A digital Finite Impulse Response (FIR) filter is a ubiquitous block in digital signal processing applications and its behavior is determined by its coefficients. To protect filter coefficients from an adversary, efficient obfuscation techniques have been proposed, either by hiding them behind decoys or replacing them by key bits. In this article, we initially introduce a query attack that can discover the secret key of such obfuscated FIR filters, which could not be broken by existing prominent attacks. Then, we propose a first of its kind hybrid technique, including both hardware obfuscation and logic locking using a point function for the protection of parallel direct and transposed forms of digital FIR filters. Experimental results show that the hybrid protection technique can lead to FIR filters with higher security while maintaining the hardware complexity competitive or superior to those locked by prominent logic locking methods. It is also shown that the protected multiplier blocks and FIR filters are resilient to existing attacks. The results on different forms and realizations of FIR filters show that the parallel direct form FIR filter has a promising potential for a secure design.

翻译：数字有限脉冲响应（FIR）滤波器是数字信号处理应用中的通用模块，其行为由其系数决定。为确保滤波器系数免遭对手窃取，现有方案提出了高效混淆技术，或通过将其隐藏在伪系数中，或通过秘钥位替代。本文首先提出一种查询攻击方法，能够发现此类混淆FIR滤波器的秘密密钥，而现有主流攻击无法破解该密钥。随后，我们首次提出一种混合技术，结合了硬件混淆与基于点函数的逻辑锁定，用于保护并行直接型和转置型数字FIR滤波器。实验结果表明，该混合保护技术可在维持或优于主流逻辑锁定方法硬件复杂度的前提下，赋予FIR滤波器更高的安全性。研究还表明，受保护的乘法器模块和FIR滤波器对现有攻击具有鲁棒性。针对不同形式与实现结构的FIR滤波器实验显示，并行直接型FIR滤波器在安全设计方面展现出显著潜力。