Deterministic encryption is well-suited for certain uses, such as full disk encryption, where the data being protected never changes once the thing being protected (in this case, the drive) falls into an attacker’s hands. The same encryption is suboptimal for protecting data flowing between a CPU and a memory chip because adversaries can observe the ciphertext each time the plaintext changes, opening the system to replay attacks and other well-known exploit techniques. Probabilistic encryption, by contrast, resists such attacks because the same plaintext can encrypt to a wide range of ciphertexts that are randomly chosen during the encryption process.
“Fundamentally, [the use of deterministic encryption] is a design trade-off,” Jesse De Meulemeester, lead author of the Battering RAM paper, wrote in an online interview. “Intel and AMD opted for deterministic encryption without integrity or freshness to keep encryption scalable (i.e., protect the entire memory range) and reduce overhead. That choice enables low-cost physical attacks like ours. The only way to fix this likely requires hardware changes, e.g., by providing freshness and integrity in the memory encryption.”
Daniel Genkin, one of the researchers behind Wiretap, agreed. “It’s a design choice made by Intel when SGX moved from client machines to server,” he said. “It offers better performance at the expense of security.” Genkin was referring to Intel’s move about five years ago to discontinue SGX for client processors—where encryption was limited to no more than 256 MB of RAM—to server processors that could encrypt terabytes of RAM. The transition required Intel to revamp the encryption to make it scale for such vast amounts of data.
“The papers are two sides of the same coin,” he added.
While both of Tuesday’s attacks exploit weaknesses related to deterministic encryption, their approaches and findings are distinct, and each comes with its own advantages and disadvantages. Both research teams said they learned of the other’s work only after privately submitting their findings to the chipmakers. The teams then synchronized the publish date for Tuesday. It’s not the first time such a coincidence has occurred. In 2018, multiple research teams independently developed attacks with names including Spectre and Meltdown. Both plucked secrets out of Intel and AMD processors by exploiting their use of performance enhancement known as speculative execution.