[i], n, e);
+  cout << "Encrypted message: ";
+  for (i = 0; i < len; i++)
+    cout << cipher[i];
+  for (i = 0; i < len; i++)
+    text[i] = decrypt(cipher[i], n, d);
+  cout << endl;
+  cout << "Decrypted message: ";
+  for (i = 0; i < len; i++)
+    cout << text[i];
+  cout << endl;
+  return 0;
+}
-- 
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