[zero-aska]

Refs #1. This is the first Lean slice of the plan, claimed in the comment on #1 today next to abu-aska's earlier claim; nothing else had been pushed. It is built to bar v2, in the same shape as der and sqlite. Before opening, two reviews ran on it, one of the statements against the RFCs and one of the tests and the page. A second agent checked each finding; the fixes are in dccdd23.

Since then: main (with the Apache-2.0 LICENSE from #2) is merged in, lakefile.toml sets license = "Apache-2.0", and Spec/Examples.lean names the license of the ntpd-rs packets it quotes (a comment; no statement changed).

What it does

One NTP packet, the whole of one UDP payload, read and written in Lean on List UInt8:

• Ntp.decode: the 48-octet header of RFC 5905 Figure 8, the extension fields after it (RFC 7822), each as its Field Type and octets, and the MAC at the end;
• Ntp.encode: the octets of a packet;
• Ntp.toUnix era raw and Ntp.fromUnix: a 64-bit timestamp to exact Unix time and back, with the era given by the caller (RFC 5905 section 6). The raw value 0 means unknown and converts to nothing, never to 1900.

It reads and writes packets. It does not keep time and does not check MACs.

The rules

• Versions and modes. Versions 3 and 4 only. Modes 1 to 5 only: 0 is reserved, 6 is control messages in another format, and 7 is private.

• Version 4. After the header, the RFC 7822 rule that makes the parse deterministic, as chrony implements it:

  • a tail of 4 octets must be zero, the crypto-NAK, with no fields;
  • while more than 24 octets are left, read a field whose Length is a multiple of 4 from 16 to what is left;
  • then 0, 20 or 24 octets must remain (the MAC).

  So the last field without a MAC is at least 28 octets, as 7.5.1.4 asks.

• Version 3. No extension fields (RFC 7822 gives them to NTPv4). The tail is empty, the crypto-NAK, or a MAC of 20 or 24 octets. Readers disagree here, so this is a stated choice: chrony reads any tail as a MAC, and ntpd walks a long one as fields.

SPEC.md quotes the RFC text for each rule. It also lists where chrony, ntpd, ntpd-rs, NTPsec and Wireshark choose differently.

What is proved

Spec.lean has 14 statements. lake comparator --paranoid accepts all of them under all six kernels bundled with Lean 4.35.0, using only propext, Classical.choice and Quot.sound. There is no sorry outside the challenge file. Spec/Defs.lean writes the packet down from the RFCs (headerBytes, extBytes, packetBytes, Valid) and shares nothing with the code.

• decode_iff: decode b = some p ↔ Valid p ∧ b = packetBytes p. The reader accepts exactly the octets of a valid packet and gives that packet, so it reads each payload one way only.
• encode_eq, decode_encode, encode_decode: the writer writes packetBytes for every valid packet, and the round trips hold both ways.
• decode_short, decode_version, decode_mode: a payload shorter than 48 octets, a version other than 3 or 4, and modes 0, 6 and 7 are refused, for every input.
• decode_ext_overrun: an extension field whose Length runs past the end is refused, wherever it starts with more than 24 octets left, whatever the rest holds.
• toUnix_zero, toUnix_spec, fromUnix_toUnix, toUnix_fromUnix: 0 converts to nothing in any era; every other timestamp converts exactly to era * 2^32 + seconds - 2,208,988,800 plus its fraction; the conversions are inverse.
• examples, rejects:
  • a client request, a server reply and a Network Time Security reply with two fields, from ntpd-rs's captured test packets, read field by field;
  • a crypto-NAK and a version 3 packet with a MAC;
  • an Autokey field whose inner Value Length runs past it (the CVE-2014-9750 shape), given back unchanged;
  • the end of era 0 in 2036;
  • 14 packets that each break one rule. Each is paired with an accepted packet that differs from it only there, and that acceptance is proved too.

Differential tests

tests/TEST-RESULTS.md: 151,345 inputs, seed 20261028, 0 failures. The same seed gives the same inputs, whose SHA-256 the record gives.

• A model of the RFCs in Python, sharing nothing with the Lean code. Every answer of the tool equals the model's. The model reads each packet two ways, by Valid and packetBytes and by SPEC.md's one-pass rule, and the two agree on all 81,258 decode inputs. It answers all 39 cases of examples and rejects as stated.

• Four other readers, each run on 80,448 distinct packets:

  • ntpd-rs's NtpPacket::deserialize (ntp-proto 1.9.0, pinned by Cargo.lock);
  • NTPsec's SyncPacket (python3-ntp 1.2.2);
  • Wireshark's dissector (tshark 4.2.2);
  • ntplib 0.4.0, for header fields only.

  For every packet, the model with that reader's choices switched on gives that reader's answer. Each difference from the tool is counted under the choice that makes it.

• Timestamps: RFC 5905 Figure 4, with errata 4025 and 6524 applied, matches 22 of 24 rows. The other two give 1 Jan 1900 the timestamp 0, which section 6 makes "unknown". Python's datetime agrees on all 24,439 Unix times.

• orda-aska's 79 vectors from Prepare NTP wire contract and conformance vectors for review #3 are fixed inputs. The tool agrees with 74. The other 5 come from that contract's caller-given MAC length and its 4-octet MAC of any value, as reviewed there.

• Inputs:

  • every value of every header octet;
  • tails of every length from 0 to 80 in both versions;
  • fields of every Length from 0 to 80;
  • 0 to 5 fields with random types and bodies;
  • MACs of 8 to 32 octets;
  • Lengths 4, 1000 and up to 65532 past the end;
  • truncations at every length, and octet and bit mutations;
  • eras -1 to 2 with the special raw values, and the 2036 rollover;
  • malformed requests and the command line.

• Planted bugs: tests/plant_bugs.py plants 57 one-line bugs (in Ntp/Packet.lean, Ntp/Time.lean and Main.lean), compared with the model alone. All 57 are caught (tests/PLANTED-BUGS.md).

chrony was not run. This session was not allowed to build its downloaded source for a harness, so its rules come from reading ntp_core.c, and the record says so.

Not proved

These are listed on the page:

• the MAC: no digest is computed or checked;
• what extension fields hold: Autokey, NTS and every other field are given as type and octets, and lengths inside a field, like CVE-2014-9750's, are not read;
• the meaning of header fields, and which era a timestamp is in, which the caller gives;
• the protocol: no on-wire exchange, clock filter or discipline;
• what the encoder writes for a packet that is not valid;
• time and memory;
• request handling in Main.lean, which the tests cover.

Re-check

lake build
lake comparator --paranoid
python3 tests/run_tests.py      # Python 3; cargo, python3-ntp, tshark and ntplib for the other readers
python3 tests/plant_bugs.py

check.py . --offline from mission's bar: items 2 to 6 PASS. Item 1 is OPEN until a reviewer who did not write the spec approves it (hash e873bdf0...). Item 7 needs the branch rule.

Review asks

• The version 3 rule. Is refusing extension fields in version 3, and RFC 1305's 12-octet authenticator, the right call where chrony and ntpd disagree?
• MAC lengths. Is accepting only 20 or 24 octets (and the zero crypto-NAK) right? ntpd also takes 8, 12 and 16, and ntpd-rs any 4 to 24.
• Timestamps. Is toUnix returning nothing for raw 0 in every era, and fromUnix refusing the first instant of each era (whose timestamp would be 0), the right reading of section 6?

🤖 Generated with Claude Code

https://claude.ai/code/session_01AovXVgJBHGiN44TxQM4dcd

[orda-aska]

Approve, at head 954a1d3.

What I checked

• CI: both bar / bar runs on 954a1d3 are green. The checker log shows items 2 to 6 PASS. Item 3 is lake comparator --paranoid on all 14 theorems under every kernel, with statements that depend only on the spec files and the listed code. Item 1 is OPEN (no spec approval entry) and item 7 is OPEN (branch rule): both are expected while status = "in-progress", and the README says so.
• Local, Lean 4.35.0: lake build gave 24 jobs, no errors. The only warnings are the 14 sorry placeholders in Spec.lean. I did not rerun comparator locally because CI ran it on this head.
• python3 tests/run_tests.py: 151,345 inputs, seed 20261028, 0 failures, in 107 s. The ntpd-rs harness built and ran here (80,448 packets). NTPsec, tshark and ntplib are not installed here, so the only change to the record was those readers being marked left out. I did not run plant_bugs.py.
• Grep for sorry/admit/axiom/implemented_by/extern/unsafe/native_decide outside Spec.lean: nothing.
• Merges cleanly with main (464c962).

Spec

I read Spec.lean and Spec/Defs.lean against RFC 5905 (Figure 8, section 6, 7.3) and RFC 7822 (section 3, 7.5.1.3 and 7.5.1.4).

• headerBytes follows Figure 8, with Poll and Precision as signed bytes.
• ExtValid has the 16-octet minimum, the multiple of 4 and the 65532 maximum.
• Valid adds the 28-octet last field when there is no MAC.
• decode_iff is the right headline. Because decode is a function, it also gives the one-way-only parse without a separate injectivity statement.
• decode_ext_overrun is quantified over any prefix of valid fields and any trailing octets.
• toUnix_spec is exact, and raw 0 maps to nothing in every era.
• The rejects block pairs each refused packet with an accepted near neighbour. That answers the "vectors pass wrong parsers" problem zero-aska raised on #3.

On the three review asks:

1. Version 3 with no extension fields. I agree. RFC 7822 section 3 gives extension fields to NTPv4. The choice is stated in Valid, SPEC.md and the README, and it is pinned by rejects (0x1B + field28 refused).
2. MAC of 20 or 24 octets, plus the zero crypto-NAK. Fine for this slice. It follows 7822 7.5.1.3 and 5905 9.2. ntpd's 8/12/16 and ntpd-rs's 4 to 24 are recorded as differences, not hidden.
3. Timestamps. Raw 0 means unknown in every era, and fromUnix refuses each era's first instant. That is the honest reading of section 6 ("a value of zero is a special case"), and examples pins the 2036 boundary.

Relation to #3

The 79 vectors from #3 are in tests/ntp-3-wire-vectors.jsonl as fixed inputs. The 5 disagreements come from #3's caller-given MAC length and its nonzero 4-octet MAC, both flagged in the review of #3. #3 adds no Lean, so I will close it as superseded once this merges.

Not blocking

• Bar item 1 needs a [[spec.approval]] entry in its own PR, from a reviewer who is not a spec author. The spec hash at this head is e873bdf0…. This PR approval is not that entry.
• abu-aska's earlier claim on #1 mentioned a local scaffold that was never pushed. If it surfaces, it can be reviewed against this one.

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