[论文解读] Encrypt Flip-Flop: A Novel Logic Encryption Technique For Sequential Circuits

Rajit Karmakar, Santanu Chattopadhyay+1 more|arXiv (Cornell University)|Jan 15, 2018

Physical Unclonable Functions (PUFs) and Hardware Security参考文献 17被引用 65

一句话总结

本论文提出 Encrypt Flip-Flop，一种基于 MUX 的逻辑加密，保护 flip-flop 输出，旨在阻止扫描型、SAT 和其他攻击，开销低，在 ISCAS’89 与 ITC’99 基准上得到验证。

ABSTRACT

Logic Encryption is one of the most popular hardware security techniques which can prevent IP piracy and illegal IC overproduction. It introduces obfuscation by inserting some extra hardware into a design to hide its functionality from unauthorized users. Correct functionality of an encrypted design depends upon the application of correct keys, shared only with the authorized users. In the recent past, extensive efforts have been devoted in extracting the secret key of an encrypted design. At the same time, several countermeasures have also been proposed by the research community to thwart different state-of-the-art attacks on logic encryption. However, most of the proposed countermeasures fail to prevent the powerful SAT attack. Although a few researchers have proposed different solutions to withstand SAT attack, those solutions suffer from several drawbacks such as high design overheads, low output corruptibility, and vulnerability against removal attack. Almost all the known logic encryption strategies are vulnerable to scan based attack. In this paper, we propose a novel encryption technique called Encrypt Flip-Flop, which encrypts the outputs of selected flip-flops by inserting multiplexers (MUX). The proposed strategy can thwart all the known attacks including SAT and scan based attacks. The scheme has low design overhead and implementation complexity. Experimental results on several ISCAS'89 and ITC'99 benchmarks show that our proposed method can produce reasonable output corruption for wrong keys.

研究动机与目标

• 动机：在外包集成电路制造中，防止知识产权盗版和过量生产，需要安全的逻辑加密。
• 提出一种翻转触发器输出加密策略，以抑制通过扫描链的信息泄露。
• 实现对多种攻击的保护（基于扫描、基于 SAT、路径敏化、逻辑圆锥攻击）且硬件开销低。
• 提供一个实用的翻转触发器选择流程，在最大化安全性同时最小化对输出的泄露。

提出的方法

• 在选定的翻转触发器输出前插入基于 MUX 的密钥门，以混淆 Q 和/或 not-Q 路径。
• 选择要加密的翻转触发器，使每个受影响的输出包含所有密钥门（基于 ICOD 的重叠分析）。
• 避免在 MUX 之外添加额外硬件；保护依赖于对 Q/ not-Q 信号的仔细放置和流量控制。
• 将翻转触发器输出视为加密单位，以限制通过扫描链的信息泄露。
• 提供一个加密算法（Algorithm 2）用于选择用于安全放置的翻转触发器。

实验结果

研究问题

• RQ1 Encrypt Flip-Flop 是否能阻止通过扫描链的信息泄露并抵抗常见的逻辑加密攻击？
• RQ2 应如何选择用于加密的翻转触发器以最大化安全性并最小化开销？
• RQ3 该方法是否在不增加额外硬件的情况下对付基于 SAT 的攻击和路径敏化攻击？
• RQ4 在实际中对广泛讨论的攻击（逻辑锥、爬山搜索等）的安全性有何影响？

主要发现

Circuit Name	Attack Complexity (Scan / Brute Force)
s5378	2^{66} / 2^{163}
s9234	2^{51} / 2^{159}
s13207	2^{56} / 2^{190}
s15850	2^{62} / 2^{205}
s38417	2^{69} / 2^{156}
s38584	2^{74} / 2^{166}
b17	2^{61} / 2^{165}
b18	2^{47} / 2^{165}
b19	2^{51} / 2^{152}
b20	2^{64} / 2^{160}
b21	2^{58} / 2^{160}
b22	2^{53} / 2^{160}

• Encrypt Flip-Flop 通过用基于 MUX 的密钥门对翻转触发器输出进行加密并进行小心的翻转触发器选择来实现保护。
• 选择策略确保每个受影响的输出包含所有密钥门，限制来自逻辑锥攻击的穷举优势。
• 由于扫描链不泄露密钥信息且可控性/可观测性受到限制，SCAN 基于的攻击情境得到缓解。
• Compared to prior schemes, the method claims low design overhead and resilience against multiple attacks without requiring additional security hardware.
• 在 ISCAS’89 和 ITC’99 基准上的实验结果证明了该方法的可行性及其安全优势。