A very, very simplified explanation of how BCrypt equality works.

simplified_bcrypt.rb

module BCrypt
  class Password
    def initialize(password_hash)
      @password_hash = password_hash
    end
    
    def self.create(password)
      @password_hash = encrypt(password)
    end
    
    # overrides the equality operator
    # This allows BCrypt to accept plaintext strings, and to match their equality against encrypted strings.
    def ==(value)
      @password_hash == BCrypt::Password.encrypt(value)
    end
    
    def to_s
      @password_hash
    end
    
    private
    
    def self.encrypt(value)
      # this is a mystery function that takes in a plaintext string and returns an encrypted string.
      #
      # e.g. BCrypt::Password.encrypt("password") #=> "$2a$10$A1sbW0NafppF58qfQqzp0eTro0h9TIQ8MSxMZR3o14ACYwtAQbVlC"
    end
  end
end