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How to Identify NFC Tag Types — ATQA, SAK & ATS Complete Detection Guide

When an NFC reader detects a tag, it receives anti-collision bytes that identify the chip before any NDEF data is read. The combination of ATQA (Answer to Request), SAK (Select Acknowledge), and ATS (Answer to Select) uniquely fingerprint the tag's manufacturer, chip model, and capabilities. This guide maps every common combination.

1. The ISO 14443-3 Anti-Collision Exchange

READER                          TAG
  │                              │
  │──── REQA (0x26) ────────────→│  "Who's there?"
  │←──── ATQA (2 bytes) ────────│  "I'm here, here's my type info"
  │                              │
  │──── ANTICOLLISION (SELECT) ─→│  "Tell me your UID"
  │←──── UID (4/7/10 bytes) ────│  "Here's my serial number"
  │                              │
  │──── SELECT (UID) ───────────→│  "I'm talking to YOU"
  │←──── SAK (1 byte) ──────────│  "Selected. Here's what I am"
  │                              │
  │──── RATS ───────────────────→│  (ISO 14443-4 only)
  │←──── ATS ───────────────────│  "Here's my protocol capabilities"

2. ATQA — Answer to Request, Type A

ATQA is 2 bytes (16 bits). Bit 8 of each byte indicates UID size. The remaining bits encode the chip platform:

3. SAK — Select Acknowledge Decode

SAK is 1 byte with critical identification bits:

SAK bit meanings (from ISO 14443-3):
Bit 6: ISO 14443-4 compliant (ATS response available)
Bit 5: NFC Forum Tag Platform compliant
Bit 4: UID complete (cascade bit)
Bit 3: Anti-collision complete

# The key bit is bit 6:
# SAK bit 6 = 1 → card speaks ISO 14443-4 (APDUs, RATS, ATS)
# SAK bit 6 = 0 → card is MIFARE Classic/Ultralight (proprietary protocol)

4. ATS — Answer to Select (ISO 14443-4 Cards)

If SAK bit 6 = 1, the reader sends RATS and the card responds with ATS. This provides more detail:

ATS structure:
[TL] [T0] [TA(1)] [TB(1)] [TC(1)] [Historical bytes (T1-Tk)] [CRC]

# Common ATS for DESFire EV3:
# 06 75 77 81 02 80  → TL=6, T0=75 (TA1+TB1+TC1 present, FSCI=5)
# TA1=77 (DRI=7, DSI=7 — fastest rates)
# TB1=81 (FWI=8, SFGI=1)
# TC1=02 (CID supported, NAD not supported)

# Common ATS for NTAG 424 DNA:
# 08 78 77 71 02  ... longer ATS with historical bytes

5. Quick Identification Table

6. Python Detection Script

def identify_tag(atqa, sak, uid):
    """Identify NFC tag chip from ATQA, SAK, UID."""
    atqa_int = (atqa[1] << 8) | atqa[0]

    if sak == 0x00 and atqa_int in (0x0004, 0x0044):
        return 'NTAG 21x / MIFARE Ultralight (Type 2)'
    elif sak == 0x08:
        if atqa_int == 0x0008:
            return 'MIFARE Classic 1K'
        else:
            return f'MIFARE Classic (ATQA=0x{atqa_int:04X})'
    elif sak == 0x18:
        return 'MIFARE Classic 4K'
    elif sak == 0x20:
        if atqa_int == 0x0344:
            return 'MIFARE DESFire EV1/EV2/EV3 (Type 4)'
        else:
            return f'ISO 14443-4 Card (ATQA=0x{atqa_int:04X})'
    return f'Unknown tag: ATQA=0x{atqa_int:04X}, SAK=0x{sak:02X}'

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Related Tools

NFC Tag Type Detector — Identify tags from ATQA/SAK/UID | NFC Capacity Calculator — Check NDEF capacity | NFC Tag Selection Guide — Choose the right tag | NTAG 213 vs 215 vs 216