class ActiveSupport::CachingKeyGenerator

key_size
re-executing the key generation process when it’s called using the same salt and
CachingKeyGenerator is a wrapper around KeyGenerator which allows users to avoid

def generate_key(salt, key_size=64)

i.e. OpenSSL::Digest::SHA1#block_length
to be compatible with the default settings of ActiveSupport::MessageVerifier.
Returns a derived key suitable for use. The default key_size is chosen 
def generate_key(salt, key_size=64)
  @cache_keys["#{salt}#{key_size}"] ||= @key_generator.generate_key(salt, key_size)
end

def initialize(key_generator) 

def initialize(key_generator)
  @key_generator = key_generator
  @cache_keys = ThreadSafe::Cache.new
end