How can I increase memory security in Delphi?

Question

Is it possible to \"wipe\" strings in Delphi? Let me explain:

I am writing an application that will include a DLL to authorise users. It will read an encrypted file

Answer 1

DLLs don't own allocated memory, processes do. The memory allocated by your specific process will be discarded once the process terminates, whether the DLL hangs around (because it is in use by another process) or not.

Answer 2

Would it be possible to load the decrypted XML into an array of char or byte rather than a string? Then there would be no copy-on-write handling, so you would be able to backfill the memory with #0's before freeing?

Be careful if assigning array of char to string, as Delphi has some smart handling here for compatibility with traditional packed array[1..x] of char.

Also, could you use ShortString?

Answer 3

Messy but you could make a note of the heap size that you've used while you've got the heap filled with sensitive data then when that is released do a GetMem to allocate you a large chunk spanning (say) 200% of that. do a Fill on that chunk and make the assumption that any fragmentation is unlinkely to be of much use to an examiner. Bri

Answer 4

If you use the FastMM memory manager in Full Debug mode, then you can force it to overwrite memory when it is being freed.

Normally that behaviour is used to detect wild pointers, but it can also be used for what your want.

On the other hand, make sure you understand what Craig Stuntz writes: do not write this authentication and authorization stuff yourself, use the underlying operating system whenever possible.

BTW: Hallvard Vassbotn wrote a nice blog about FastMM: http://hallvards.blogspot.com/2007/05/use-full-fastmm-consider-donating.html

Regards,

Jeroen Pluimers

Answer 5

How about keeping the password as a hash value in the XML and verify by comparing the hash of the input password with the hashed password in your XML.

Edit: You can keep all the sensitive data encrypted and decrypt only at the last possible moment.

Answer 6

I don't think it is worth bothering with, because if a user can read the memory of the process using the DLL, the same user can also halt the execution at any given point in time. Halting the execution before the memory is wiped will still give the user full access to the unencrypted data.

IMO any user sufficiently interested and able to do what you describe will not be seriously inconvenienced by your DLL wiping the memory.