Many smartphone applications use near field communication (NFC) systems to guarantee that two smartphones appear in near field when interactions take place. 'Google Wallet' is one such example. This kind of guarantee is called near field authentication, which is to authenticate whether two smartphones stay closely to each other. Using NFC systems is a natural option for near field authentication. However, NFC systems rely on NFC chips, which are not available on many smartphones, especially the low-end smartphones. This obstructs the popularization of NFC applications. In this paper, we propose a simple and lightweight system to perform near field authentication without using NFC chips. The idea is to put two smartphones side by side and let the user slide his finger across the two smartphone screens. The two smartphones then extract correlated feature values from the movement on each screen to authenticate each other. When the two smartphones are in near field, our system generates the same cryptographic key for both. The key can be used by another upper system to carry out confidential communications. Our system is proved to be secure in the random oracle model. We demonstrate its efficiency on Motorola Droid smartphones in our evaluations.