frequency_analysis.rb

ciphertexts = [
  "VGYURYPEDFNCEPGEWELLPVNWRDCNFGMGXQEPPRG-CBFIGLEMG",
  "EQ’PMEIGQNVBAGBTFEGMSYNRKMNWEPBLWBYPSNWMQNFBDWEQVYNR-CBFIGLEMG"
]

english_freqs = "ETAOINSHRDLCUMWFGYPBVKJXQZ".chars

# count letter frequencies in ciphertexts
char_freqs = Hash.new(0)
(ciphertexts.join).each_char {|c| char_freqs[c] += 1}

# sort by letter frequency freqs (descending)
ciphertext_freqs = Hash[char_freqs.sort_by{|k,v| -v}]

# remove non-alphanumeric ciphertext chars (e.g. punctuation)
(ciphertext_freqs.keys - english_freqs).each {|p| ciphertext_freqs.delete(p)}

# display the letter and frequency mappings
puts "Ciphertext => English (count)"
ciphertext_freqs.each_with_index do |ct_freq,index|
 puts "#{ct_freq.first} => #{english_freqs[index]} (#{ct_freq.last})"
end

used_english_freqs = english_freqs[0..ciphertext_freqs.count-1].join
cipher_char_freqs =  ciphertext_freqs.keys.join

ciphertexts.each do |ciphertext|
  ciphertext_before = ciphertext
  ciphertext_after  = ciphertext.tr(cipher_char_freqs, used_english_freqs)
  puts "Ciphertext before: #{ciphertext_before}"
  puts "Ciphertext after:  #{ciphertext_after}"
end