Cryptography assignment or something?

gistfile1.rb

lines = [
  "ABCDEFGHIJFG",
  "ABIKLAKGCMAIHDJACKNKCKNMDH",
  "MADHDPACLDHIKH",
  "AILDQIGCRIPACKNGHPACLDSDKLDPD",
  "ALCIHPIQTIDCEAPAHIG"
]

ct = lines.join("")

freq1 = Hash.new { 0 }
freq2 = Hash.new { 0 }
freq3 = Hash.new { 0 }
freq4 = Hash.new { 0 }

ct.each_char do |c|
  freq1[c] += 1
end

ct.each_char.to_a.each_cons(2) do |cons|
  cons = cons.join("")
  freq2[cons] += 1
end
ct.each_char.to_a.each_cons(3) do |cons|
  cons = cons.join("")
  freq3[cons] += 1
end
ct.each_char.to_a.each_cons(4) do |cons|
  cons = cons.join("")
  freq4[cons] += 1
end

freq1a = freq1.map { |k,v| [k,v] }
freq2a = freq2.map { |k,v| [k,v] }
freq3a = freq3.map { |k,v| [k,v] }
freq4a = freq4.map { |k,v| [k,v] }

[freq1a, freq2a, freq3a, freq4a].each do |array|
  array.sort! { |a, b| -a[1] <=> -b[1] }
end

[freq1a, freq2a, freq3a, freq4a].each do |array|
  puts "---------------------------------------"
  array.each do |ary|
    puts "#{ary[0]}: #{ary[1]}"
  end
end


changes = Hash.new { |h,x| x }
loop do
  puts (ct.each_char.to_a.map { |c| changes[c] }.join(""))
  sub = gets
  a = sub[0]
  b = sub[1]
  changes[a] = b
end