encrypt-decrypt-file.rb

require 'optparse'

class Practice
	def initialize
		@fname = 'data.txt'
		# predefined key for the demo (32-byte)
		@key = "12345678901234567890123456789012"
		# prepage libraries in bg for faster start
		@libs_loading_thr = Thread.new do
			require 'openssl'
			require 'base64'
		end

		#-- parse cmd options
		opt_parser = OptionParser.new do |opts|
		    opts.banner = 'Accepted cmd options:'

		    opts.on '--demo', 'Interactive demo' do
		        demo
		    end
		    opts.on '-e', '--encrypt', 'Encrypt '+@fname do
				ensure_file_exists!
				puts "Encrypting #{@fname}..."
				@libs_loading_thr.join
		        encrypt_file
		    end
		    opts.on '-d', '--decrypt', 'Decrypt '+@fname do
				ensure_file_exists!
				puts "Decrypting #{@fname}..."
				@libs_loading_thr.join
		        decrypt_file
		    end
		end
		# if no options -- show help
		if ARGV.empty?
			puts opt_parser.help
			exit
		end
		opt_parser.parse!
	rescue RuntimeError
		puts 'Error: '+$!.to_s
	end

	def demo
		# create the file if not exists
		unless File.exist? @fname
			File.write @fname, 'This is a secret message.'
		end

		puts "Prepare #{@fname} in the current folder."
		print 'Press Enter when ready -- file content will be encrypted...'
		@libs_loading_thr.join
		gets

		encrypt_file
		print 'Press Enter to decrypt the text...'
		gets
		
		decrypt_file
	end

	def encrypt_file
		text = File.read @fname
		encrypted_text = aes_encrypt(text, @key)
		File.write @fname, encrypted_text
		puts 'Now, text in the file is encrypted.'
	end

	def decrypt_file
		encrypted_text = File.read @fname
		decrypted_text = aes_decrypt(encrypted_text, @key)
		File.write @fname, decrypted_text
		puts 'Now, text in the file is decrypted.'
	end

	def ensure_file_exists!
		raise "File not found: #{@fname}" unless File.exist? @fname
	end

	# AES256 encryption
	def aes_encrypt(text, key)
		cipher = get_cipher.tap { _1.encrypt; _1.key = key }
		iv = cipher.random_iv  # 16 bytes
		cipher_text = cipher.update(text) + cipher.final
		# *ensure there is no newline char in the encrypted text
		Base64.strict_encode64(iv + cipher_text)
	end

	def aes_decrypt(encrypted_text, key)
		decrypted_text = Base64.decode64(encrypted_text)
		iv, cipher_text = decrypted_text.then { [_1[0..15], _1[16..]] }
		cipher = get_cipher.tap { _1.decrypt; _1.key = key; _1.iv = iv }
		cipher.update(cipher_text) + cipher.final
	rescue
		raise 'Looks like file was not encrypted'
	end

	def get_cipher
		OpenSSL::Cipher::AES256.new(:CBC)
	end
end

Practice.new