meshcore-analyzer/cmd/ingestor/decoder.go

package main

import (
	"crypto/aes"
	"crypto/hmac"
	"crypto/sha256"
	"encoding/binary"
	"encoding/hex"
	"encoding/json"
	"fmt"
	"math"
	"strings"
	"unicode/utf8"

	"github.com/meshcore-analyzer/packetpath"
	"github.com/meshcore-analyzer/sigvalidate"
)

// Route type constants (header bits 1-0)
const (
	RouteTransportFlood   = 0
	RouteFlood            = 1
	RouteDirect           = 2
	RouteTransportDirect  = 3
)

// Payload type constants (header bits 5-2)
const (
	PayloadREQ       = 0x00
	PayloadRESPONSE  = 0x01
	PayloadTXT_MSG   = 0x02
	PayloadACK       = 0x03
	PayloadADVERT    = 0x04
	PayloadGRP_TXT   = 0x05
	PayloadGRP_DATA  = 0x06
	PayloadANON_REQ  = 0x07
	PayloadPATH      = 0x08
	PayloadTRACE     = 0x09
	PayloadMULTIPART = 0x0A
	PayloadCONTROL   = 0x0B
	PayloadRAW_CUSTOM = 0x0F
)

var routeTypeNames = map[int]string{
	0: "TRANSPORT_FLOOD",
	1: "FLOOD",
	2: "DIRECT",
	3: "TRANSPORT_DIRECT",
}

var payloadTypeNames = map[int]string{
	0x00: "REQ",
	0x01: "RESPONSE",
	0x02: "TXT_MSG",
	0x03: "ACK",
	0x04: "ADVERT",
	0x05: "GRP_TXT",
	0x06: "GRP_DATA",
	0x07: "ANON_REQ",
	0x08: "PATH",
	0x09: "TRACE",
	0x0A: "MULTIPART",
	0x0B: "CONTROL",
	0x0F: "RAW_CUSTOM",
}

// Header is the decoded packet header.
type Header struct {
	RouteType      int    `json:"routeType"`
	RouteTypeName  string `json:"routeTypeName"`
	PayloadType    int    `json:"payloadType"`
	PayloadTypeName string `json:"payloadTypeName"`
	PayloadVersion int    `json:"payloadVersion"`
}

// TransportCodes are present on TRANSPORT_FLOOD and TRANSPORT_DIRECT routes.
type TransportCodes struct {
	Code1 string `json:"code1"`
	Code2 string `json:"code2"`
}

// Path holds decoded path/hop information.
type Path struct {
	HashSize      int      `json:"hashSize"`
	HashCount     int      `json:"hashCount"`
	Hops          []string `json:"hops"`
	HopsCompleted *int     `json:"hopsCompleted,omitempty"`
}

// AdvertFlags holds decoded advert flag bits.
type AdvertFlags struct {
	Raw         int  `json:"raw"`
	Type        int  `json:"type"`
	Chat        bool `json:"chat"`
	Repeater    bool `json:"repeater"`
	Room        bool `json:"room"`
	Sensor      bool `json:"sensor"`
	HasLocation bool `json:"hasLocation"`
	HasFeat1    bool `json:"hasFeat1"`
	HasFeat2    bool `json:"hasFeat2"`
	HasName     bool `json:"hasName"`
}

// Payload is a generic decoded payload. Fields are populated depending on type.
type Payload struct {
	Type          string       `json:"type"`
	DestHash      string       `json:"destHash,omitempty"`
	SrcHash       string       `json:"srcHash,omitempty"`
	MAC           string       `json:"mac,omitempty"`
	EncryptedData string       `json:"encryptedData,omitempty"`
	ExtraHash     string       `json:"extraHash,omitempty"`
	// Extended ACK fields per firmware 1.16.0 (issue #1610) —
	// firmware/src/helpers/BaseChatMesh.cpp:218-234. ACK payloads grew from
	// always-4 bytes to 4/5/6 (4-byte truncated sha256 CRC, optional 1-byte
	// attempt counter, optional 1-byte RNG byte added in commit a130a95a).
	// AckLen is the wire payload length; AckAttempt/AckRand are surfaced
	// only when the sender included them (legacy 4-byte ACKs leave them nil).
	AckLen        *int   `json:"ackLen,omitempty"`
	AckAttempt    *int   `json:"ackAttempt,omitempty"`
	AckRand       *int   `json:"ackRand,omitempty"`
	PubKey        string       `json:"pubKey,omitempty"`
	Timestamp     uint32       `json:"timestamp,omitempty"`
	TimestampISO  string       `json:"timestampISO,omitempty"`
	Signature     string       `json:"signature,omitempty"`
	SignatureValid *bool       `json:"signatureValid,omitempty"`
	Flags         *AdvertFlags `json:"flags,omitempty"`
	Lat           *float64     `json:"lat,omitempty"`
	Lon           *float64     `json:"lon,omitempty"`
	Name          string       `json:"name,omitempty"`
	Feat1         *int         `json:"feat1,omitempty"`
	Feat2         *int         `json:"feat2,omitempty"`
	BatteryMv     *int         `json:"battery_mv,omitempty"`
	TemperatureC  *float64     `json:"temperature_c,omitempty"`
	ChannelHash   int          `json:"channelHash,omitempty"`
	ChannelHashHex   string    `json:"channelHashHex,omitempty"`
	DecryptionStatus string    `json:"decryptionStatus,omitempty"`
	Channel          string    `json:"channel,omitempty"`
	// GRP_DATA (PAYLOAD_TYPE_GRP_DATA=0x06) inner fields, decoded after
	// channel decrypt per firmware/src/helpers/BaseChatMesh.cpp:382-385.
	DataType         *int      `json:"dataType,omitempty"`
	DataLen          *int      `json:"dataLen,omitempty"`
	DecryptedBlob    string    `json:"decryptedBlob,omitempty"`
	Text             string    `json:"text,omitempty"`
	Sender           string    `json:"sender,omitempty"`
	SenderTimestamp  uint32    `json:"sender_timestamp,omitempty"`
	EphemeralPubKey string     `json:"ephemeralPubKey,omitempty"`
	PathData      string       `json:"pathData,omitempty"`
	SNRValues     []float64    `json:"snrValues,omitempty"`
	Tag           uint32       `json:"tag,omitempty"`
	AuthCode      uint32       `json:"authCode,omitempty"`
	TraceFlags    *int         `json:"traceFlags,omitempty"`
	RawHex        string       `json:"raw,omitempty"`
	Error         string       `json:"error,omitempty"`
	// MULTIPART (PAYLOAD_TYPE_MULTIPART=0x0A) inner fields, decoded per
	// firmware/src/Mesh.cpp:289 — byte0 = (remaining<<4) | inner_type.
	Remaining     *int    `json:"remaining,omitempty"`
	InnerType     *int    `json:"innerType,omitempty"`
	InnerTypeName string  `json:"innerTypeName,omitempty"`
	InnerAckCrc   string  `json:"innerAckCrc,omitempty"`
	// Extended ACK inner fields (issue #1610) — when the multipart inner
	// blob is a v1.16+ extended ACK (5 or 6 bytes after the byte0 header),
	// surface the same attempt/rand bytes as the top-level decoder.
	InnerAckLen     *int  `json:"innerAckLen,omitempty"`
	InnerAckAttempt *int  `json:"innerAckAttempt,omitempty"`
	InnerAckRand    *int  `json:"innerAckRand,omitempty"`
	InnerPayload  string  `json:"innerPayload,omitempty"`
	// CONTROL (PAYLOAD_TYPE_CONTROL=0x0B) byte0 flags, per
	// firmware/src/Mesh.cpp:69 — byte0 high-bit marks zero-hop direct subset.
	CtrlFlags     string  `json:"ctrlFlags,omitempty"`
	CtrlZeroHop   *bool   `json:"ctrlZeroHop,omitempty"`
	CtrlLength    *int    `json:"ctrlLength,omitempty"`
	// CONTROL DISCOVER_REQ / DISCOVER_RESP body fields (#1802). Subtype is
	// "DISCOVER_REQ" | "DISCOVER_RESP" | "UNKNOWN". For REQ: filter, tag, and
	// optional since. For RESP: node_type (low nibble of byte0), snr, tag, and
	// pubkey (hex; 32 bytes or 8 bytes when prefix_only). All optional —
	// emitted only when the body length is sufficient.
	CtrlSubtype  string  `json:"ctrlSubtype,omitempty"`
	CtrlFilter   *int    `json:"ctrlFilter,omitempty"`
	CtrlTag      *uint32 `json:"ctrlTag,omitempty"`
	CtrlSince    *uint32 `json:"ctrlSince,omitempty"`
	CtrlNodeType *int    `json:"ctrlNodeType,omitempty"`
	CtrlSNR      *int    `json:"ctrlSNR,omitempty"`
	CtrlPubKey   string  `json:"ctrlPubKey,omitempty"`
	// RAW_CUSTOM (PAYLOAD_TYPE_RAW_CUSTOM=0x0F) — application-defined per
	// firmware/src/Mesh.cpp:577 (createRawData). Exposes the bare envelope
	// shape (length + leading tag) so consumers can triage by app id.
	RawLength    *int   `json:"rawLength,omitempty"`
	FirstByteTag string `json:"firstByteTag,omitempty"`
}

// DecodedPacket is the full decoded result.
type DecodedPacket struct {
	Header         Header          `json:"header"`
	TransportCodes *TransportCodes `json:"transportCodes"`
	Path           Path            `json:"path"`
	Payload        Payload         `json:"payload"`
	Raw            string          `json:"raw"`
	Anomaly        string          `json:"anomaly,omitempty"`
	payloadRaw     []byte
}

func decodeHeader(b byte) Header {
	rt := int(b & 0x03)
	pt := int((b >> 2) & 0x0F)
	pv := int((b >> 6) & 0x03)

	rtName := routeTypeNames[rt]
	if rtName == "" {
		rtName = "UNKNOWN"
	}
	ptName := payloadTypeNames[pt]
	if ptName == "" {
		ptName = "UNKNOWN"
	}

	return Header{
		RouteType:       rt,
		RouteTypeName:   rtName,
		PayloadType:     pt,
		PayloadTypeName: ptName,
		PayloadVersion:  pv,
	}
}

// Firmware-derived limits — see firmware/src/MeshCore.h:19,21.
const (
	maxPathSize      = 64  // MAX_PATH_SIZE — total path bytes allowed
	maxPacketPayload = 184 // MAX_PACKET_PAYLOAD — max raw payload bytes
)

// isValidPathLen mirrors firmware Packet::isValidPathLen
// (firmware/src/Packet.cpp:13-18). hash_size==4 is reserved; total path bytes
// must fit within MAX_PATH_SIZE.
func isValidPathLen(pathByte byte) bool {
	hashCount := int(pathByte & 0x3F)
	hashSize := int(pathByte>>6) + 1
	if hashSize == 4 {
		return false // reserved
	}
	return hashCount*hashSize <= maxPathSize
}

func decodePath(pathByte byte, buf []byte, offset int) (Path, int, error) {
	hashSize := int(pathByte>>6) + 1
	hashCount := int(pathByte & 0x3F)
	// Exact mirror of firmware Packet::isValidPathLen (Packet.cpp:13-18).
	// hash_size==4 is reserved and is rejected by firmware regardless of
	// hash_count, so we must reject 0xC0 etc even on zero-hop packets —
	// firmware never emits them, so an on-wire pathByte with the upper
	// 2 bits set to 11 is by definition malformed/adversarial.
	if !isValidPathLen(pathByte) {
		return Path{}, 0, fmt.Errorf("invalid path encoding: pathByte 0x%02X (hash_size=%d hash_count=%d) violates firmware validity (Packet.cpp:13-18, MAX_PATH_SIZE=%d)", pathByte, hashSize, hashCount, maxPathSize)
	}
	totalBytes := hashSize * hashCount
	hops := make([]string, 0, hashCount)

	for i := 0; i < hashCount; i++ {
		start := offset + i*hashSize
		end := start + hashSize
		if end > len(buf) {
			break
		}
		hops = append(hops, strings.ToUpper(hex.EncodeToString(buf[start:end])))
	}

	return Path{
		HashSize:  hashSize,
		HashCount: hashCount,
		Hops:      hops,
	}, totalBytes, nil
}

// isTransportRoute delegates to packetpath.IsTransportRoute.
func isTransportRoute(routeType int) bool {
	return packetpath.IsTransportRoute(routeType)
}

func decodeEncryptedPayload(typeName string, buf []byte) Payload {
	if len(buf) < 4 {
		return Payload{Type: typeName, Error: "too short", RawHex: hex.EncodeToString(buf)}
	}
	return Payload{
		Type:          typeName,
		DestHash:      hex.EncodeToString(buf[0:1]),
		SrcHash:       hex.EncodeToString(buf[1:2]),
		MAC:           hex.EncodeToString(buf[2:4]),
		EncryptedData: hex.EncodeToString(buf[4:]),
	}
}

func decodeAck(buf []byte) Payload {
	if len(buf) < 4 {
		return Payload{Type: "ACK", Error: "too short", RawHex: hex.EncodeToString(buf)}
	}
	checksum := binary.LittleEndian.Uint32(buf[0:4])
	ackLen := len(buf)
	if ackLen > 6 {
		ackLen = 6
	}
	p := Payload{
		Type:      "ACK",
		ExtraHash: fmt.Sprintf("%08x", checksum),
		AckLen:    &ackLen,
	}
	// Firmware 1.16.0 extended ACK (issue #1610): 5th byte is the attempt
	// counter (commit f6e6fdaa), 6th byte is a random byte added so identical
	// attempts still hash uniquely (commit a130a95a).
	if len(buf) >= 5 {
		attempt := int(buf[4])
		p.AckAttempt = &attempt
	}
	if len(buf) >= 6 {
		rnd := int(buf[5])
		p.AckRand = &rnd
	}
	return p
}

func decodeAdvert(buf []byte, validateSignatures bool) Payload {
	if len(buf) < 100 {
		return Payload{Type: "ADVERT", Error: "too short for advert", RawHex: hex.EncodeToString(buf)}
	}

	pubKey := hex.EncodeToString(buf[0:32])
	timestamp := binary.LittleEndian.Uint32(buf[32:36])
	signature := hex.EncodeToString(buf[36:100])
	appdata := buf[100:]

	p := Payload{
		Type:         "ADVERT",
		PubKey:       pubKey,
		Timestamp:    timestamp,
		TimestampISO: fmt.Sprintf("%s", epochToISO(timestamp)),
		Signature:    signature,
	}

	if validateSignatures {
		valid, err := sigvalidate.ValidateAdvert(buf[0:32], buf[36:100], timestamp, appdata)
		if err != nil {
			f := false
			p.SignatureValid = &f
		} else {
			p.SignatureValid = &valid
		}
	}

	if len(appdata) > 0 {
		flags := appdata[0]
		advType := int(flags & 0x0F)
		hasFeat1 := flags&0x20 != 0
		hasFeat2 := flags&0x40 != 0
		p.Flags = &AdvertFlags{
			Raw:         int(flags),
			Type:        advType,
			Chat:        advType == 1,
			Repeater:    advType == 2,
			Room:        advType == 3,
			Sensor:      advType == 4,
			HasLocation: flags&0x10 != 0,
			HasFeat1:    hasFeat1,
			HasFeat2:    hasFeat2,
			HasName:     flags&0x80 != 0,
		}

		off := 1
		if p.Flags.HasLocation && len(appdata) >= off+8 {
			latRaw := int32(binary.LittleEndian.Uint32(appdata[off : off+4]))
			lonRaw := int32(binary.LittleEndian.Uint32(appdata[off+4 : off+8]))
			lat := float64(latRaw) / 1e6
			lon := float64(lonRaw) / 1e6
			p.Lat = &lat
			p.Lon = &lon
			off += 8
		}
		if hasFeat1 && len(appdata) >= off+2 {
			feat1 := int(binary.LittleEndian.Uint16(appdata[off : off+2]))
			p.Feat1 = &feat1
			off += 2
		}
		if hasFeat2 && len(appdata) >= off+2 {
			feat2 := int(binary.LittleEndian.Uint16(appdata[off : off+2]))
			p.Feat2 = &feat2
			off += 2
		}
		if p.Flags.HasName {
			// Find null terminator to separate name from trailing telemetry bytes
			nameEnd := len(appdata)
			for i := off; i < len(appdata); i++ {
				if appdata[i] == 0x00 {
					nameEnd = i
					break
				}
			}
			name := string(appdata[off:nameEnd])
			name = sanitizeName(name)
			// Firmware writes the node name into a 32-byte buffer
			// (MAX_ADVERT_DATA_SIZE, firmware/src/MeshCore.h:11). Truncate
			// here so adversarial on-wire adverts can't pollute Payload.Name
			// with bytes firmware would never emit.
			if len(name) > 32 {
				name = name[:32]
			}
			p.Name = name
			off = nameEnd
			// Skip null terminator(s)
			for off < len(appdata) && appdata[off] == 0x00 {
				off++
			}
		}

		// Telemetry bytes after name: battery_mv(2 LE) + temperature_c(2 LE, signed, /100)
		// Only sensor nodes (advType=4) carry telemetry bytes.
		//
		// Firmware derivation (see firmware/src/helpers/SensorMesh.h and the
		// SensorHost::handleAdvert path in firmware/src/helpers/SensorMesh.cpp:
		// the sensor builds appdata as <flags+adv_type><pubkey?><name\0>
		// followed by two little-endian uint16 fields appended verbatim:
		//   appdata[name_end+0..1] = battery voltage in millivolts (uint16 LE,
		//                            valid 0 < mv ≤ 10000)
		//   appdata[name_end+2..3] = temperature × 100 (int16 LE, divide by 100
		//                            for °C; valid raw -5000..10000 → -50..100 °C)
		// We accept only adverts whose flags.Sensor bit is set (firmware
		// AdvertDataHelpers.h:7-12, ADV_TYPE_SENSOR=4) before parsing telemetry.
		if p.Flags.Sensor && off+4 <= len(appdata) {
			batteryMv := int(binary.LittleEndian.Uint16(appdata[off : off+2]))
			tempRaw := int16(binary.LittleEndian.Uint16(appdata[off+2 : off+4]))
			tempC := float64(tempRaw) / 100.0
			if batteryMv > 0 && batteryMv <= 10000 {
				p.BatteryMv = &batteryMv
			}
			// Raw int16 / 100 → °C; accept -50°C to 100°C (raw: -5000 to 10000)
			if tempRaw >= -5000 && tempRaw <= 10000 {
				p.TemperatureC = &tempC
			}
		}
	}

	return p
}

// channelDecryptResult holds the decrypted channel message fields.
type channelDecryptResult struct {
	Timestamp uint32
	Flags     byte
	Sender    string
	Message   string
}

// countNonPrintable counts characters that are non-printable (< 0x20 except \n, \t).
func countNonPrintable(s string) int {
	count := 0
	for _, r := range s {
		if r < 0x20 && r != '\n' && r != '\t' {
			count++
		} else if r == utf8.RuneError {
			count++
		}
	}
	return count
}

// decryptChannelMessage implements MeshCore channel decryption:
// HMAC-SHA256 MAC verification followed by AES-128-ECB decryption.
func decryptChannelMessage(ciphertextHex, macHex, channelKeyHex string) (*channelDecryptResult, error) {
	channelKey, err := hex.DecodeString(channelKeyHex)
	if err != nil || len(channelKey) != 16 {
		return nil, fmt.Errorf("invalid channel key")
	}

	macBytes, err := hex.DecodeString(macHex)
	if err != nil || len(macBytes) != 2 {
		return nil, fmt.Errorf("invalid MAC")
	}

	ciphertext, err := hex.DecodeString(ciphertextHex)
	if err != nil || len(ciphertext) == 0 {
		return nil, fmt.Errorf("invalid ciphertext")
	}

	// 32-byte channel secret: 16-byte key + 16 zero bytes
	channelSecret := make([]byte, 32)
	copy(channelSecret, channelKey)

	// Verify HMAC-SHA256 (first 2 bytes must match provided MAC)
	h := hmac.New(sha256.New, channelSecret)
	h.Write(ciphertext)
	calculatedMac := h.Sum(nil)
	if calculatedMac[0] != macBytes[0] || calculatedMac[1] != macBytes[1] {
		return nil, fmt.Errorf("MAC verification failed")
	}

	// AES-128-ECB decrypt (block-by-block, no padding)
	if len(ciphertext)%aes.BlockSize != 0 {
		return nil, fmt.Errorf("ciphertext not aligned to AES block size")
	}
	block, err := aes.NewCipher(channelKey)
	if err != nil {
		return nil, fmt.Errorf("AES cipher: %w", err)
	}
	plaintext := make([]byte, len(ciphertext))
	for i := 0; i < len(ciphertext); i += aes.BlockSize {
		block.Decrypt(plaintext[i:i+aes.BlockSize], ciphertext[i:i+aes.BlockSize])
	}

	// Parse: timestamp(4 LE) + flags(1) + message(UTF-8, null-terminated)
	if len(plaintext) < 5 {
		return nil, fmt.Errorf("decrypted content too short")
	}
	timestamp := binary.LittleEndian.Uint32(plaintext[0:4])
	flags := plaintext[4]
	messageText := string(plaintext[5:])
	if idx := strings.IndexByte(messageText, 0); idx >= 0 {
		messageText = messageText[:idx]
	}

	// Validate decrypted text is printable UTF-8 (not binary garbage)
	if !utf8.ValidString(messageText) || countNonPrintable(messageText) > 2 {
		return nil, fmt.Errorf("decrypted text contains non-printable characters")
	}

	result := &channelDecryptResult{Timestamp: timestamp, Flags: flags}

	// Parse "sender: message" format
	colonIdx := strings.Index(messageText, ": ")
	if colonIdx > 0 && colonIdx < 50 {
		potentialSender := messageText[:colonIdx]
		if !strings.ContainsAny(potentialSender, ":[]") {
			result.Sender = potentialSender
			result.Message = messageText[colonIdx+2:]
		} else {
			result.Message = messageText
		}
	} else {
		result.Message = messageText
	}

	return result, nil
}

// knownChannelCasing maps known channel keys to their canonical display names.
// Only well-known channels are normalized — custom/user channels are left as-is.
var knownChannelCasing = map[string]string{
	"public": "Public",
}

// normalizeChannelName fixes casing for well-known channel names.
// Only normalizes names that appear in knownChannelCasing (e.g. "public" → "Public").
// Custom channel names are left untouched since we can't know the intended casing.
func normalizeChannelName(name string) string {
	if corrected, ok := knownChannelCasing[strings.ToLower(name)]; ok {
		return corrected
	}
	return name
}

func decodeGrpTxt(buf []byte, channelKeys map[string]string) Payload {
	if len(buf) < 3 {
		return Payload{Type: "GRP_TXT", Error: "too short", RawHex: hex.EncodeToString(buf)}
	}

	channelHash := int(buf[0])
	channelHashHex := fmt.Sprintf("%02X", buf[0])
	mac := hex.EncodeToString(buf[1:3])
	encryptedData := hex.EncodeToString(buf[3:])

	hasKeys := len(channelKeys) > 0
	// Match Node.js: only attempt decryption if encrypted data >= 5 bytes (10 hex chars)
	if hasKeys && len(encryptedData) >= 10 {
		for name, key := range channelKeys {
			result, err := decryptChannelMessage(encryptedData, mac, key)
			if err != nil {
				continue
			}
			text := result.Message
			if result.Sender != "" && result.Message != "" {
				text = result.Sender + ": " + result.Message
			}
			return Payload{
				Type:             "CHAN",
				Channel:          normalizeChannelName(name),
				ChannelHash:      channelHash,
				ChannelHashHex:   channelHashHex,
				DecryptionStatus: "decrypted",
				Sender:           result.Sender,
				Text:             text,
				SenderTimestamp:  result.Timestamp,
			}
		}
		return Payload{
			Type:             "GRP_TXT",
			ChannelHash:      channelHash,
			ChannelHashHex:   channelHashHex,
			DecryptionStatus: "decryption_failed",
			MAC:              mac,
			EncryptedData:    encryptedData,
		}
	}

	return Payload{
		Type:             "GRP_TXT",
		ChannelHash:      channelHash,
		ChannelHashHex:   channelHashHex,
		DecryptionStatus: "no_key",
		MAC:              mac,
		EncryptedData:    encryptedData,
	}
}

// decodeGrpData decodes PAYLOAD_TYPE_GRP_DATA (0x06). Outer envelope is the
// same shape as GRP_TXT (channel_hash(1)+MAC(2)+ciphertext) — see
// firmware/src/helpers/BaseChatMesh.cpp:476,500. When the channel key matches,
// the decrypted inner is parsed per firmware/src/helpers/BaseChatMesh.cpp:382-385
// as data_type(uint16 LE) + data_len(1) + blob(data_len).
func decodeGrpData(buf []byte, channelKeys map[string]string) Payload {
	if len(buf) < 3 {
		return Payload{Type: "GRP_DATA", Error: "too short", RawHex: hex.EncodeToString(buf)}
	}
	channelHash := int(buf[0])
	channelHashHex := fmt.Sprintf("%02X", buf[0])
	mac := hex.EncodeToString(buf[1:3])
	encryptedData := hex.EncodeToString(buf[3:])

	hasKeys := len(channelKeys) > 0
	if hasKeys && len(encryptedData) >= 10 {
		for name, key := range channelKeys {
			plain, err := decryptChannelBlock(encryptedData, mac, key)
			if err != nil {
				continue
			}
			// Inner: data_type(uint16 LE) + data_len(1) + blob (firmware:382-385).
			if len(plain) < 3 {
				return Payload{
					Type:             "GRP_DATA",
					Channel:          name,
					ChannelHash:      channelHash,
					ChannelHashHex:   channelHashHex,
					DecryptionStatus: "decrypted",
					Error:            "inner too short",
				}
			}
			dataType := int(binary.LittleEndian.Uint16(plain[0:2]))
			dataLen := int(plain[2])
			if 3+dataLen > len(plain) {
				return Payload{
					Type:             "GRP_DATA",
					Channel:          name,
					ChannelHash:      channelHash,
					ChannelHashHex:   channelHashHex,
					DecryptionStatus: "decrypted",
					DataType:         &dataType,
					DataLen:          &dataLen,
					Error:            "inner data_len exceeds buffer",
				}
			}
			blob := hex.EncodeToString(plain[3 : 3+dataLen])
			return Payload{
				Type:             "GRP_DATA",
				Channel:          name,
				ChannelHash:      channelHash,
				ChannelHashHex:   channelHashHex,
				DecryptionStatus: "decrypted",
				DataType:         &dataType,
				DataLen:          &dataLen,
				DecryptedBlob:    blob,
			}
		}
		return Payload{
			Type:             "GRP_DATA",
			ChannelHash:      channelHash,
			ChannelHashHex:   channelHashHex,
			DecryptionStatus: "decryption_failed",
			MAC:              mac,
			EncryptedData:    encryptedData,
		}
	}

	return Payload{
		Type:             "GRP_DATA",
		ChannelHash:      channelHash,
		ChannelHashHex:   channelHashHex,
		DecryptionStatus: "no_key",
		MAC:              mac,
		EncryptedData:    encryptedData,
	}
}

// decodeMultipart decodes PAYLOAD_TYPE_MULTIPART (0x0A) per
// firmware/src/Mesh.cpp:287-310. byte0 = (remaining<<4) | inner_type;
// when inner_type == PAYLOAD_TYPE_ACK the next 4 bytes are an ack_crc.
func decodeMultipart(buf []byte) Payload {
	if len(buf) < 1 {
		return Payload{Type: "MULTIPART", Error: "too short", RawHex: hex.EncodeToString(buf)}
	}
	remaining := int(buf[0] >> 4)
	innerType := int(buf[0] & 0x0F)
	innerName := payloadTypeNames[innerType]
	if innerName == "" {
		innerName = "UNKNOWN"
	}
	p := Payload{
		Type:          "MULTIPART",
		Remaining:     &remaining,
		InnerType:     &innerType,
		InnerTypeName: innerName,
	}
	if innerType == PayloadACK && len(buf) >= 5 {
		// ack_crc is little-endian; surface as canonical big-endian hex
		// to match decodeAck's extraHash convention.
		crc := binary.LittleEndian.Uint32(buf[1:5])
		p.InnerAckCrc = fmt.Sprintf("%08x", crc)
		// Firmware 1.16.0 extended ACK (issue #1610): inner ACK blob may be
		// 5 or 6 bytes (payload_len = 1 + ack_len) instead of always 4.
		ackLen := len(buf) - 1
		if ackLen > 6 {
			ackLen = 6
		}
		p.InnerAckLen = &ackLen
		if len(buf) >= 6 {
			attempt := int(buf[5])
			p.InnerAckAttempt = &attempt
		}
		if len(buf) >= 7 {
			rnd := int(buf[6])
			p.InnerAckRand = &rnd
		}
	} else if len(buf) > 1 {
		p.InnerPayload = hex.EncodeToString(buf[1:])
	}
	return p
}

// decodeControl decodes PAYLOAD_TYPE_CONTROL (0x0B).
//
// byte0 high nibble is the control subtype (per firmware/src/Mesh.cpp:69
// and firmware/examples/simple_repeater/MyMesh.cpp:773-820):
//   0x80 = CTL_TYPE_NODE_DISCOVER_REQ  — body: filter:u8 | tag:u32 LE | since:u32 LE (optional)
//   0x90 = CTL_TYPE_NODE_DISCOVER_RESP — body: snr:i8 | tag:u32 LE | pubkey (32B full, or 8B prefix)
//                                        low nibble of byte0 is node_type
//
// The legacy CtrlZeroHop bool (bit7 of byte0) is retained for backwards
// compatibility — it is misleading (bit7 is also set for DISCOVER_RESP)
// and is flagged for follow-up renaming. Length checks gate every field
// extraction; short/truncated bodies emit the subtype only and never panic.
func decodeControl(buf []byte) Payload {
	if len(buf) < 1 {
		return Payload{Type: "CONTROL", Error: "too short", RawHex: hex.EncodeToString(buf)}
	}
	zeroHop := buf[0]&0x80 != 0
	length := len(buf)
	p := Payload{
		Type:        "CONTROL",
		CtrlFlags:   fmt.Sprintf("%02x", buf[0]),
		CtrlZeroHop: &zeroHop,
		CtrlLength:  &length,
		RawHex:      hex.EncodeToString(buf),
	}

	switch buf[0] & 0xF0 {
	case 0x80:
		p.CtrlSubtype = "DISCOVER_REQ"
		// REQ body: filter:u8 | tag:u32 LE | since:u32 LE (optional).
		// Total body (incl. byte0) >= 6 = 1 + 1 + 4.
		if len(buf) >= 6 {
			filter := int(buf[1])
			p.CtrlFilter = &filter
			tag := binary.LittleEndian.Uint32(buf[2:6])
			p.CtrlTag = &tag
			if len(buf) >= 10 {
				since := binary.LittleEndian.Uint32(buf[6:10])
				p.CtrlSince = &since
			}
		}
	case 0x90:
		p.CtrlSubtype = "DISCOVER_RESP"
		nodeType := int(buf[0] & 0x0F)
		p.CtrlNodeType = &nodeType
		// RESP body: snr:i8 | tag:u32 LE | pubkey. Header is 6 bytes
		// (byte0 + snr + 4B tag). Pubkey is 32B full or 8B prefix.
		if len(buf) >= 6 {
			snr := int(int8(buf[1]))
			p.CtrlSNR = &snr
			tag := binary.LittleEndian.Uint32(buf[2:6])
			p.CtrlTag = &tag
			remaining := len(buf) - 6
			if remaining >= 32 {
				p.CtrlPubKey = hex.EncodeToString(buf[6:38])
			} else if remaining >= 8 && remaining < 32 {
				p.CtrlPubKey = hex.EncodeToString(buf[6:14])
			}
			// Other lengths (e.g. 4B): omit pubkey rather than emit a
			// partial/ambiguous value.
		}
	default:
		p.CtrlSubtype = "UNKNOWN"
	}
	return p
}

// decodeRawCustom decodes PAYLOAD_TYPE_RAW_CUSTOM (0x0F). Application-defined
// payload per firmware/src/Mesh.cpp:577 (createRawData); we only surface the
// envelope shape (total length + leading tag byte).
func decodeRawCustom(buf []byte) Payload {
	length := len(buf)
	p := Payload{
		Type:      "RAW_CUSTOM",
		RawLength: &length,
		RawHex:    hex.EncodeToString(buf),
	}
	if length > 0 {
		p.FirstByteTag = fmt.Sprintf("%02X", buf[0])
	}
	return p
}

// decryptChannelBlock performs the MAC verify + AES-128-ECB decrypt step shared
// by GRP_TXT and GRP_DATA, returning the raw plaintext block (no further
// parsing). See firmware/src/helpers/BaseChatMesh.cpp:376-391.
func decryptChannelBlock(ciphertextHex, macHex, channelKeyHex string) ([]byte, error) {
	channelKey, err := hex.DecodeString(channelKeyHex)
	if err != nil || len(channelKey) != 16 {
		return nil, fmt.Errorf("invalid channel key")
	}
	macBytes, err := hex.DecodeString(macHex)
	if err != nil || len(macBytes) != 2 {
		return nil, fmt.Errorf("invalid MAC")
	}
	ciphertext, err := hex.DecodeString(ciphertextHex)
	if err != nil || len(ciphertext) == 0 {
		return nil, fmt.Errorf("invalid ciphertext")
	}
	channelSecret := make([]byte, 32)
	copy(channelSecret, channelKey)
	h := hmac.New(sha256.New, channelSecret)
	h.Write(ciphertext)
	calc := h.Sum(nil)
	if calc[0] != macBytes[0] || calc[1] != macBytes[1] {
		return nil, fmt.Errorf("MAC verification failed")
	}
	if len(ciphertext)%aes.BlockSize != 0 {
		return nil, fmt.Errorf("ciphertext not aligned to AES block size")
	}
	block, err := aes.NewCipher(channelKey)
	if err != nil {
		return nil, err
	}
	plain := make([]byte, len(ciphertext))
	for i := 0; i < len(ciphertext); i += aes.BlockSize {
		block.Decrypt(plain[i:i+aes.BlockSize], ciphertext[i:i+aes.BlockSize])
	}
	return plain, nil
}

func decodeAnonReq(buf []byte) Payload {
	if len(buf) < 35 {
		return Payload{Type: "ANON_REQ", Error: "too short", RawHex: hex.EncodeToString(buf)}
	}
	return Payload{
		Type:            "ANON_REQ",
		DestHash:        hex.EncodeToString(buf[0:1]),
		EphemeralPubKey: hex.EncodeToString(buf[1:33]),
		MAC:             hex.EncodeToString(buf[33:35]),
		EncryptedData:   hex.EncodeToString(buf[35:]),
	}
}

func decodePathPayload(buf []byte) Payload {
	if len(buf) < 4 {
		return Payload{Type: "PATH", Error: "too short", RawHex: hex.EncodeToString(buf)}
	}
	return Payload{
		Type:     "PATH",
		DestHash: hex.EncodeToString(buf[0:1]),
		SrcHash:  hex.EncodeToString(buf[1:2]),
		MAC:      hex.EncodeToString(buf[2:4]),
		PathData: hex.EncodeToString(buf[4:]),
	}
}

func decodeTrace(buf []byte) Payload {
	if len(buf) < 9 {
		return Payload{Type: "TRACE", Error: "too short", RawHex: hex.EncodeToString(buf)}
	}
	tag := binary.LittleEndian.Uint32(buf[0:4])
	authCode := binary.LittleEndian.Uint32(buf[4:8])
	flags := int(buf[8])
	p := Payload{
		Type:       "TRACE",
		Tag:        tag,
		AuthCode:   authCode,
		TraceFlags: &flags,
	}
	if len(buf) > 9 {
		p.PathData = hex.EncodeToString(buf[9:])
	}
	return p
}

func decodePayload(payloadType int, buf []byte, channelKeys map[string]string, validateSignatures bool) Payload {
	switch payloadType {
	case PayloadREQ:
		return decodeEncryptedPayload("REQ", buf)
	case PayloadRESPONSE:
		return decodeEncryptedPayload("RESPONSE", buf)
	case PayloadTXT_MSG:
		return decodeEncryptedPayload("TXT_MSG", buf)
	case PayloadACK:
		return decodeAck(buf)
	case PayloadADVERT:
		return decodeAdvert(buf, validateSignatures)
	case PayloadGRP_TXT:
		return decodeGrpTxt(buf, channelKeys)
	case PayloadGRP_DATA:
		return decodeGrpData(buf, channelKeys)
	case PayloadANON_REQ:
		return decodeAnonReq(buf)
	case PayloadPATH:
		return decodePathPayload(buf)
	case PayloadTRACE:
		return decodeTrace(buf)
	case PayloadMULTIPART:
		return decodeMultipart(buf)
	case PayloadCONTROL:
		return decodeControl(buf)
	case PayloadRAW_CUSTOM:
		return decodeRawCustom(buf)
	default:
		return Payload{Type: "UNKNOWN", RawHex: hex.EncodeToString(buf)}
	}
}

// DecodePacket decodes a hex-encoded MeshCore packet.
func DecodePacket(hexString string, channelKeys map[string]string, validateSignatures bool) (*DecodedPacket, error) {
	hexString = strings.ReplaceAll(hexString, " ", "")
	hexString = strings.ReplaceAll(hexString, "\n", "")
	hexString = strings.ReplaceAll(hexString, "\r", "")

	buf, err := hex.DecodeString(hexString)
	if err != nil {
		return nil, fmt.Errorf("invalid hex: %w", err)
	}
	if len(buf) < 2 {
		return nil, fmt.Errorf("packet too short (need at least header + pathLength)")
	}

	header := decodeHeader(buf[0])
	offset := 1

	var tc *TransportCodes
	if isTransportRoute(header.RouteType) {
		if len(buf) < offset+4 {
			return nil, fmt.Errorf("packet too short for transport codes")
		}
		tc = &TransportCodes{
			Code1: strings.ToUpper(hex.EncodeToString(buf[offset : offset+2])),
			Code2: strings.ToUpper(hex.EncodeToString(buf[offset+2 : offset+4])),
		}
		offset += 4
	}

	if offset >= len(buf) {
		return nil, fmt.Errorf("packet too short (no path byte)")
	}
	pathByte := buf[offset]
	offset++

	path, bytesConsumed, decodeErr := decodePath(pathByte, buf, offset)
	if decodeErr != nil {
		return nil, decodeErr
	}
	offset += bytesConsumed

	// Bounds check: pathByte is wire-supplied (hash_size in upper 2 bits,
	// hash_count in lower 6 bits → up to 4*63=252 claimed path bytes). A
	// malformed packet can claim more bytes than the buffer holds — without
	// this guard `buf[offset:]` panics with `slice bounds out of range
	// [offset:len(buf)]`. See issue #1211 (prod observed [218:15]).
	if offset > len(buf) {
		return nil, fmt.Errorf("packet path length (%d bytes claimed by pathByte 0x%02X) exceeds buffer (%d bytes)", bytesConsumed, pathByte, len(buf))
	}

	payloadBuf := buf[offset:]
	// Firmware caps payload at MAX_PACKET_PAYLOAD=184 (firmware/src/MeshCore.h:19).
	if len(payloadBuf) > maxPacketPayload {
		return nil, fmt.Errorf("packet payload (%d bytes) exceeds firmware MAX_PACKET_PAYLOAD=%d (MeshCore.h:19)", len(payloadBuf), maxPacketPayload)
	}
	payload := decodePayload(header.PayloadType, payloadBuf, channelKeys, validateSignatures)

	// TRACE packets store hop IDs in the payload (buf[9:]) rather than the header
	// path field. Firmware always sends TRACE as DIRECT (route_type 2 or 3);
	// FLOOD-routed TRACEs are anomalous but handled gracefully (parsed, but
	// flagged). The TRACE flags byte (payload offset 8) encodes path_sz in
	// bits 0-1 as a power-of-two exponent: hash_bytes = 1 << path_sz.
	// NOT the header path byte's hash_size bits. The header path contains SNR
	// bytes — one per hop that actually forwarded.
	// We expose hopsCompleted (count of SNR bytes) so consumers can distinguish
	// how far the trace got vs the full intended route.
	var anomaly string
	if header.PayloadType == PayloadTRACE && payload.Error != "" {
		anomaly = fmt.Sprintf("TRACE payload decode failed: %s", payload.Error)
	}
	if header.PayloadType == PayloadTRACE && payload.PathData != "" {
		// Flag anomalous routing — firmware only sends TRACE as DIRECT
		if header.RouteType != RouteDirect && header.RouteType != RouteTransportDirect {
			anomaly = "TRACE packet with non-DIRECT routing (expected DIRECT or TRANSPORT_DIRECT)"
		}
		// The header path hops count represents SNR entries = completed hops
		hopsCompleted := path.HashCount
		// Extract per-hop SNR from header path bytes (int8, quarter-dB encoding).
		// Mirrors cmd/server/decoder.go — must be done at ingest time so SNR
		// values are persisted in decoded_json (server endpoint serves DB as-is).
		if hopsCompleted > 0 && len(path.Hops) >= hopsCompleted {
			snrVals := make([]float64, 0, hopsCompleted)
			for i := 0; i < hopsCompleted; i++ {
				b, err := hex.DecodeString(path.Hops[i])
				if err == nil && len(b) == 1 {
					snrVals = append(snrVals, float64(int8(b[0]))/4.0)
				}
			}
			if len(snrVals) > 0 {
				payload.SNRValues = snrVals
			}
		}
		pathBytes, err := hex.DecodeString(payload.PathData)
		if err == nil && payload.TraceFlags != nil {
			// path_sz from flags byte is a power-of-two exponent per firmware:
			// hash_bytes = 1 << (flags & 0x03)
			pathSz := 1 << (*payload.TraceFlags & 0x03)
			hops := make([]string, 0, len(pathBytes)/pathSz)
			for i := 0; i+pathSz <= len(pathBytes); i += pathSz {
				hops = append(hops, strings.ToUpper(hex.EncodeToString(pathBytes[i:i+pathSz])))
			}
			path.Hops = hops
			path.HashCount = len(hops)
			path.HashSize = pathSz
			path.HopsCompleted = &hopsCompleted
		}
	}

	// Zero-hop direct packets have hash_count=0 (lower 6 bits of pathByte),
	// which makes the generic formula yield a bogus hashSize. Reset to 0
	// (unknown) so API consumers get correct data. We mask with 0x3F to check
	// only hash_count, matching the JS frontend approach — the upper hash_size
	// bits are meaningless when there are no hops. Skip TRACE packets — they
	// use hashSize to parse hops from the payload above.
	if (header.RouteType == RouteDirect || header.RouteType == RouteTransportDirect) && pathByte&0x3F == 0 && header.PayloadType != PayloadTRACE {
		path.HashSize = 0
	}

	return &DecodedPacket{
		Header:         header,
		TransportCodes: tc,
		Path:           path,
		Payload:        payload,
		Raw:            strings.ToUpper(hexString),
		Anomaly:        anomaly,
		payloadRaw:     payloadBuf,
	}, nil
}

// ComputeContentHash computes the SHA-256-based content hash (first 16 hex chars).
// It hashes the payload-type nibble + payload (skipping path bytes) to produce a
// route-independent identifier for the same logical packet. For TRACE packets,
// path_len is included in the hash to match firmware behavior.
func ComputeContentHash(rawHex string) string {
	buf, err := hex.DecodeString(rawHex)
	if err != nil || len(buf) < 2 {
		if len(rawHex) >= 16 {
			return rawHex[:16]
		}
		return rawHex
	}

	headerByte := buf[0]
	offset := 1
	if isTransportRoute(int(headerByte & 0x03)) {
		offset += 4
	}
	if offset >= len(buf) {
		if len(rawHex) >= 16 {
			return rawHex[:16]
		}
		return rawHex
	}
	pathByte := buf[offset]
	offset++
	hashSize := int((pathByte>>6)&0x3) + 1
	hashCount := int(pathByte & 0x3F)
	pathBytes := hashSize * hashCount

	payloadStart := offset + pathBytes
	if payloadStart > len(buf) {
		if len(rawHex) >= 16 {
			return rawHex[:16]
		}
		return rawHex
	}

	payload := buf[payloadStart:]

	// Hash payload-type byte only (bits 2-5 of header), not the full header.
	// Firmware: SHA256(payload_type + [path_len for TRACE] + payload)
	// Using the full header caused different hashes for the same logical packet
	// when route type or version bits differed. See issue #786.
	payloadType := (headerByte >> 2) & 0x0F
	toHash := []byte{payloadType}
	if int(payloadType) == PayloadTRACE {
		// Firmware uses uint16_t path_len (2 bytes, little-endian)
		toHash = append(toHash, pathByte, 0x00)
	}
	toHash = append(toHash, payload...)

	h := sha256.Sum256(toHash)
	return hex.EncodeToString(h[:])[:16]
}

// PayloadJSON serializes the payload to JSON for DB storage.
func PayloadJSON(p *Payload) string {
	b, err := json.Marshal(p)
	if err != nil {
		return "{}"
	}
	return string(b)
}

// ValidateAdvert checks decoded advert data before DB insertion.
func ValidateAdvert(p *Payload) (bool, string) {
	if p == nil || p.Error != "" {
		reason := "null advert"
		if p != nil {
			reason = p.Error
		}
		return false, reason
	}

	pk := p.PubKey
	if len(pk) < 16 {
		return false, fmt.Sprintf("pubkey too short (%d hex chars)", len(pk))
	}
	allZero := true
	for _, c := range pk {
		if c != '0' {
			allZero = false
			break
		}
	}
	if allZero {
		return false, "pubkey is all zeros"
	}

	if p.Lat != nil {
		if math.IsInf(*p.Lat, 0) || math.IsNaN(*p.Lat) || *p.Lat < -90 || *p.Lat > 90 {
			return false, fmt.Sprintf("invalid lat: %f", *p.Lat)
		}
	}
	if p.Lon != nil {
		if math.IsInf(*p.Lon, 0) || math.IsNaN(*p.Lon) || *p.Lon < -180 || *p.Lon > 180 {
			return false, fmt.Sprintf("invalid lon: %f", *p.Lon)
		}
	}

	if p.Name != "" {
		for _, c := range p.Name {
			if (c >= 0x00 && c <= 0x08) || c == 0x0b || c == 0x0c || (c >= 0x0e && c <= 0x1f) || c == 0x7f {
				return false, "name contains control characters"
			}
		}
		if len(p.Name) > 64 {
			return false, fmt.Sprintf("name too long (%d chars)", len(p.Name))
		}
	}

	if p.Flags != nil {
		role := advertRole(p.Flags)
		// Accept canonical labels plus "none" (ADV_TYPE_NONE=0) and the
		// "type-N" placeholders we now return for ADV_TYPE 5-15 (FUTURE)
		// — see firmware/src/helpers/AdvertDataHelpers.h:7-12.
		validRoles := map[string]bool{
			"repeater": true, "companion": true, "room": true, "sensor": true, "none": true,
		}
		if !validRoles[role] && !strings.HasPrefix(role, "type-") {
			return false, fmt.Sprintf("unknown role: %s", role)
		}
	}

	return true, ""
}

// sanitizeName strips non-printable characters (< 0x20 except tab/newline) and DEL.
func sanitizeName(s string) string {
	var b strings.Builder
	b.Grow(len(s))
	for _, c := range s {
		if c == '\t' || c == '\n' || (c >= 0x20 && c != 0x7f) {
			b.WriteRune(c)
		}
	}
	return b.String()
}

// advertRole returns a stable role label for an advert. Follows firmware
// ADV_TYPE_* constants in firmware/src/helpers/AdvertDataHelpers.h:7-12:
//   0 NONE, 1 CHAT, 2 REPEATER, 3 ROOM, 4 SENSOR, 5-15 FUTURE.
// Previously this coerced both 0 (NONE) and 5-15 (FUTURE) to "companion",
// silently relabelling unknown/reserved types — see issue #1279 P1 #3.
func advertRole(f *AdvertFlags) string {
	if f == nil {
		return "companion"
	}
	switch f.Type {
	case 0:
		return "none"
	case 1:
		return "companion"
	case 2:
		return "repeater"
	case 3:
		return "room"
	case 4:
		return "sensor"
	default:
		return fmt.Sprintf("type-%d", f.Type)
	}
}

func epochToISO(epoch uint32) string {
	// Go time from Unix epoch
	t := unixTime(int64(epoch))
	return t.UTC().Format("2006-01-02T15:04:05.000Z")
}