NFC (Near Field Communication，近場通訊)，其為RFID (Radio Frequency Identification) 技術的延伸，電子裝置透過此技術可以簡單快速地在近距離內相互連結並傳遞資料。NFC具有短距離傳輸的原生安全性及相當直覺的操作方式。因此，該技術被廣泛的應用在門禁管理、票券服務、電子付款、裝置配對等應用上。然而隨著NFC應用的普及和使用上的需求，安全性的議題便成為了重要的考量。尤其是針對於沒有處理器能夠執行加密機制來建立安全通道的Passive Tag而言，目前都沒有很完善的機制保護使用者的資料不被竊取。

因此，在本研究中，提出一個防止越權存取的加密機制，透過對稱式加密演算法來保護NFC Tag中的個人資料，並藉由非對稱式加密演算法的特性達到金鑰交換 (Key Exchange) 的目的，藉此擴大NFC Tag應用的範疇。經由具有NFC功能的Android智慧型手機實作加解密功能，將NFC Tag上所載有的明文資料透過加密演算法處理後，再將加密後的密文寫回NFC Tag中。因此若有惡意的攻擊者想使用非法的讀取裝置 (Reader) 對NFC進行越權存取時，將會得到無法辨識的資訊，藉以保護使用者的個人資料安全。

經由實測數據得知，在現今的NFC智慧型手機上對NFC Tag中700個位元組的個人資料進行金鑰長度256位元的AES (Advanced Encryption Standard) 加解密動作，僅需約20ms的時間內即可完成資料的加解密。而實測RSA演算法得知，使用2048位元的金鑰對32個位元組( 256位元)的AES Key進行加解密動作，約在140ms內即可完成。並且，透過壓縮技術 (Huffman Coding)，可將存放在NFC Tag中700個位元組的個人資料壓縮至400個位元組左右。藉此減少NFC Tag中42%的記憶體空間佔用，並減少約400ms的存取時間。

NFC (Near Field Communication) refers to a standards-based connectivity technology, is an extension of existing RFID (Radio Frequency Identification) standards. NFC technology provides wireless connectivity in close proximity, enabling communication and data transfer between electronic devices. Given the inherent security of transmission over short distances and its fairly intuitive operation, NFC technology is extensively used in access control, ticketing, electronic payment and device pairing.

With increasing use of NFC applications, security issues have become more serious. In particular, the “Passive Tag” has no CPU to establish a secure channel by performing encryption. Currently, no significant mechanisms for protecting sensitive data stored in tags.

This study, a security mechanism is proposed for preventing unauthorized access. Encryption and decryption functions are implemented on an Android smartphone application to store encrypted data in NFC tags. Malicious attackers see only unreadable and unrecognizable “Cipher text” from NFC tags when gain unauthorized access. Therefore, the proposed security mechanism can be used to secure personal data in NFC tags.

Measurements made using a currently available Android smart phone reveal that, 700bytes of data can be encrypted/decrypted by AES Encryption in less than 20ms, and that 32bytes of data can be encrypted/decrypted by RSA encryption/decryption in less than 140ms. The data can be compressed using Huffman coding before they are stored in NFC Tags. 700bytes of uncompressed data can be compressed to 400bytes. The time for Tag access is also reduced.

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