The challenge
"A macOS deployment includes thin and universal binaries mixed with archives. Enumerate every CPU type and identify what truly supports the advertised fleet."
| answer_id | Question | Format |
|---|---|---|
d20- |
"Determine macho.cpu_type_set for macos-001.bin." (artifact metadata-) |
JSON: "compact valid JSON array with exact values and order; no Markdown or explanatory text"; trimmed |
The timed modes asked this field alongside the earlier days' Metadata fields: 25 questions in the Time Trial, 125 across 5 sets in the Mastery.
Reading the prompt: what it said we'd need
| Clue | What it pointed to | What I did about it |
|---|---|---|
| "thin and universal binaries" | Mach-O comes in two shapes: a thin file with one header, and a fat (universal) file with a table of slices, one per CPU | The solver reads the thin header's cputype, or every slice's cputype in a fat file (checking each slice header against its table entry) |
| "mixed with archives" | Static libraries (ar, !<arch>) whose members are Mach-O objects |
The solver walks BSD and GNU ar members, skips symbol tables, and reads each Mach-O member |
| "Enumerate every CPU type" | The answer is a list of cputype values, not one name |
A JSON array, one entry per CPU type |
cpu_ |
A set: no duplicates, and an order the question calls "exact" | Duplicates removed and sorted ascending (unconfirmed, see below) |
| "what truly supports" | Trust the header bytes, not the file name | Identified files by their magic bytes |
The format was the hard part. "cpu_type" could mean the name (arm64), the number (16777228), or a hex constant. Day 19's ELF field had been accepted as 0x003e (0x plus the field's full width in hex), so that form was on the list from the start.
What was in the files
- Single Solve:
macos-001.bin, 56 bytes, inmetadata-day-20-evidence.zip: a thin 64-bit little-endian Mach-O (magic bytescf fa ed fe),cputype0x0100000c(CPU_TYPE_ARM64),cpusubtype0,MH_EXECUTE, two load commands includingLC_UUID. - Time Trial and Mastery: the same template with the earlier days' fields. Every Mach-O in the six timed sets was thin arm64 too, so the fat and
arpaths were never exercised by a real answer.
Single Solve
The solver started it and stopped cleanly. It flagged the field as new ("don't know what this question asks"), and its Metadata candidate search had no candidate list for cpu-type-set, so it sent nothing.
Claude read the header from the bytes (the solver had parsed Mach-O since Day 17; only this field name was missing), drew up a ranked candidate list, and sent the candidates one at a time, stopping at the first CORRECT:
| # | Candidate | Why it was on the list | Result |
|---|---|---|---|
| 1 | ["arm64"] |
The solver's CPU name, as lipo - prints it |
incorrect (13:01:38 UTC) |
| 2 | [16777228] |
The raw cputype as a JSON number |
incorrect (13:02:11) |
| 3 | ["ARM64"] |
The upper-case name some libraries use | incorrect (13:02:25) |
| 4 | ["0x0100000c"] |
0x plus 8 hex digits, the same form as Day 19's accepted 0x003e |
CORRECT (13:02:38) |
Not sent: ["CPU_TYPE_ARM64"] and ["16777228"].
Rule: macho.cpu_type_set is the file's Mach-O cputype values, each as a JSON string 0x%08x.
started_at 2026-10-20T13:00:10.410Z (started by the solver)
deadline none (Single Solve)
answers 1, sent four times (three rejected, see above)
start / artifact / solve timings: not recorded (hand-sent candidates)
last submit 13:02:37 → 13:02:38
TOTAL 2 min 28 s from the start reply to CORRECT
result CORRECT flag 404d29fd…ed61
Our solver: builds on the Metadata solver we had prepared the evening before and adds one field, macho-cpu-type-set, for every question-ID prefix. It collects every cputype from a thin Mach-O, every slice of a fat one, and every Mach-O member of an ar archive; formats each as 0x%08x; removes duplicates; sorts ascending; and writes compact JSON. It refuses a file that isn't Mach-O. Bytes are parsed, never executed.
Tests (8, all pass): the accepted Single value; the saved Single through the whole chain; a fat file; i386; BSD and GNU ar; a non-Mach-O refused; the field name kept; timing.
Rehearsal (before any timed start): a regression rehearsal over 89 saved Metadata attempts, slowest 0.08 s, per-set answers identical: 88 identical, 1 newly solved (today's), 0 changed. It passed, and the new solver became the one the scheduled job uses.
Time Trial
CORRECT on the first attempt, ranked (50 s clock, 25 answers).
started_at 2026-10-20T13:05:40.457Z
deadline 2026-10-20T13:06:30.457Z (50 s)
answers 25 (Mach-O field: ["0x0100000c"])
start 797 ms
artifact 460 ms
solve 11 ms
submit 1214 ms
TOTAL 2482 ms (submit +1.82 s after started_at)
result CORRECT flag 46dc75bd…cc7c
Mastery
CORRECT on the first attempt, ranked (5 sets, 170 s clock, 125 answers).
started_at 2026-10-20T13:05:48.402Z
deadline 2026-10-20T13:08:38.402Z (170 s)
answers 125 across 5 sets (every Mach-O field: ["0x0100000c"])
start ~606 ms (TOTAL minus the logged phases)
artifact 778 ms
solve 72 ms
submit 776 ms
TOTAL 2233 ms (submit +1.79 s after started_at)
result CORRECT flag 0f967e66…6bd6
Time to solve
All three modes were solved by 09:05:50 ET, 5 min 40 s after the solver's first start request at 09:00:10 ET. The Single Solve went through on the fourth candidate at 09:02:38; about three minutes of solver work, tests, and rehearsal followed, and the ranked Time Trial and Mastery each went through on the first submit.
Takeaways
- Answer forms carry over between days. Day 19's
0x003etaught that header fields want0xplus the field's full width in hex. Ranking that form first would have saved three sends and about a minute. - The fat and
arpaths are unconfirmed. Every timed set was thin arm64, so the ordering of a multi-CPU set (sorted ascending, duplicates removed) has never been checked by the Game Master. If a later set holds a universal binary and is rejected, slice order is the first thing to try. - The rehearsal held. No timed start happened until 89 saved attempts replayed unchanged.