require "openssl"
require "base64"
require "active_support/core_ext/array/extract_options"
require "active_support/message_verifier"

module ActiveSupport
  # MessageEncryptor is a simple way to encrypt values which get stored
  # somewhere you don't trust.
  #
  # The cipher text and initialization vector are base64 encoded and returned
  # to you.
  #
  # This can be used in situations similar to the <tt>MessageVerifier</tt>, but
  # where you don't want users to be able to determine the value of the payload.
  #
  #   salt  = SecureRandom.random_bytes(64)
  #   key   = ActiveSupport::KeyGenerator.new('password').generate_key(salt, 32) # => "\x89\xE0\x156\xAC..."
  #   crypt = ActiveSupport::MessageEncryptor.new(key)                           # => #<ActiveSupport::MessageEncryptor ...>
  #   encrypted_data = crypt.encrypt_and_sign('my secret data')                  # => "NlFBTTMwOUV5UlA1QlNEN2xkY2d6eThYWWh..."
  #   crypt.decrypt_and_verify(encrypted_data)                                   # => "my secret data"
  class MessageEncryptor
    DEFAULT_CIPHER = "aes-256-cbc"

    module NullSerializer #:nodoc:
      def self.load(value)
        value
      end

      def self.dump(value)
        value
      end
    end

    module NullVerifier #:nodoc:
      def self.verify(value)
        value
      end

      def self.generate(value)
        value
      end
    end

    class InvalidMessage < StandardError; end
    OpenSSLCipherError = OpenSSL::Cipher::CipherError

    # Initialize a new MessageEncryptor. +secret+ must be at least as long as
    # the cipher key size. For the default 'aes-256-cbc' cipher, this is 256
    # bits. If you are using a user-entered secret, you can generate a suitable
    # key by using <tt>ActiveSupport::KeyGenerator</tt> or a similar key
    # derivation function.
    #
    # Options:
    # * <tt>:cipher</tt>     - Cipher to use. Can be any cipher returned by
    #   <tt>OpenSSL::Cipher.ciphers</tt>. Default is 'aes-256-cbc'.
    # * <tt>:digest</tt> - String of digest to use for signing. Default is
    #   +SHA1+. Ignored when using an AEAD cipher like 'aes-256-gcm'.
    # * <tt>:serializer</tt> - Object serializer to use. Default is +Marshal+.
    def initialize(secret, *signature_key_or_options)
      options = signature_key_or_options.extract_options!
      sign_secret = signature_key_or_options.first
      @secret = secret
      @sign_secret = sign_secret
      @cipher = options[:cipher] || "aes-256-cbc"
      @digest = options[:digest] || "SHA1" unless aead_mode?
      @verifier = resolve_verifier
      @serializer = options[:serializer] || Marshal
    end

    # Encrypt and sign a message. We need to sign the message in order to avoid
    # padding attacks. Reference: http://www.limited-entropy.com/padding-oracle-attacks.
    def encrypt_and_sign(value)
      verifier.generate(_encrypt(value))
    end

    # Decrypt and verify a message. We need to verify the message in order to
    # avoid padding attacks. Reference: http://www.limited-entropy.com/padding-oracle-attacks.
    def decrypt_and_verify(value)
      _decrypt(verifier.verify(value))
    end

    # Given a cipher, returns the key length of the cipher to help generate the key of desired size
    def self.key_len(cipher = DEFAULT_CIPHER)
      OpenSSL::Cipher.new(cipher).key_len
    end

    private

      def _encrypt(value)
        cipher = new_cipher
        cipher.encrypt
        cipher.key = @secret

        # Rely on OpenSSL for the initialization vector
        iv = cipher.random_iv
        cipher.auth_data = "" if aead_mode?

        encrypted_data = cipher.update(@serializer.dump(value))
        encrypted_data << cipher.final

        blob = "#{::Base64.strict_encode64 encrypted_data}--#{::Base64.strict_encode64 iv}"
        blob << "--#{::Base64.strict_encode64 cipher.auth_tag}" if aead_mode?
        blob
      end

      def _decrypt(encrypted_message)
        cipher = new_cipher
        encrypted_data, iv, auth_tag = encrypted_message.split("--".freeze).map { |v| ::Base64.strict_decode64(v) }

        # Currently the OpenSSL bindings do not raise an error if auth_tag is
        # truncated, which would allow an attacker to easily forge it. See
        # https://github.com/ruby/openssl/issues/63
        raise InvalidMessage if aead_mode? && auth_tag.bytes.length != 16

        cipher.decrypt
        cipher.key = @secret
        cipher.iv  = iv
        if aead_mode?
          cipher.auth_tag = auth_tag
          cipher.auth_data = ""
        end

        decrypted_data = cipher.update(encrypted_data)
        decrypted_data << cipher.final

        @serializer.load(decrypted_data)
      rescue OpenSSLCipherError, TypeError, ArgumentError
        raise InvalidMessage
      end

      def new_cipher
        OpenSSL::Cipher.new(@cipher)
      end

      def verifier
        @verifier
      end

      def aead_mode?
        @aead_mode ||= new_cipher.authenticated?
      end

      def resolve_verifier
        if aead_mode?
          NullVerifier
        else
          MessageVerifier.new(@sign_secret || @secret, digest: @digest, serializer: NullSerializer)
        end
      end
  end
end
