From: Theodore Y. Ts'o > Sent: 29 March 2020 22:42 > On Sun, Mar 29, 2020 at 05:41:22PM +0000, George Spelvin wrote: > > > Using xor was particularly stupid. > > > The whole generator was then linear and trivially reversable. > > > Just using addition would have made it much stronger. > > > > I considered changing it to addition (actually, add pairs and XOR the > > sums), but that would break its self-test. And once I'd done that, > > there are much better possibilities. > > > > Actually, addition doesn't make it *much* stronger. To start > > with, addition and xor are the same thing at the lsbit, so > > observing 113 lsbits gives you a linear decoding problem. > > David, > > If anyone is trying to rely on prandom_u32() as being "strong" in any > sense of the word in terms of being reversable by attacker --- they > shouldn't be using prandom_u32(). That's going to be true no matter > *what* algorithm we use. Indeed, but xor merging of 4 LFSR gives an appearance of an improvements (over a single LFSR) but gives none and just increases the complexity. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
