GoKawiilResearchers forge RSA signatures without factoring the underlying key
Cryptography researchers demonstrated a method to forge certain RSA signatures without recovering the private key, using a variant of the special number field sieve algorithm combined with an oracle found in some blind-signature protocols. The attack is practical against deprecated 1,024-bit keys and reduces the estimated security margin of 2,048-bit and 4,096-bit keys in vulnerable blind-signature systems, while standard RSA implementations using PKCS or PSS padding appear unaffected.
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The result challenges the long-held assumption that breaking RSA signatures requires factoring the underlying large number, which has been the theoretical basis for RSA's security. Karsten Nohl, a cryptography expert at Allurity, said the finding would be a conceptual breakthrough if confirmed by peer review, since it suggests RSA can be practically broken without cracking its key in certain configurations. The attack's efficiency—reportedly far fewer operations and core-years than factoring—could prompt closer scrutiny of blind-signature systems still relying on larger key sizes.
- The forgery method skips factoring and instead exploits an oracle in blind-signature protocols using a variant of the special number field sieve.
- 1,024-bit RSA keys are already deprecated, but the technique also weakens confidence in 2,048-bit and 4,096-bit keys in vulnerable blind-signature setups.
- Commonly used RSA implementations with PKCS or PSS padding are not currently reported to be affected, and the work still awaits peer review.
Source: techspot.com — Skye Jacobs, 2026-09-28
Published there as: “Researchers forged RSA signatures without ever cracking the key”
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