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.