move cryptography to its own folder/files

pkg/cryptography/curve25519.go

@@ -0,0 +1,25 @@
+package cryptography
+
+import (
+	"crypto/rand"
+
+	"golang.org/x/crypto/curve25519"
+)
+
+func GenerateKeyPair() (privateKey, publicKey []byte, err error) {
+	privateKey = make([]byte, curve25519.ScalarSize)
+	if _, err := rand.Read(privateKey); err != nil {
+		return nil, nil, err
+	}
+
+	publicKey, err = curve25519.X25519(privateKey, curve25519.Basepoint)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	return privateKey, publicKey, nil
+}
+
+func DeriveSharedSecret(privateKey, peerPublicKey []byte) ([]byte, error) {
+	return curve25519.X25519(privateKey, peerPublicKey)
+}