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Informational 17 Bitcoin

hashes: Interleave two independent hashes for 2-way SHA256d

Public commit record

What the developer wrote

Authored by jrakibi

81/100 · Strong
hashes: Interleave two independent hashes for 2-way SHA256d

x86 SHA256RNDS2 has a latency of 4-6 cycles depending on
the architecture. the next SHA256RNDS2 needs the result
of the current one as input, so the CPU has to wait until
it is ready. (in Transform 2 for eg, 3 out of every 4 cycles
are wasted doing nothing)

we fill those wasted cycles by computing a second independent
hash alongside the first.

See https://uops.info/html-instr/SHA256RNDS2_XMM_XMM.html
for per-architecture instruction timings.
✓ Specific, descriptive subject✓ Names a concrete action or component✓ Provides detailed explanatory context✓ Links an issue, advisory, or supporting reference
The short version

What changed, and why it matters

This commit is a performance optimization for Bitcoin's double-SHA256 hashing on modern Intel/AMD processors. It rewrites a low-level routine to compute two independent hashes at once instead of one, filling idle CPU cycles. There is no security fix or vulnerability here; it is purely about making hashing faster.

Recommended action

No security action required. Treat as a normal performance improvement. Reviewers may want to verify that callers are updated to pass two inputs and that the new function is only invoked under the same `sha,sse2,ssse3,sse4.1` target-feature guards as before.

Security signals we found

01

No security-relevant signals detected

02

Performance-only optimization using CPU SHA-NI intrinsics

03

No changes to input validation, bounds checking, or caller contracts

04

No use of unsafe blocks beyond the pre-existing target-feature-gated unsafe function

Risk score

Why this scored 17/100

Our methodology →
Potential impact 0/30
Exploitability 0/25
Stealth signal 0/15
Affected reach 5/15
Confidence 8/10
Evidence quality 4/5
Human-validated context

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