cscamp ctf 2012 - crypto 100

Crypto 100 challenge requires decryption of a given ciphertext.

There is also an input field that takes in a plain text and responds with a ciphertext. By encrypting 'a' and 'A' we see that it provides the same output. Hence the cipher is case insensitive. The following code solves the challenge by generating a table of cipher text for a-z and then decrypts the given ciphertext using the mapping.

	require 'uri'
	require 'net/http'
	def odd_positions_in_string( str )
	str.scan(/(.).?/).join
	end
	def even_positions_in_string ( str )
	str.scan(/.?(.)/).join
	end
	uri = URI.parse('http://176.9.193.13/CrYpt0ch4l12554222426.php')
	http = Net::HTTP.new(uri.hostname, uri.port)
	res = http.request_get(uri.path)
	enc_ver = res.body.delete('^01').scan(/.{54}/)
	# vertical to horiztal conversion
	enc_hor_temp = ''
	54.times {|i| enc_ver.collect{|x| enc_hor_temp << x[i]} }
	enc_hor = enc_hor_temp.scan(/....../)
	# create a mapping of 'a'..'z' with its respective encoding
	char_to_hex_map = Hash.new
	("a".."z").each do |alpha|
	uri = URI("http://176.9.193.13/CrYpt0ch4l12554222426.php?key=#{alpha}")
	html = Net::HTTP.get(uri)
	enc = html.split(/\r?\n/).last.delete('^01')
	char_to_hex_map[alpha] = odd_positions_in_string(enc) + even_positions_in_string(enc)
	end
	# find the decrypted text
	decrypted_text = ''
	enc_hor.each do |i|
	decrypted_text << char_to_hex_map.key(i)
	end
	puts decrypted_text.upcase

view raw cscamp_crypto100.rb hosted with ❤ by GitHub

$ ruby cscamp_crypto100.rb
JUSTASMALLBLINDYCRYPTOGAMEZ