Simple Encryption in Ruby without external gems

If you don't need real encryption, you can use a simple cipher. (This can be used when you don't need security, or to encrypt short random/one-off strings.)

ALPHABET = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"

#generated with ALPHABET.split('').shuffle.join
ENCODING = "MOhqm0PnycUZeLdK8YvDCgNfb7FJtiHT52BrxoAkas9RWlXpEujSGI64VzQ31w"

def encode(text)
  text.tr(ALPHABET, ENCODING)
end

def decode(text)
  text.tr(ENCODING, ALPHABET)
end

You could use OpenSSL::Cipher

# for more info, see http://ruby-doc.org/stdlib-1.9.3/libdoc/openssl/rdoc/OpenSSL/Cipher.html

require 'openssl'
require 'digest/sha1'

# create the cipher for encrypting
cipher = OpenSSL::Cipher::Cipher.new("aes-256-cbc")
cipher.encrypt

# you will need to store these for later, in order to decrypt your data
key = Digest::SHA1.hexdigest("yourpass")
iv = cipher.random_iv

# load them into the cipher
cipher.key = key
cipher.iv = iv

# encrypt the message
encrypted = cipher.update('This is a secure message, meet at the clock-tower at dawn.')
encrypted << cipher.final
puts "encrypted: #{encrypted}\n"

# now we create a sipher for decrypting
cipher = OpenSSL::Cipher::Cipher.new("aes-256-cbc")
cipher.decrypt
cipher.key = key
cipher.iv = iv

# and decrypt it
decrypted = cipher.update(encrypted)
decrypted << cipher.final
puts "decrypted: #{decrypted}\n"

But the intermediate form doesn't lend itself well to printing

---

Given your thought that it would be nice if the intermediate form was the same length, you might just use a simple map of one char to another.

PLEASE UNDERSTAND THAT THIS IS NOT SECURE

You can easily brute force the key, but it seems to be congruent with your requirements.

class Cipher
  
  def initialize(shuffled)
    normal = ('a'..'z').to_a + ('A'..'Z').to_a + ('0'..'9').to_a + [' ']
    @map = normal.zip(shuffled).inject(:encrypt => {} , :decrypt => {}) do |hash,(a,b)|
      hash[:encrypt][a] = b
      hash[:decrypt][b] = a
      hash
    end
  end

  def encrypt(str)
    str.split(//).map { |char| @map[:encrypt][char] }.join
  end
  
  def decrypt(str)
    str.split(//).map { |char| @map[:decrypt][char] }.join
  end
  
end

# pass the shuffled version to the cipher
cipher = Cipher.new ["K", "D", "w", "X", "H", "3", "e", "1", "S", "B", "g", "a", "y", "v", "I", "6", "u", "W", "C", "0", "9", "b", "z", "T", "A", "q", "U", "4", "O", "o", "E", "N", "r", "n", "m", "d", "k", "x", "P", "t", "R", "s", "J", "L", "f", "h", "Z", "j", "Y", "5", "7", "l", "p", "c", "2", "8", "M", "V", "G", "i", " ", "Q", "F"]

msg = "howdy pardner"

crypted = cipher.encrypt msg
crypted # => "1IzXAF6KWXvHW"

decrypted = cipher.decrypt crypted
decrypted # => "howdy pardner"

Optional method for encryption and decryption

gem 'activesupport'

require 'active_support'

key = SecureRandom.random_bytes(32)
crypt = ActiveSupport::MessageEncryptor.new(key)
encrypted_data = crypt.encrypt_and_sign("your password")
password = crypt.decrypt_and_verify(encrypted_data)