resolving missing cipher test from issue #272 (#283)

* resolving missing cipher test from issue #272

Signed-off-by: Zzocker <[email protected]>

* coorecting test file name

Signed-off-by: Zzocker <[email protected]>

Author: Zzocker

ciphers/diffieHellmanKeyExchange.go

@@ -0,0 +1,41 @@
+// Package diffiehelkeyexchange implements Deffie Hellman Key Exchange Algorithm
+// for more information watch : https://www.youtube.com/watch?v=NmM9HA2MQGI
+package diffiehelkeyexchange
+
+const (
+	generator         = 3
+	primeNumber int64 = 6700417 // prime number discovered by Leonhard Euler
+)
+
+// GenerateShareKey : generates a key using client private key , generator and primeNumber
+// this key can be made public
+// shareKey = (g^key)%primeNumber
+func GenerateShareKey(prvKey int64) int64 {
+	return modularExponentiation(generator, prvKey, primeNumber)
+}
+
+// GenerateMutualKey : generates a mutual key that can be used by only alice and bob
+// mutualKey = (shareKey^prvKey)%primeNumber
+func GenerateMutualKey(prvKey, shareKey int64) int64 {
+	return modularExponentiation(shareKey, prvKey, primeNumber)
+}
+
+// r = (b^e)%mod
+func modularExponentiation(b, e, mod int64) int64 {
+
+	//runs in O(log(n)) where n = e
+	//uses exponentiation by squaring to speed up the process
+	if mod == 1 {
+		return 0
+	}
+	var r int64 = 1
+	b = b % mod
+	for e > 0 {
+		if e&1 == 1 {
+			r = (r * b) % mod
+		}
+		e = e >> 1
+		b = (b * b) % mod
+	}
+	return r
+}

ciphers/diffiehelkeyexchange/diffieHellmanKeyExchange.go

@@ -0,0 +1,41 @@
+package diffiehelkeyexchange
+
+import (
+	"crypto/rand"
+	"crypto/rsa"
+	"testing"
+)
+
+func TestDiffieHellmanKeyExchange(t *testing.T) {
+	t.Run("Test 1: modularExponentiation", func(t *testing.T) {
+		var want int64 = 9 // (3^5)mod13 = 243mod13 = 9
+		var prvKey int64 = 5
+		var generator int64 = 3
+		var primeNumber int64 = 13
+		got := modularExponentiation(generator, prvKey, primeNumber)
+		if got != want {
+			t.Errorf(`with privateKey=%d, generator=%d and primeNumber=%d  
+			modularExponentiation should result=%d, but resulted=%d`, prvKey, generator, primeNumber, want, got)
+		}
+	})
+
+	t.Run("Test 2: Key Exchange", func(t *testing.T) {
+		// generating a small sized rsa key for testing
+		alicePrvKey, _ := rsa.GenerateKey(rand.Reader, 31)
+		bobPrvKey, _ := rsa.GenerateKey(rand.Reader, 31)
+
+		// alice and bob generates their respective share key with their privateKey
+		shareKeyByAlice := GenerateShareKey(alicePrvKey.D.Int64())
+		shareKeyByBob := GenerateShareKey(bobPrvKey.D.Int64())
+
+		// generated share key now can be exchanged even via insecure channel
+
+		// mutualKey can be computed using shared key
+		mutualKeyComputedByAlice := GenerateMutualKey(alicePrvKey.D.Int64(), shareKeyByBob)
+		mutualKeyComputedByBob := GenerateMutualKey(bobPrvKey.D.Int64(), shareKeyByAlice)
+
+		if mutualKeyComputedByAlice != mutualKeyComputedByBob {
+			t.Errorf("mutual key computed by alice and bob should be same, but got un-equal")
+		}
+	})
+}