Near Field Authentication

• Introduction
• Near Field Communication (NFC) system provide smartphones the ability to let two smart phones communicate with each other only when they are within a very small distance, usually a few centimeters. 
• For example, the Google wallet system uses NFC systems to establish the connection between a user and a checkout machine. 
• For example, the secure file transfer between two smartphones. It uses NFC systems to first exchange a cryptographic key and then transfer encrypted files over wireless local area network (WLN). 
• Authenticate whether two smartphones stay closely to each other. 
• It is natural to use NFC chips, however it is not available on many smart phones. 
• Another approach proposed by Li et al in ICC 2013.
• Put two smart phones side by side and let the use slide his finger across the two smartphone screens. 
• When two smart phones are in near field, their designed system will generate the same cryptographic key for both.
• The key can be used by another system to carry out confidential communications. 
• Background
• The Diffie–Hellman key exchange method allows two parties that have no prior knowledge of each other to jointly establish a shared secret key over an insecure channel. This key can then be used to encrypt subsequent communications using a symmetric key cipher.
• Alice and Bob agree to use a modulus p = 23 and base g = 5 
• They have their own secret integer
• They send message (g^theirSecret) mod p
• They descrypt the message (message)^their own secret, the results would be the same. Thus they can established a shared key.
• Man in the middle attack
• The attacker secretly relays and possibly alters the communication between two parties who believe they are directly communicating with each other