-- Packing/unpacking for Eulora's communication protocol:
  -- Serpent Message to/from Serpent Packet
  -- RSA Message to/from RSA Packet
  -- S.MG, 2018

package body Packing is

  -- Packing a Serpent message into Serpent package, using the given key
  function Pack( Msg : in Raw_Types.Serpent_Msg;
                 K   : in Serpent.Key ) 
               return Raw_Types.Serpent_Pkt is

    -- single Serpent blocks containing plain / encrypted data
    Plain    : Serpent.Block;
    Encr     : Serpent.Block;

    -- Serpent Key Schedule - needed for direct encr/decr calls
    KS       : Serpent.Key_Schedule;

    -- final resulting Serpent package
    Pkt      : Raw_Types.Serpent_Pkt := (others => 0);
  begin
    -- prepare the Serpent key schedule based on given key
    Serpent.Prepare_Key( K, KS );

    -- encrypt message block by block and copy result in packet
    for I in 1 .. S_Blocks loop
      -- get current block to encrypt
      Plain := Msg( Msg'First + (I-1) * Block_Len .. 
                    Msg'First +  I    * Block_Len - 1 );
      -- encrypt with Serpent
      Serpent.Encrypt( KS, Plain, Encr );
      -- copy result to output packet
      Pkt( Pkt'First + (I-1) * Block_Len ..
           Pkt'First +  I    * Block_Len - 1 ) 
         := Encr;
    end loop;

    -- return result
    return Pkt;
  end Pack;

  -- Unpacking a Serpent packet into contained message, using the given key
  function Unpack( Pkt : in Raw_Types.Serpent_Pkt;
                   K   : in Serpent.Key) 
                 return Raw_Types.Serpent_Msg is
    -- single Serpent blocks containing plain / encrypted data
    Plain    : Serpent.Block;
    Encr     : Serpent.Block;

    -- Serpent Key Schedule - needed for direct encr/decr calls
    KS       : Serpent.Key_Schedule;

    -- the message extracted from the given packet
    Msg : Raw_Types.Serpent_Msg := (others => 0);
  begin
    -- prepare the Serpent key for use
    Serpent.Prepare_Key( K, KS );

    -- decrypt the Serpent packet block by block
    for I in 1 .. S_Blocks loop
      -- get current block from input and decrypt
      Encr := Pkt( Pkt'First + (I-1) * Block_Len ..
                   Pkt'First +  I    * Block_Len - 1 );
      Serpent.Decrypt( KS, Encr, Plain );

      -- copy result to its correct position in final output
      Msg( Msg'First + (I-1) * Block_Len ..
           Msg'First +  I    * Block_Len - 1 )
         := Plain;
    end loop;

    -- return the result - the message content of given package
    return Msg;
  end Unpack;

end Packing;