暗号化の鍵生成が遅いとのことなので検証用に毎回鍵生成と鍵使い回しの両メソッドを書いた。使い方は$ encrypt_test [メソッド名] [ファイル名]

encrypt_test.rb

require 'openssl'

# 暗号化
def encrypt(in_file)
  in_filename = File::basename(in_file, '.txt')
  out_file = open(in_filename + "_encrypted" + ".txt", "w")

  File::open(in_file, "r") {|f|
    f.each do |line|
      str = line.chomp!
      enc = OpenSSL::Cipher::Cipher.new("aes256")
      enc.encrypt
      enc.pkcs5_keyivgen("test")
      encrypted = (enc.update(str) + enc.final).unpack("H*").to_s
      out_file.puts encrypted
    end
  }
end

# 暗号化（鍵生成共通化）
def encrypt_shared(in_file)
  in_filename = File::basename(in_file, '.txt')
  out_file = open(in_filename + "_encrypted" + ".txt", "w")

  enc = OpenSSL::Cipher::Cipher.new("aes256")
  enc.encrypt
  enc.pkcs5_keyivgen("test")

  File::open(in_file, "r") {|f|
    f.each do |line|
      str = line.chomp!
      enc2 = enc.clone
      encrypted = (enc2.update(str) + enc2.final).unpack("H*").to_s
      out_file.puts encrypted
    end
  }
end

# 復号化
def decrypt(in_file)
  in_filename = File::basename(in_file, '.txt')
  out_file = open(in_filename + "_decrypted" + ".txt", "w")

  File::open(in_file, "r") {|f|
    f.each do |line|
      str = line.chomp!
      dec = OpenSSL::Cipher::Cipher.new("aes256")
      dec.decrypt
      dec.pkcs5_keyivgen("test")
      decrypted = dec.update(Array.new([str]).pack("H*")) + dec.final
      out_file.puts decrypted
    end
  }
end

# 復号化（鍵生成共通化）
def decrypt_shared(in_file)
  in_filename = File::basename(in_file, '.txt')
  out_file = open(in_filename + "_decrypted" + ".txt", "w")

  dec = OpenSSL::Cipher::Cipher.new("aes256")
  dec.decrypt
  dec.pkcs5_keyivgen("test")

  File::open(in_file, "r") {|f|
    f.each do |line|
      str = line.chomp!
      dec2 = dec.clone
      decrypted = dec2.update(Array.new([str]).pack("H*")) + dec2.final
      out_file.puts decrypted
    end
  }
end

# 実行
send(ARGV[0], ARGV[1])