Re: A question about modifying the buffer under authenticated encryption

[Mikulas Patocka <mpatocka@redhat.com>]

On Tue, 6 Feb 2024, Eric Biggers wrote:

> On Tue, Feb 06, 2024 at 10:46:59PM +0100, Mikulas Patocka wrote:
> > Hi
> > 
> > I'm trying to fix some problems in dm-crypt that it may report 
> > authentication failures when the user reads data with O_DIRECT and 
> > modifies the read buffer while it is being read.
> > 
> > I'd like to ask you:
> > 
> > 1. If the authenticated encryption encrypts a message, reading from 
> >    buffer1 and writing to buffer2 - and buffer1 changes while reading from 
> >    it - is it possible that it generates invalid authentication tag?
> > 
> > 2. If the authenticated encryption decrypts a message, reading from 
> >    buffer1 and writing to buffer2 - and buffer2 changes while writing to 
> >    it - is is possible that it reports authentication tag mismatch?
> > 
> 
> Yes, both scenarios are possible.  But it depends on the AEAD algorithm and how
> it happens to be implemented, and on whether the data overlaps or not.
> 
> This is very much a "don't do that" sort of thing.
> 
> - Eric

I see. So I will copy the data to a kernel buffer before encryption or 
decryption.

I assume that authenticated encryption or decryption using the same buffer 
as a source and as a destination should be ok. Right?

Mikulas