Access violation if GCM mode is set after cipher is initialized

[danielmarschall]

To Reproduce

1. Open demo project Cipher_Console
2. Change Cipher.Mode to cmGCM
3. Run the code. You receive EAccessViolation

---

Usually, if you change the cipher mode after the cipher has been initialized, then TDECCipher.SetMode should call Done on the cipher and re-initialize it.

GCM Encode requires that TGCM.Init is called and has set FEncryptionMethod := EncryptionMethod. However, if Ciper.Mode is set after the cipher has been initialized, then TGCM is created but TGCM.Init has not been called.

procedure TDECCipherModes.InitMode;
begin
  if FMode = TCipherMode.cmGCM then
  begin
    if Context.BlockSize = 16 then
      FGCM := TGCM.Create
    else
      // GCM requires a cipher with 128 bit block size
      raise EDECCipherException.CreateResFmt(@sInvalidBlockSize,
                                             [128, GetEnumName(TypeInfo(TCipherMode),
                                             Integer(FMode))]);
  end
  else
    if Assigned(FGCM) then
      FreeAndNil(FGCM);
end;

The program flow if Mode has been set before init:

1. TDECCipher.Init()
2. TDECCipher.Init()
3. TDECCipherModes.OnAfterInitVectorInitialized ( calls FGCM.Init(self.DoEncode, OriginalInitVector) )
4. TGCM.Init

The program flow if Mode has been set after init:

1. TDECCipher.SetMode
2. TDECCipherModes.InitMode
3. TGCM.Create, but not TGCM.Init ( I am not sure where I get OriginalInitVector from )

So... what do we do? We probably need to preserve the InitVector, so we can call TGCM.Init with the correct parameters.

The other solution is to forbid to change the mode after the cipher has been initialized. But this is a code breaking change.