From: Matthew Kerwin <matthew@...>
Date: 2012-05-29T16:57:03+09:00
Subject: Re: does this leak more than the size of the string via timing side channels

Er, sorry, I misused the variable names. Obviously I meant
string1.chars... and string2....

On 29 May 2012 17:54, Matthew Kerwin <matthew@kerwin.net.au> wrote:
> I'd suggest something like:
>
>  if lengths match:
>    if checksums match:
>      char-by-char comparison
>
> .. although I can't prove that it doesn't have an equivalent weakness.
>  The char-by-char is just in case the two strings have the same
> checksum, obviously.
>
> An alternative could be to transpose the strings from index=>char to
> char=>[indices], then check that each char's indices are the same for
> both strings:
>
>  allc = {}
>  in1 = Hash.new []
>  in2 = Hash.new []
>  in1.chars.each_with_index do |c, i|
>    allc[c] = true
>    in1[c] ||= []
>    in1[c] << i
>  end
>  in2.chars.each_with_index do |c, i|
>    allc[c] = true
>    in2[c] ||= []
>    in2[c] << i
>  end
>  allc.each do |c|
>    return false if in1[c] != in2[c]
>  end
>  true
>
> There might be a neater way to write it. And it's much slower, but
> should be more securerer.
>
> --
>   Matthew Kerwin, B.Sc (CompSci) (Hons)
>   http://matthew.kerwin.net.au/
>   ABN: 59-013-727-651
>
>   "You'll never find a programming language that frees
>   you from the burden of clarifying your ideas." - xkcd
>



-- 
  Matthew Kerwin, B.Sc (CompSci) (Hons)
  http://matthew.kerwin.net.au/
  ABN: 59-013-727-651

  "You'll never find a programming language that frees
  you from the burden of clarifying your ideas." - xkcd