Add subdirectory for cryptography examples

+ Add example using columnar transposition for encrypting / decrypting messages
+ Fix cmake build error from duplicate lib-bst when building from project root

cpp/cryptography/columnar-transposition/lib-cipher.hpp

@@ -0,0 +1,48 @@
+/*##############################################################################
+## Author: Shaun Reed                                                         ##
+## Legal: All Content (c) 2021 Shaun Reed, all rights reserved                ##
+## About: An example of using columnar transposition cipher                   ##
+## + Using concept at www.braingle.com/brainteasers/codes/columnartrans.php   ##
+##                                                                            ##
+## Contact: shaunrd0@gmail.com  | URL: www.shaunreed.com | GitHub: shaunrd0   ##
+################################################################################
+*/
+
+#ifndef COLUMNARTRANSPOSITION_LIB_CIPHER_HPP
+#define COLUMNARTRANSPOSITION_LIB_CIPHER_HPP
+
+#include <iostream>
+#include<algorithm>
+#include<vector>
+
+/******************************************************************************/
+// CipherData struct to hold data and methods related to encrypting / decrypting
+struct CipherData {
+  // Can't create this object without providing a keyWord to initialize order
+  explicit CipherData(std::string keyWord) : keyWord_(std::move(keyWord))
+  {
+    InitOrder(keyWord_);
+  }
+  CipherData() : keyWord_(GetKey()) { InitOrder(keyWord_);}
+
+  /****************************************************************************/
+  // Member functions
+
+  // Initializes orderVect, pass by value to create local copy of keyWord
+  void InitOrder(std::string temp);
+
+  static std::string GetKey();
+
+  static void
+  Transpose(const std::vector<std::string> &in, std::vector<std::string> &out);
+
+  // Default values for param allows calling with no message
+  std::string Encrypt(std::string message="");
+  std::string Decrypt(std::string message="");
+
+private:
+  std::string keyWord_;
+  std::vector<int> orderVect_;
+};
+
+#endif // COLUMNARTRANSPOSITION_LIB_CIPHER_HPP