A precursor of http://github.com/henrik/autho. Simple Rails-less authentication model. Assumes an API compatible with ActiveRecord and Rails' has_secure_password.

authentication.rb

require "bcrypt"
require "attr_extras"

class Authentication
  pattr_initialize :finder, :email, :password

  def self.digest(unencrypted_password)
    BCrypt::Password.create(unencrypted_password)
  end

  def user
    user = finder.find_by_email(email)
    user && authenticate(user, password)
  end

  private

  def authenticate(user, unencrypted_password)
    if BCrypt::Password.new(user.password_digest) == unencrypted_password
      user
    else
      nil
    end
  end
end

authentication_spec.rb

require "spec_helper"

describe Authentication do
  describe "#user" do
    let(:finder) { double }

    it "returns the user with matching email and password" do
      user = double(password_digest: digest("sesame"))
      expect(finder).to receive(:find_by_email).with("user@example.com").and_return(user)

      auth = Authentication.new(finder, "user@example.com", "sesame")
      expect(auth.user).to eq user
    end

    it "finds nothing if email does not match" do
      expect(finder).to receive(:find_by_email).with("user@example.com").and_return(nil)

      auth = Authentication.new(finder, "user@example.com", "sesame")
      expect(auth.user).to be_nil
    end

    it "finds nothing if password does not match" do
      user = double(password_digest: digest("sesame"))
      expect(finder).to receive(:find_by_email).with("user@example.com").and_return(user)

      auth = Authentication.new(finder, "user@example.com", "nomagic")
      expect(auth.user).to be_nil
    end
  end

  def digest(unencrypted_password)
    BCrypt::Password.create(unencrypted_password)
  end
end