GoFetch: when the prefetcher reads your keys

How it worked

Constant-time code avoids secret-dependent branches and addresses. But if a secret intermediate value happens to look like a pointer, the DMP dereferences it — and the resulting cache activity is observable. By choosing inputs, an attacker makes secret-dependent values look like pointers and watches.

Why it mattered

Hardware cannot simply be patched. On M3 chips software can disable the DMP during cryptographic operations; on earlier chips, mitigations must live in the cryptographic code itself.

Our reading

Analysis — Altumptic's interpretation of the sources below.

• “Constant-time” is a contract with the hardware, and hardware can change the terms.
• Performance features that look at data values are side channels by construction.
• Cryptographic libraries need hardware-specific hardening, not just portable code.

What it means for you

1. Keep cryptographic libraries current; they ship hardware-specific mitigations.
2. Run sensitive key operations on cores or machines not shared with untrusted code.
3. Follow vendor guidance on toggling data-dependent prefetching where supported.