words by patrick

I’ve been asked to lead a Ruby session at my school for a small group of students interested in learning the language. As such, I’ve been brushing up on it lately, and I came up with a fun little script I thought I’d share.

Our topic is cryptography, and in the spirit of fun I thought I would show how easy it is to write a ROT13 cipher in Ruby. Here it is:

def uppercase_swap(byte)
  if /[A-M]/ === byte.chr then byte += 13
  else byte -= 13 end
end

def lowercase_swap(byte)
  if /[a-m]/ === byte.chr then byte += 13
  else byte -= 13 end
end

ARGV.shift.each_byte do |byte|
  byte = uppercase_swap byte if /[A-Z]/ =~ byte.chr
  byte = lowercase_swap byte if /[a-z]/ =~ byte.chr
  print byte.chr.to_s
end
print "\n"

As you can see, it’s not very big. Ignoring the 2 function definitions at the top momentarily, all we are doing is taking the first argument (ARGV.shift), and iterating through each byte –each character of the string. With each byte, we are checking to see if it is an upper or lowercase letter and calling the appropriate function to swap it with a different letter if it is.

Simple. Nice.

If anyone has any input on how to improve this code, please let me know. UPDATE: I made it better, here’s how:

def swapLetter(byte)
  if /[A-M]|[a-m]/ === byte.chr then byte += 13
  else byte -= 13 end
end

ARGV.shift.each_byte do |byte|
  byte = swapLetter(byte) if /[A-Z]|[a-z]/ =~ byte.chr
  print byte.chr.to_s
end
print "\n"

Merging functions is goood.

This entry was posted on Thursday, October 5th, 2006 at 9:32 am and is filed under crypto, ruby. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.