Rails - Aug 2006 - NEWBIE QUESTION - recode in Ruby

I wish to recode the following php code to ruby:

function text_decrypt_symbol($s, $i) {
# $s is a text-encoded string, $i is index of 2-char code. function  
returns number in range 0-255

         return (ord(substr($s, $i, 1)) - 100)*16 + ord(substr($s, $i 
+1, 1)) - 100;
}

function text_decrypt($s) {

     if ($s == "")
         return $s;
     $enc = 85 ^ text_decrypt_symbol($s, 0);
     for ($i = 2; $i < strlen($s); $i+=2) { # $i=2 to skip salt
         $result .= chr(text_decrypt_symbol($s, $i) ^ $enc++);
         if ($enc > 255)
             $enc = 0;
     }
     return $result;
}

As I have stated I am a ruby newbie but thus far I have produced this:

   def text_decrypt(value)
     enc = 85 ^ text_decrypt_symbol(value, 0);

     i = 2

     while i < value.length
         curchar = text_decrypt_symbol(value, i) ^ enc++
         result += curchar.chr
         if enc > 255
             enc = 0
         end

         i+=2
     end
     return result;
   end
   def text_decrypt_symbol(value, value2)
     char1 = value[value2, 1]
     char2 = value[value2+1, 1]

     return (char1.chr - 100)*16 + char2.chr - 100;
   end

Apparently the .chr method requires an integer to work.  Anyone have  
any advice?