session cookie decrypter for Rails

rails_42_with_active_support.rb

require 'cgi'
require 'json'
require 'active_support'

def verify_and_decrypt_session_cookie(cookie, secret_key_base)
  cookie = CGI::unescape(cookie)
  salt         = 'encrypted cookie'
  signed_salt  = 'signed encrypted cookie'
  key_generator = ActiveSupport::KeyGenerator.new(secret_key_base, iterations: 1000)
  secret = key_generator.generate_key(salt)
  sign_secret = key_generator.generate_key(signed_salt)
  encryptor = ActiveSupport::MessageEncryptor.new(secret, sign_secret, serializer: JSON)
  encryptor.decrypt_and_verify(cookie)
end

rails_42_with_pure_ruby.rb

require 'openssl'
require 'base64'
require 'cgi'
require 'json'

def verify_and_decrypt_session_cookie cookie, secret_key_base
  cookie = CGI.unescape(cookie)

  #################
  # generate keys #
  #################
  encrypted_cookie_salt = 'encrypted cookie' # default: Rails.application.config.action_dispatch.encrypted_cookie_salt
  encrypted_signed_cookie_salt = 'signed encrypted cookie' # default: Rails.application.config.action_dispatch.encrypted_signed_cookie_salt
  iterations = 1000
  key_size = 64
  secret = OpenSSL::PKCS5.pbkdf2_hmac_sha1(secret_key_base, encrypted_cookie_salt, iterations, key_size)
  sign_secret = OpenSSL::PKCS5.pbkdf2_hmac_sha1(secret_key_base, encrypted_signed_cookie_salt, iterations, key_size)

  ##########
  # Verify #
  ##########
  data, digest = cookie.split('--')
  raise 'invalid message' unless digest == OpenSSL::HMAC.hexdigest(OpenSSL::Digest::SHA1.new, sign_secret, data)
  # you better use secure compare instead of `==` to prevent time based attact,
  # ref: ActiveSupport::SecurityUtils.secure_compare

  ###########
  # Decrypt #
  ###########
  encrypted_message = Base64.strict_decode64(data)
  encrypted_data, iv = encrypted_message.split('--').map{|v| Base64.strict_decode64(v) }
  cipher = OpenSSL::Cipher::Cipher.new('aes-256-cbc')
  cipher.decrypt
  cipher.key = secret
  cipher.iv  = iv
  decrypted_data = cipher.update(encrypted_data)
  decrypted_data << cipher.final

  JSON.load(decrypted_data)
end

rails_5_with_active_support.rb

require 'bundler/setup'
require 'yaml'
require 'uri'
require 'json'
require 'active_support'

def verify_and_decrypt_session_cookie(cookie, secret_key_base)
  cookie = URI.decode_www_form_component(cookie)
  salt         = 'encrypted cookie'
  signed_salt  = 'signed encrypted cookie'
  key_generator = ActiveSupport::KeyGenerator.new(secret_key_base, iterations: 1000)
  secret = key_generator.generate_key(salt)[0, ActiveSupport::MessageEncryptor.key_len]
  sign_secret = key_generator.generate_key(signed_salt)
  encryptor = ActiveSupport::MessageEncryptor.new(secret, sign_secret, serializer: JSON)
  encryptor.decrypt_and_verify(cookie)
end

rails_5_with_pure_ruby.rb

require 'openssl'
require 'base64'
require 'uri'
require 'json'
require 'yaml'

def verify_and_decrypt_session_cookie(cookie, secret_key_base)
  cookie = URI.decode_www_form_component(cookie)

  #################
  # generate keys #
  #################
  # Rails.application.config.action_dispatch.encrypted_cookie_salt
  encrypted_cookie_salt = 'encrypted cookie'
  # Rails.application.config.action_dispatch.encrypted_signed_cookie_salt
  encrypted_signed_cookie_salt = 'signed encrypted cookie'
  iterations = 1000
  key_size = 64
  secret = OpenSSL::PKCS5.pbkdf2_hmac_sha1(secret_key_base, encrypted_cookie_salt, iterations, key_size)[0, 32]
  sign_secret = OpenSSL::PKCS5.pbkdf2_hmac_sha1(secret_key_base, encrypted_signed_cookie_salt, iterations, key_size)

  ##########
  # Verify #
  ##########
  data, digest = cookie.split('--')
  raise 'invalid message' unless digest == OpenSSL::HMAC.hexdigest(OpenSSL::Digest::SHA1.new, sign_secret, data)
  # you better use secure compare instead of `==` to prevent time based attact,
  # ref: ActiveSupport::SecurityUtils.secure_compare

  ###########
  # Decrypt #
  ###########
  encrypted_message = Base64.strict_decode64(data)
  encrypted_data, iv = encrypted_message.split('--').map{|v| Base64.strict_decode64(v) }
  cipher = OpenSSL::Cipher.new('aes-256-cbc')
  cipher.decrypt
  cipher.key = secret
  cipher.iv  = iv
  decrypted_data = cipher.update(encrypted_data)
  decrypted_data << cipher.final
  JSON.parse(decrypted_data)
end

FYI: your scripts didn't work for me under rails 5.2.4, but they got me started. Here's what I wound up with: https://gist.github.com/nevans/558b69f227c243f63552a6f91915424f

Hi, @nevans, Thanks for your work.

As the file name called rails_42_with_active_support.rb, this gist is for Rails v4 instead of v5. I checked your gist and the SO thread here: https://stackoverflow.com/questions/41474176/how-to-decrypt-a-rails-5-session-cookie-manually

They are really helpful, thank you.