require 'concurrent/map'
require 'openssl'

module ActiveSupport
  # KeyGenerator is a simple wrapper around OpenSSL's implementation of PBKDF2.
  # It can be used to derive a number of keys for various purposes from a given secret.
  # This lets Rails applications have a single secure secret, but avoid reusing that
  # key in multiple incompatible contexts.
  class KeyGenerator
    def initialize(secret, options = {})
      @secret = secret
      # The default iterations are higher than required for our key derivation uses
      # on the off chance someone uses this for password storage
      @iterations = options[:iterations] || 2**16
    end

    # Returns a derived key suitable for use.  The default key_size is chosen
    # to be compatible with the acceptable key length of aes-256-cbc, the default cipher.
    def generate_key(salt, key_size=32)
      OpenSSL::PKCS5.pbkdf2_hmac_sha1(@secret, salt, @iterations, key_size)
    end
  end

  # CachingKeyGenerator is a wrapper around KeyGenerator which allows users to avoid
  # re-executing the key generation process when it's called using the same salt and
  # key_size.
  class CachingKeyGenerator
    def initialize(key_generator)
      @key_generator = key_generator
      @cache_keys = Concurrent::Map.new
    end

    # Returns a derived key suitable for use.  The default key_size is chosen
    # to be compatible with the acceptable key length of aes-256-cbc, the default cipher.
    def generate_key(salt, key_size=32)
      @cache_keys["#{salt}#{key_size}"] ||= @key_generator.generate_key(salt, key_size)
    end
  end

  class LegacyKeyGenerator # :nodoc:
    SECRET_MIN_LENGTH = 30 # Characters

    def initialize(secret)
      ensure_secret_secure(secret)
      @secret = secret
    end

    def generate_key(salt)
      @secret
    end

    private

    # To prevent users from using something insecure like "Password" we make sure that the
    # secret they've provided is at least 30 characters in length.
    def ensure_secret_secure(secret)
      if secret.blank?
        raise ArgumentError, "A secret is required to generate an integrity hash " \
          "for cookie session data. Set a secret_key_base of at least " \
          "#{SECRET_MIN_LENGTH} characters in config/secrets.yml."
      end

      if secret.length < SECRET_MIN_LENGTH
        raise ArgumentError, "Secret should be something secure, " \
          "like \"#{SecureRandom.hex(16)}\". The value you " \
          "provided, \"#{secret}\", is shorter than the minimum length " \
          "of #{SECRET_MIN_LENGTH} characters."
      end
    end
  end
end