Update README.md

Author: usecodelee

README.md

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-# present
+# PRESENT
 
 **present 轻量级密码 C# 实现**
 
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 ### An Ultra-Lightweight Block Cipher
 With the establishment of the AES the need for new block ciphers has been greatly diminished; for almost all block cipher appli- cations the AES is an excellent and preferred choice. However, despite recent implementation advances, the AES is not suitable for extremely constrained environments such as RFID tags and sensor networks. In this paper we describe an ultra-lightweight block cipher, present. Both security and hardware efficiency have been equally important during the design of the cipher and at 1570 GE, the hardware requirements for present are competitive with today’s leading compact stream ciphers.
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+[PRESENT论文](https://github.com/usecodelee/Present/blob/master/present_spec.pdf)