src/derails/passive_resistance/key_generator.rb
# frozen_string_literal: true

require "concurrent/map"
require "openssl"

module PassiveResistance
  # = Key Generator
  #
  # 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
    class << self
      def hash_digest_class=(klass)
        if klass.kind_of?(Class) && klass < OpenSSL::Digest
          @hash_digest_class = klass
        else
          raise ArgumentError, "#{klass} is expected to be an OpenSSL::Digest subclass"
        end
      end

      def hash_digest_class
        @hash_digest_class ||= OpenSSL::Digest::SHA1
      end
    end

    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
      # Also allow configuration here so people can use this to build a rotation
      # scheme when switching the digest class.
      @hash_digest_class = options[:hash_digest_class] || self.class.hash_digest_class
    end

    # Returns a derived key suitable for use.  The default +key_size+ is chosen
    # to be compatible with the default settings of PassiveResistance::MessageVerifier.
    # i.e. <tt>OpenSSL::Digest::SHA1#block_length</tt>
    def generate_key(salt, key_size = 64)
      OpenSSL::PKCS5.pbkdf2_hmac(@secret, salt, @iterations, key_size, @hash_digest_class.new)
    end

    def inspect # :nodoc:
      "#<#{self.class.name}:#{'%#016x' % (object_id << 1)}>"
    end
  end

  # = Caching Key Generator
  #
  # 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.
    def generate_key(*args)
      @cache_keys[args.join("|")] ||= @key_generator.generate_key(*args)
    end
  end
end
