Re: HalfSipHash Acceptable Usage

["Jason A. Donenfeld" <Jason@zx2c4.com>]

Hi Eric,

I computed performance numbers for both 32-bit and 64-bit using the
actual functions in which talking about replacing MD5 with SipHash.
The basic harness is here [1] if you're curious. SipHash was a pretty
clear winner for both cases.

x86_64:
[    1.714302] secure_tcpv6_sequence_number_md5# cycles: 102373398
[    1.747685] secure_tcp_sequence_number_md5# cycles: 92042258
[    1.773522] secure_tcpv6_sequence_number_siphash# cycles: 70786533
[    1.798701] secure_tcp_sequence_number_siphash# cycles: 68941043

x86:
[    1.635749] secure_tcpv6_sequence_number_md5# cycles: 106016335
[    1.670259] secure_tcp_sequence_number_md5# cycles: 95670512
[    1.708387] secure_tcpv6_sequence_number_siphash# cycles: 105988635
[    1.740264] secure_tcp_sequence_number_siphash# cycles: 88225395

>>> 102373398 > 70786533
True
>>> 92042258 > 68941043
True
>>> 106016335 > 105988635
True
>>> 95670512 > 88225395
True

While MD5 is probably faster for some kind of large-data
cycles-per-byte, due to its 64-byte internal state, SipHash -- the
"Sip" part standing "Short Input PRF" -- is fast for shorter inputs.
In practice with the functions we're talking about replacing, there's
no need to hash 64-bytes. So, SipHash comes out faster and more
secure.

I also haven't begun to look focusedly at the assembly my SipHash
implemention is generating, which means there's still window for even
more performance improvements.

Jason


[1] https://git.zx2c4.com/linux-dev/tree/net/core/secure_seq.c?h=siphash-bench#n194