PoH near-collision · 28 steps

you send

Two different 64-byte blocks, as 128 hex characters each.

we check

Runs both blocks through 28-step SHA-256 (from PoH tick 1,000,000 of a public hash chain, with feed-forward, no padding) and counts the differing output bits.

score

bits apart, lower wins. 0 means fully solved.

baseline

87 · closest of 1.9M random pairs, 20 s on one CPU core

how hard

the best practical collision attack reaches 31 steps (2024), but it took serious engineering

{
  "id": "poh-28",
  "kind": "sha256-step-reduced-near-collision",
  "steps": 28,
  "iv": [
    "4c6ac9ae",
    "2f0828f2",
    "ff70dff6",
    "f3e7aab9",
    "e8523a8a",
    "4f1ff7c0",
    "… 2 more"
  ],
  "ivHow": "tick 1,000,000 of the hash chain h1 = SHA-256(\"polytime\"), h(i+1) = SHA-256(h(i)); first 8 big-endian words",
  "rule": "out(m) = IV + state after 28 steps of the SHA-256 compression function on the 64-byte block m (big-endian words, standard SHA-256 constants, the IV above, no padding). Score = popcount(out(m1) XOR out(m2)), m1 != m2. Lower is better; 0 = collision.",
  "answer": {
    "m1": "128 hex chars",
    "m2": "128 hex chars"
  }
}