SRAM Security Concerns Grow

SRAM security concerns are intensifying as a combination of new and existing techniques allow hackers to tap into data for longer periods of time after a device is powered down.

This is particularly alarming as the leading edge of design shifts from planar SoCs to heterogeneous systems in package, such as those used in AI or edge processing, where chiplets frequently have their own memory hierarchy. Until now, most cybersecurity concerns involving volatile memory have focused on DRAM, because it is often external and easier to attack. SRAM, in contrast, does not contain a component as obviously vulnerable as a heat-sensitive capacitor, and in the past it has been harder to pinpoint. But as SoCs are disaggregated and more features are added into devices, SRAM is becoming a much bigger security concern.

The attack scheme is well understood. Known as cold boot, it was first identified in 2008, and is essentially a variant of a side-channel attack. In a cold boot approach, an attacker dumps data from internal SRAM to an external device, and then restarts the system from the external device with some code modification. “Cold boot is primarily targeted at SRAM, with the two primary defenses being isolation and in-memory encryption,” said Vijay Seshadri, distinguished engineer at Cycuity.

Compared with network-based attacks, such as DRAM’s rowhammer, cold boot is relatively simple. It relies on physical proximity and a can of compressed air.

The vulnerability was first described by Edward Felton, director of Princeton University’s Center for Information Technology Policy, J. Alex Halderman, currently director of the Center for Computer Security & Society at the University of Michigan, and colleagues. The breakthrough in their research was based on the growing realization in the engineering research community that data does not vanish from memory the moment a device is turned off, which until then was a common assumption. Instead, data in both DRAM and SRAM has a brief “remanence.”[1]

Using a cold boot approach, data can be retrieved, especially if an attacker sprays the chip with compressed air, cooling it enough to slow the degradation of the data. As the researchers described their approach, “We obtained surface temperatures of approximately −50°C with a simple cooling technique — discharging inverted cans of ‘canned air’ duster spray directly onto the chips. At these temperatures, we typically found that fewer than 1% of bits decayed even after 10 minutes without power.”

Unfortunately, despite nearly 20 years of security research since the publication of the Halderman paper, the authors’ warning still holds true. “Though we discuss several strategies for mitigating these risks, we know of no simple remedy that would eliminate them.”

However unrealistic, there is one simple and obvious remedy to cold boot — never leave a device unattended. But given human behavior, it’s safer to assume that every device is vulnerable, from smart watches to servers, as well as automotive chips used for increasingly autonomous driving.

While the original research exclusively examined DRAM, within the last six years cold boot has proven to be one of the most serious vulnerabilities for SRAM. In 2018, researchers at Germany’s Technische Universität Darmstadt published a paper describing a cold boot attack method that is highly resistant to memory erasure techniques, and which can be used to manipulate the cryptographic keys produced by the SRAM physical unclonable function (PUF).

As with so many security issues, it’s been a cat-and-mouse game between remedies and counter-attacks. And because cold boot takes advantage of slowing down memory degradation, in 2022 Yang-Kyu Choi and colleagues at the Korea Advanced Institute of Science and Technology (KAIST), described a way to undo the slowdown with an ultra-fast data sanitization method that worked within 5 ns, using back bias to control the device parameters of CMOS.

Fig. 1: Asymmetric forward back-biasing scheme for permanent erasing. (a) All the data are reset to 1. (b) All the data are reset to 0. Whether all the data where reset to 1 or 0 is determined by the asymmetric forward back-biasing scheme. Source: KAIST/Creative Commons [2]

Their paper, as well as others, have inspired new approaches to combating cold boot attacks.

“To mitigate the risk of unauthorized access from unknown devices, main devices, or servers, check the authenticated code and unique identity of each accessing device,” said Jongsin Yun, memory technologist at Siemens EDA. “SRAM PUF is one of the ways to securely identify each device. SRAM is made of two inverters cross-coupled to each other. Although each inverter is designed to be the same device, normally one part of the inverter has a somewhat stronger NMOS than the other due to inherent random dopant fluctuation. During the initial power-on process, SRAM data will be either data 1 or 0, depending on which side has a stronger device. In other words, the initial data state of the SRAM array at the power on is decided by this unique random process variation and most of the bits maintain this property for life. One can use this unique pattern as a fingerprint of a device. The SRAM PUF data is reconstructed with other coded data to form a cryptographic key. SRAM PUF is a great way to anchor its secure data into hardware. Hackers may use a DFT circuit to access the memory. To avoid insecurely reading the SRAM information through DFT, the security-critical design makes DFT force delete the data as an initial process of TEST mode.”

However, there can be instances where data may be required to be kept in a non-volatile memory (NVM). “Data is considered insecure if the NVM is located outside of the device,” said Yun. “Therefore, secured data needs to be stored within the device with write protection. One-time programmable (OTP) memory or fuses are good storage options to prevent malicious attackers from tampering with the modified information. OTP memory and fuses are used to store cryptographic keys, authentication information, and other critical settings for operation within the device. It is useful for anti-rollback, which prevents hackers from exploiting old vulnerabilities that have been fixed in newer versions.”

Chiplet vulnerabilities
Chiplets also could present another vector for attack, due to their complexity and interconnections. “A chiplet has memory, so it’s going to be attacked,” said Cycuity’s Seshadri. “Chiplets, in general, are going to exacerbate the problem, rather than keeping it status quo, because you’re going to have one chiplet talking to another. Could an attack on one chiplet have a side effect on another? There need to be standards to address this. In fact, they’re coming into play already. A chiplet provider has to say, ‘Here’s what I’ve done for security. Here’s what needs to be done when interfacing with another chiplet.”

Yun notes there is a further physical vulnerability for those working with chiplets and SiPs. “When multiple chiplets are connected to form a SiP, we have to trust data coming from an external chip, which creates further complications. Verification of the chiplet’s authenticity becomes very important for SiPs, as there is a risk of malicious counterfeit chiplets being connected to the package for hacking purposes. Detection of such counterfeit chiplets is imperative.”

These precautions also apply when working with DRAM. In all situations, Seshardi said, thinking about security has to go beyond device-level protection. “The onus of protecting DRAM is not just on the DRAM designer or the memory designer,” he said. “It has to be secured by design principles when you are developing. In addition, you have to look at this holistically and do it at a system level. You must consider all the other things that communicate with DRAM or that are placed near DRAM. You must look at a holistic solution, all the way from software down to things like the memory controller and then finally, the DRAM itself.”

Encryption as a backup
Data itself always must be encrypted as second layer of protection against known and novel attacks, so an organization’s assets will still be protected even if someone breaks in via cold boot or another method.

“The first and primary method of preventing a cold boot attack is limiting physical access to the systems, or physically modifying the systems case or hardware preventing an attacker’s access,” said Jim Montgomery, market development director, semiconductor at TXOne Networks. “The most effective programmatic defense against an attack is to ensure encryption of memory using either a hardware- or software-based approach. Utilizing memory encryption will ensure that regardless of trying to dump the memory, or physically removing the memory, the encryption keys will remain secure.”

Montgomery also points out that TXOne is working with the Semiconductor Manufacturing Cybersecurity Consortium (SMCC) to develop common criteria based upon SEMI E187 and E188 standards to assist DM’s and OEM’s to implement secure procedures for systems security and integrity, including controlling the physical environment.

Another potential defense is to use smarter device approaches.

According to Jayson Bethurem, vice president marketing and business development of Flex Logix, “SRAM devices are subject to cold boot side channel attacks. Adaptable devices with fast loading eFPGA can mitigate these kinds of attacks by masking cryptography keys and algorithms as well as data shuffling to confuse cryptanalysis attackers.”

What kind and how much encryption will depend on use cases, said Jun Kawaguchi, global marketing executive for Winbond. “Encryption strength for a traffic signal controller is going to be different from encryption for nuclear plants or medical devices, critical applications where you need much higher levels,” he said. “There are different strengths and costs to it.”

Another problem, in the post-quantum era, is that encryption itself may be vulnerable. To defend against those possibilities, researchers are developing post-quantum encryption schemes. One way to stay a step ahead is homomorphic encryption [HE], which will find a role in data sharing, since computations can be performed on encrypted data without first having to decrypt it.

Homomorphic encryption could be in widespread use as soon as the next few years, according to Ronen Levy, senior manager for IBM’s Cloud Security & Privacy Technologies Department, and Omri Soceanu, AI Security Group manager at IBM.  However, there are still challenges to be overcome.

“There are three main inhibitors for widespread adoption of homomorphic encryption — performance, consumability, and standardization,” according to Levy. “The main inhibitor, by far, is performance. Homomorphic encryption comes with some latency and storage overheads. FHE hardware acceleration will be critical to solving these issues, as well as algorithmic and cryptographic solutions, but without the necessary expertise it can be quite challenging.”

An additional issue is that most consumers of HE technology, such as data scientists and application developers, do not possess deep cryptographic skills, HE solutions that are designed for cryptographers can be impractical. A few HE solutions require algorithmic and cryptographic expertise that inhibit adoption by those who lack these skills.

Finally, there is a lack of standardization. “Homomorphic encryption is in the process of being standardized,” said Soceanu. “But until it is fully standardized, large organizations may be hesitant to adopt a cryptographic solution that has not been approved by standardization bodies.”

Once these issues are resolved, they predicted widespread use as soon as the next few years. “Performance is already practical for a variety of use cases, and as hardware solutions for homomorphic encryption become a reality, more use cases would become practical,” said Levy. “Consumability is addressed by creating more solutions, making it easier and hopefully as frictionless as possible to move analytics to homomorphic encryption. Additionally, standardization efforts are already in progress.”

A new attack and an old problem
Unfortunately, security never will be as simple as making users more aware of their surroundings. Otherwise, cold boot could be completely eliminated as a threat. Instead, it’s essential to keep up with conference talks and the published literature, as graduate students keep probing SRAM for vulnerabilities, hopefully one step ahead of genuine attackers.

For example, SRAM-related cold boot attacks originally targeted discrete SRAM. The reason is that it’s far more complicated to attack on-chip SRAM, which is isolated from external probing and has minimal intrinsic capacitance. However, in 2022, Jubayer Mahmod, then a graduate student at Virginia Tech and his advisor, associate professor Matthew Hicks, demonstrated what they dubbed “Volt Boot,” a new method that could penetrate on-chip SRAM. According to their paper, “Volt Boot leverages asymmetrical power states (e.g., on vs. off) to force SRAM state retention across power cycles, eliminating the need for traditional cold boot attack enablers, such as low-temperature or intrinsic data retention time…Unlike other forms of SRAM data retention attacks, Volt Boot retrieves data with 100% accuracy — without any complex post-processing.”

Conclusion
While scientists and engineers continue to identify vulnerabilities and develop security solutions, decisions about how much security to include in a design is an economic one. Cost vs. risk is a complex formula that depends on the end application, the impact of a breach, and the likelihood that an attack will occur.

“It’s like insurance,” said Kawaguchi. “Security engineers and people like us who are trying to promote security solutions get frustrated because, similar to insurance pitches, people respond with skepticism. ‘Why would I need it? That problem has never happened before.’ Engineers have a hard time convincing their managers to spend that extra dollar on the costs because of this ‘it-never-happened-before’ attitude. In the end, there are compromises. Yet ultimately, it’s going to cost manufacturers a lot of money when suddenly there’s a deluge of demands to fix this situation right away.”

References

1. S. Skorobogatov, “Low temperature data remanence in static RAM”, Technical report UCAM-CL-TR-536, University of Cambridge Computer Laboratory, June 2002.
2. Han, SJ., Han, JK., Yun, GJ. et al. Ultra-fast data sanitization of SRAM by back-biasing to resist a cold boot attack. Sci Rep 12, 35 (2022). https://doi.org/10.1038/s41598-021-03994-2