/*
Decrypt files encrypted with the 'encrypt' example application.

I'm being lazy and writing the output to stdout rather than stripping off the
".enc" suffix and writing it there. So all diagnostics go to stderr so there is
no confusion.

Written by Jack Lloyd (lloyd@randombit.net) on August 5, 2002

This file is in the public domain
*/
#include <fstream>
#include <iostream>
#include <string>
#include <vector>
#include <cstring>
#include <memory>

#include <botan/botan.h>

#if defined(BOTAN_EXT_COMPRESSOR_ZLIB)
  #include <botan/zlib.h>
#else
  #error "You didn't compile the zlib module into Botan"
#endif

using namespace Botan;

SecureVector<byte> b64_decode(const std::string&);

int main(int argc, char* argv[])
   {
   if(argc < 2)
      {
      std::cout << "Usage: " << argv[0] << " [-p passphrase] file\n"
                << "   -p : Use this passphrase to decrypt\n";
      return 1;
      }

   std::string filename, passphrase;

   for(int j = 1; argv[j] != 0; j++)
      {
      if(std::strcmp(argv[j], "-p") == 0)
         {
         if(argv[j+1])
            {
            passphrase = argv[j+1];
            j++;
            }
         else
            {
            std::cout << "No argument for -p option" << std::endl;
            return 1;
            }
         }
      else
         {
         if(filename != "")
            {
            std::cout << "You can only specify one file at a time\n";
            return 1;
            }
         filename = argv[j];
         }
      }

   if(passphrase == "")
      {
      std::cout << "You have to specify a passphrase!" << std::endl;
      return 1;
      }

   std::ifstream in(filename.c_str());
   if(!in)
      {
      std::cout << "ERROR: couldn't open " << filename << std::endl;
      return 1;
      }

   std::string algo;

   try {
      std::string header, salt_str, mac_str;
      std::getline(in, header);
      std::getline(in, algo);
      std::getline(in, salt_str);
      std::getline(in, mac_str);

      if(header != "-------- ENCRYPTED FILE --------")
         {
         std::cout << "ERROR: File is missing the usual header" << std::endl;
         return 1;
         }

      if(!have_block_cipher(algo))
         {
         std::cout << "Don't know about the block cipher \"" << algo << "\"\n";
         return 1;
         }

      const u32bit key_len = max_keylength_of(algo);
      const u32bit iv_len = block_size_of(algo);

      std::auto_ptr<S2K> s2k(get_s2k("PBKDF2(SHA-1)"));
      s2k->set_iterations(8192);
      s2k->change_salt(b64_decode(salt_str));

      SymmetricKey bc_key = s2k->derive_key(key_len, "BLK" + passphrase);
      InitializationVector iv = s2k->derive_key(iv_len, "IVL" + passphrase);
      SymmetricKey mac_key = s2k->derive_key(16, "MAC" + passphrase);

      Pipe pipe(new Base64_Decoder,
                get_cipher(algo + "/CBC", bc_key, iv, DECRYPTION),
                new Zlib_Decompression,
                new Fork(
                   0,
                   new Chain(new MAC_Filter("HMAC(SHA-1)", mac_key),
                             new Base64_Encoder)
                   )
         );

      pipe.start_msg();
      in >> pipe;
      pipe.end_msg();

      std::string our_mac = pipe.read_all_as_string(1);
      if(our_mac != mac_str)
         std::cout << "WARNING: MAC in message failed to verify\n";

      std::cout << pipe.read_all_as_string(0);
   }
   catch(Algorithm_Not_Found)
      {
      std::cout << "Don't know about the block cipher \"" << algo << "\"\n";
      return 1;
      }
   catch(Decoding_Error)
      {
      std::cout << "Bad passphrase or corrupt file\n";
      return 1;
      }
   catch(std::exception& e)
      {
      std::cout << "Exception caught: " << e.what() << std::endl;
      return 1;
      }
   return 0;
   }

SecureVector<byte> b64_decode(const std::string& in)
   {
   Pipe pipe(new Base64_Decoder);
   pipe.process_msg(in);
   return pipe.read_all();
   }