Storing Hashed User Passwords in the Database






Storing Hashed User Passwords in the Database

Problem

The database table defined in Recipe 15.8 stores users' passwords as plain text. This is a bad idea: if someone compromises the database, she will have all of your users' passwords. It's best to store a secure hash of the password instead. That way, you don't have the password (so no one can steal it), but you can verify that a user knows his password.

Solution

Recreate the users table from Recipe 15.8 so that instead of a password field, it has a hashed_password field. Here's some MySQL code to do that:

	use mywebapp_development;
	DROP TABLE IF EXISTS 'users';
	CREATE TABLE 'users' (
	  'id' INT(11) NOT NULL AUTO_INCREMENT,
	  'username' VARCHAR(255) NOT NULL,
	  'hashed_password' VARCHAR(40) NOT NULL,
	  PRIMARY KEY (id)
	);

Open the file app/models/user.rb created in Recipe 15.8, and edit it to look like this:

	require 'sha1'
	
	class User < ActiveRecord::Base
	  attr_accessor :password
	  attr_protected :hashed_password
	  validates_uniqueness_of :username
	  validates_confirmation_of : 
password,
	    :if => lambda { |user| user.new_record? or not user.password.blank? }
	  validates_length_of :password, :within => 5..40,
	    :if => lambda { |user| user.new_record? or not user.password.blank? }

	  def self.hashed(str)
	    SHA1.new(str).to_s
	  end

	  # If a user matching the credentials is found, returns the User object.
	  # If no matching user is found, returns nil.
	  def self.authenticate(user_info)
	    user = find_by_username(user_info[:username])
	    if user && user.hashed_password == hashed(user_info[:password])
	      return user
	    end
	  end

	  private
	  before_save :update_password

	  # Updates the hashed_password if a plain password was provided.
	  def update_password
	    if not password.blank?
	      self.hashed_password = self.class.hashed(password)
	    end
	  end
	end

Once you do this, your application will work as before (though you'll have to convert any preexisting user accounts to the new password format). You don't need to modify any of the controller or view code, because the User.authenticate method works the same way it did before. This is one of the benefits of separating business logic from presentation logic.

Discussion

There are now three pieces to our user model. The first is the enhanced validation code. The user model now:

  • Provides getters and setters for the password attribute.

  • Makes sure that the hashed_password field in the database can't be accessed from the outside.

  • Ensures that each user has a unique username.

When a new user is created, or when the password is changed, User ensures:

  • That the value of the password_confirmation attribute is equal to the value of the password attribute.

  • That the password is between 5 and 40 characters long.

The second section of code defines User class methods as before. We add one new class-level method, hashed, which performs the hashing function on a plaintext password. If we want to change hashing mechanisms in the future, we only have to change this method (and migrate any existing passwords).

The third piece of code in the model is a private instance method, update_password, which synchronizes the plaintext password attribute with the hashed version in the database. The call to before_save sets up this method to be called before a User object is saved to the database. This way you can change a user's password by setting password to its plaintext value, instead of doing the hash yourself.

See Also

  • Recipe 13.14, "Validating Data with ActiveRecord"

  • Recipe 15.8, "Creating a Login System"



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