Re: HalfSipHash Acceptable Usage

Hi George,

On Wed, Dec 21, 2016 at 7:34 AM, George Spelvin
[off-list ref] wrote:

In fact, I have an idea.  Allow me to make the following concrete
suggestion for using HalfSipHash with 128 bits of key material:

- 64 bits are used as the key.
- The other 64 bits are used as an IV which is prepended to
  the message to be hashed.

As a matter of practical implementation, we precompute the effect
of hashing the IV and store the 128-bit HalfSipHash state, which
is used just like a 128-bit key.

Because of the way it is constructed, it is obviously no weaker than
standard HalfSipHash's 64-bit security claim.

I don't know the security of this, and it's almost certainly weaker than
128 bits, but I *hope* it's at least a few bits stronger than 64 bits.
80 would be enough to dissuade any attacker without a six-figure budget
(that's per attack, not a one-time capital investment).  96 would be
ample for our purposes.

What I do know is that it makes a brute-force attack without
significant cryptanalytic effort impossible.

Depends who's doing the cryptanalytic effort I guess.

Please don't roll your own crypto. It's a dangerous road. Putting
homebrew crypto into the kernel would be an error. Let's stick with
the constructions and security margins that the cryptographers give
us. JP made that fairly clear, I thought.

There are already people working on this problem who undergo peer
review and a career devoted to solving these problems. One result for
small systems that need 128-bit security is Chaskey, which you can go
read about if you're curious.

Jason