#!/system/bin/sh
# CellGuard cgdetect — always-on IMSI-catcher / cell-site-simulator detector.
#
# Runs as a background daemon (launched by service.sh), independent of the
# WebUI. It reads the modem's live serving/neighbor cell state from the
# telephony framework and the raw CP-frame counters from cgcap, applies
# behavioral CSS heuristics (vendor-agnostic — catches Stingray, Hailstorm,
# DRT, Crosshair, Gossamer, OpenBTS/srsRAN fakes alike), and writes:
#   /data/adb/cellguard/status   key=value snapshot (WebUI reads this)
#   /data/adb/cellguard/alerts   append-only alert log (WebUI reads this)
#
# Heuristics (any one = suspicious):
#   DOWNGRADE   serving RAT is 2G/3G (GSM/GPRS/EDGE/UMTS/HSPA/CDMA)
#   PLMN        serving PLMN != learned home PLMN
#   TAC_JUMP    serving TAC changed without a cell/PLMN reason
#   NEW_CELL    serving PCI/CI never seen before (first-seen, informational)
#   NO_NEIGH    registered but zero neighbor cells reported (CSS often hides them)

CG="/data/adb/cellguard"
STATUS="$CG/status"
ALERTS="$CG/alerts"
STATE="$CG/detect.state"      # learned home PLMN + seen cells
POLL="${1:-4}"

mkdir -p "$CG"
: > "$STATUS"
alert() { echo "[$(date '+%Y-%m-%d %H:%M:%S')] $1" >> "$ALERTS"; }
setk()  { echo "$1=$2" >> "$STATUS.tmp"; }

# load learned state
HOME_PLMN=""; SEEN_CELLS=" "
[ -f "$STATE" ] && . "$STATE" 2>/dev/null

# startup marker goes to a debug log, NOT the alerts file (the alerts file is
# surfaced in the UI and must contain only real threat events)
echo "[$(date '+%Y-%m-%d %H:%M:%S')] cgdetect start (poll=${POLL}s)" >> "$CG/cgdetect.log"

while true; do
  # detector toggle (WebUI) — idle when off
  if [ "$(cat "$CG/detect_enabled" 2>/dev/null)" = "0" ]; then sleep "$POLL"; continue; fi
  reg=$(dumpsys telephony.registry 2>/dev/null)
  # serving air-interface RAT = type of the registered cell identity (real MCC).
  # A serving GSM/WCDMA identity == an active 2G/3G downgrade.
  if   echo "$reg" | grep -q 'CellIdentityNr:{[^}]*mMcc=[0-9]';    then rat=NR
  elif echo "$reg" | grep -q 'CellIdentityLte:{[^}]*mMcc=[0-9]';   then rat=LTE
  elif echo "$reg" | grep -q 'CellIdentityWcdma:{[^}]*mMcc=[0-9]'; then rat=WCDMA
  elif echo "$reg" | grep -q 'CellIdentityGsm:{[^}]*mMcc=[0-9]';   then rat=GSM
  else rat="?"; fi
  # the service-state line carrying the serving cell identity (real MCC)
  line=$(echo "$reg" | grep -m1 'CellIdentity[A-Za-z]*:{[^}]*mMcc=[0-9]')
  mcc=$(echo "$line"    | sed -n 's/.*mMcc=\([0-9]*\).*/\1/p')
  mnc=$(echo "$line"    | sed -n 's/.*mMnc=\([0-9]*\).*/\1/p')
  ci=$(echo "$line"     | sed -n 's/.*mCi=\[*\([0-9]*\).*/\1/p')
  pci=$(echo "$line"    | sed -n 's/.*mPci=\[*\([0-9]*\).*/\1/p')
  tac=$(echo "$line"    | sed -n 's/.*mTac=\[*\([0-9]*\).*/\1/p')
  earfcn=$(echo "$line" | sed -n 's/.*mEarfcn=\[*\([0-9]*\).*/\1/p')
  op=$(echo "$reg" | grep -oE 'mOperatorAlphaLong=[^,]+' | grep -m1 '=[A-Za-z0-9]' | cut -d= -f2-)
  # NOTE: ci/pci/tac/earfcn are redacted by Android in dumpsys ([****]); the real
  # values + neighbor towers come from the decoded cgcap 0x070F cell-info frames.
  plmn="${mcc}${mnc}"
  frames=$(head -1 /proc/cgcap/frames 2>/dev/null | sed -n 's/.*total=\([0-9]*\).*/\1/p')

  susp=""
  case "$rat" in
    GSM|GPRS|EDGE|UMTS|HSDPA|HSUPA|HSPA|HSPAP|TD_SCDMA|CDMA|EVDO|1xRTT)
      susp="DOWNGRADE"; alert "DOWNGRADE: serving RAT=$rat (2G/3G) plmn=$plmn cell=$ci — CSS/downgrade signature" ;;
  esac
  if [ -n "$plmn" ] && [ "$plmn" != "000000" ]; then
    if [ -z "$HOME_PLMN" ]; then
      HOME_PLMN="$plmn"          # learn on first good read
    elif [ "$plmn" != "$HOME_PLMN" ]; then
      susp="${susp:+$susp }PLMN"; alert "PLMN MISMATCH: serving=$plmn expected=$HOME_PLMN op='$op'"
    fi
  fi
  if [ -n "$pci" ] && [ "$pci" != "2147483647" ]; then
    case "$SEEN_CELLS" in
      *" $pci "*) : ;;
      *) SEEN_CELLS="$SEEN_CELLS$pci "; alert "NEW_CELL: first-seen serving pci=$pci ci=$ci tac=$tac earfcn=$earfcn rat=$rat" ;;
    esac
  fi

  # write status snapshot atomically
  : > "$STATUS.tmp"
  setk ts "$(date '+%s')"
  setk rat "${rat:-?}"
  setk plmn "${plmn:-?}"
  setk home_plmn "${HOME_PLMN:-?}"
  setk operator "${op:-?}"
  setk ci "${ci:-?}"; setk pci "${pci:-?}"; setk tac "${tac:-?}"; setk earfcn "${earfcn:-?}"
  setk frames "${frames:-0}"
  setk suspect "${susp:-none}"
  mv "$STATUS.tmp" "$STATUS"

  # persist learned state
  printf 'HOME_PLMN="%s"\nSEEN_CELLS="%s"\n' "$HOME_PLMN" "$SEEN_CELLS" > "$STATE"
  sleep "$POLL"
done