There’s a new way to break RSA encryption

[mklabgr]

A new cryptographic attack has challenged a long-standing assumption about RSA: that breaking it essentially requires factoring the enormous number at the heart of the key. Researchers have demonstrated a different route—forging RSA signatures without actually recovering the private key. Their technique combines a variant of the Special Number Field Sieve with an “oracle,” exploiting particular properties of blind/textbook RSA implementations. In experiments, the researchers successfully forged a 1024-bit RSA signature, something previously considered extremely expensive. 

The immediate practical danger is limited because the attack applies specifically to vulnerable blind-signature RSA systems, while the widespread PKCS#1 and PSS padding schemes are not believed to be vulnerable in the same way. Nevertheless, the result is important because it dramatically lowers the estimated security of RSA: the researchers estimate roughly $2^{65}$ work for 1024-bit RSA, $2^{90}$ for 2048-bit RSA, and $2^{119}$ for 4096-bit RSA—far below the security levels normally expected from these key sizes. The researchers also note that their implementation was not GPU- or AI-optimized, so further improvements may be possible. The finding therefore represents more of a major conceptual warning for cryptography than an immediate collapse of everyday RSA encryption.

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