Researchers at UC San Diego and France's Institute for Research in Computer Science have demonstrated an attack that forges RSA signatures within a hardware security module—a tamper-resistant device used by institutions to store and sign with private keys—without extracting the key itself. The findings were detailed in a paper submitted to the IACR Cryptology ePrint Archive on September 20.
The attack required approximately 4 billion signing requests and 1,380 CPU core-years of computational work. Researchers disabled the device's FIPS mode, a certified security setting, and used a test key of their own, asking the module to sign numbers they selected and analyzing the results to forge signatures.
Limited Impact on Cryptocurrency
The attack does not pose a direct threat to Bitcoin or Ethereum. Bitcoin uses elliptic-curve digital signature algorithms (ECDSA), as do most major blockchains. This research concerns RSA, a different signature scheme created in 1977 by Ron Rivest, Leonard Adleman, and Adi Shamir.
Protection in Modern Systems
The researchers emphasize that their attack likely poses no immediate operational threat to most modern RSA deployments. Standard RSA signing applies padding—a scrambling and formatting step such as PKCS#1 v1.5 or PSS—before the mathematical operation. Padded signatures do not create the exploitable weakness the researchers demonstrated.
Some systems intentionally use unpadded RSA signatures, such as RSA-based blind signatures, which allow a server to sign something without seeing its contents. One variant of Privacy Pass, used by Cloudflare and reportedly adopted by Apple for CAPTCHA verification, relies on this technique.
Quantum Considerations
The authors describe their result as classical evidence for transitioning away from RSA during the shift to post-quantum cryptography. For Bitcoin, the quantum threat centers on elliptic-curve signatures. Caltech researchers estimated that 10,000 to 20,000 qubits could be sufficient to run Shor's algorithm, the method that threatens these signatures. Google has set 2029 as its target deadline to migrate its systems to post-quantum cryptography.
