On the complexity of side-channel attacks on AES-256 -- methodology and quantitative results on cache attacks
Larger key lengths translate into an exponential increase in the complexity of an exhaustive search. Side-channel attacks, however, use a divide-and-conquer approach and hence it is generally assumed that increasing the key length cannot be used as mitigation. Yet, the internal round structure of AES-256 and its key-scheduling seem to hinder a direct extension of the existing attacks on AES-128 and thus challenge the proposition above. Indeed two consecutives round keys are required to infer the secret key and the MixColumns operation, not present in the last round, apparently increases the key search complexity from to 2^8 to 2^32. Additionally, it is unclear what the impact of the different round structures is on the number of required measurements. In this paper, we explore this question and show how to attack AES-256 with a key search complexity of O(2^8). This work confirms with practical experiments that AES-256 only offers a marginal increase in resistance against the attacks –both in the required number of measurements and in the required processing time. As an example, we quantify this increase for the case of cache-based side-channel attacks: AES-256 only provides an increase in complexity of 6 to 7 compared to cache-based attacks on AES-128.
A Dynamic and Differential CMOS Logic Style to Resist Power and Timing Attacks on Security IC?s
We present a dynamic and differential CMOS logic style, which has a signal independent switching behavior. It is shown that during each clock cycle, power consumption and all circuit characteristics, such as leakage current, instantaneous current and input-output delay are identical and independent of the logic value and the sequence of the input data. Implementing the encryption module in this logic will protect it against any Side Channel Attack that takes advantage of power, timing and leakage information. We have built a set of logic gates and a flip-flop needed for cryptographic functions and implemented a larger module, for which area, total power consumption and variation on the power consumption have been compared with implementations in Static Complementary CMOS logic, genuine Dynamic and Differential Logic and Current Mode Logic.
Charge Recycling Sense Amplifier Based Logic: Securing Low Power Security IC?s against Differential Power Analysis
Charge Recycling Sense Amplifier Based Logic is presented. This logic is derived from Sense Amplifier Based Logic, which is a logic style with signal independent power consumption that is capable to protect security devices such as Smart Cards against power attacks. Experimental results show that utilization of advanced circuit techniques save 20% in power consumption and 63% in peak supply current and that the logic style preserves the energy masking behavior.
Synthesis of Secure FPGA Implementations
This paper describes the synthesis of Dynamic Differential Logic to increase the resistance of FPGA implementations against Differential Power Analysis. The synthesis procedure is developed and a detailed description is given of how EDA tools should be used appropriately to implement a secure digital design flow. Compared with an existing technique to implement Dynamic Differential Logic on FPGA, the technique saves a factor 2 in slice utilization. Experimental results also indicate that a secure version of the AES encryption algorithm can now be implemented with a mere 50% increase in time delay and 90% increase in slice utilization when compared with a normal non-secure single ended implementation.