Time Invariant String Comparison

[Jason A. Donenfeld]

Hi guys,

Over at cgit [1] we use Lua for our authentication framework [2]. One
thing we're doing wrong is lines like these:

	if password == post["password"] then

Since an attacker can control the post params, this test is vulnerable
to a timing attack, by which an attacker could determine the password
one character at a time by analysis of response time.

What I'm looking for is some clever way in Lua to compare two strings
in a time invariant way. Any suggestions?

Thanks,
Jason


[1] http://git.zx2c4.com/cgit/
[2] http://git.zx2c4.com/cgit/tree/filters/simple-authentication.lua