From: Alex Young <alex@...>
Date: 2007-01-29T09:25:04+09:00
Subject: Re: "a string".xor("another string")

Erik Veenstra wrote:
> You're padding the shortest string with low values. When using
> XOR to encrypt a string (my goal...), uh, you're not encrypting
> most of the data:
> 
>  "very long secret data".xor("secret")   # ==> "\005\000\021\vE\030ong 
> secret data"
> 
> The secret key should be repeated several times, so every byte
> in the data gets XOR'ed.
Robert's solution can be modified:

> irb(main):008:0> a="a string"
> => "a string"
> irb(main):009:0> b="another string"
> => "another string"
> irb(main):014:0> s="";[a.size, b.size].max.times {|i| s << ((a[i]||0) ^ (b[i]||0))}
> => 14
This becomes:

s=""
[a.size, b.size].max.times {|i|
   s << ((a[i%a.size]||0)^(b[i%b.size]||0))
}

That takes care of the looping.  << doesn't create a new string (at 
least, not to my knowledge...) but it will (I presume) cause 
reallocation.  This should get around that:

s = " " * [a.size,b.size].max
[a.size, b.size].max.times {|i|
   s[i] = ((a[i%a.size]||0)^(b[i%b.size]||0))
}

No idea if it's faster or slower, and it's past my bedtime so I'm not 
going to benchmark it right now :-)

-- 
Alex