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This document summarizes the functions of the NFCTagWriter app, organized by tag type support.

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Part 1: NTAG4242 (NTAG 424 DNA) Functions

The NTAG 424 DNA tag support is implemented using the NTAG424DNAScanner class, which utilizes the NfcDnaKit library for communication with ISO 7816-compliant tags.

1. Set Password

• Function: beginSettingPassword(password:)
• Purpose: Set or change the AES-128 encryption password on the tag
• Features:
  • Supports setting password on new tags (using default key)
  • Supports changing existing password (requires current password authentication)
  • Automatically verifies password after setting
  • Verifies that default key no longer works after password is set
  • Uses AES-128 encryption (16-byte key)
• Process:
  1. Authenticate with default key (for new tags) or current password
  2. Change key 0 to new password
  3. Verify new password by authenticating with it
  4. Confirm default key is disabled

2. Read Data

• Function: beginReadingData(password:)
• Purpose: Read NDEF data from the tag's NDEF file
• Features:
  • Optional password authentication (if tag is protected)
  • Reads up to 256 bytes from NDEF file (file number 0x02)
  • Parses NDEF message using NLEN format (NFC Forum Type 4 Tag compliant)
  • Extracts text or URI from NDEF records
  • Supports both URI and text record types
• Data Format: NLEN structure [NLEN(2 bytes)] [NDEF Data]

3. Write Data

• Function: beginWritingData(data:password:)
• Purpose: Write NDEF data to the tag's NDEF file
• Features:
  • Requires password authentication if tag is protected
  • Creates NDEF message from text/URL string
  • Writes in chunks of 128 bytes (datasheet maximum for tearing protection)
  • Uses NLEN format for NFC Forum Type 4 Tag compliance
  • Verifies write operation by reading back data
• Data Format: Automatically creates NDEF URI or text records

4. Configure File Access

• Function: beginConfiguringFileAccess(password:)
• Purpose: Configure NDEF file access permissions for security
• Features:
  • Sets read access to ALL (0xE) - allows all readers (critical for iOS background detection)
  • Sets write access to KEY_0 (0x0) - requires authentication to write
  • Sets change access to KEY_0 (0x0) - requires authentication to change settings
  • Uses PLAIN communication mode for third-party tool compatibility
  • Disables SDM (Secure Unique NFC) for compatibility
  • Verifies configuration after applying changes
• Security: Prevents unauthorized writes while allowing reads

5. Configure CC File

• Function: beginConfiguringCCFile(password:)
• Purpose: Configure Capability Container (CC) file for iOS background detection
• Features:
  • Writes CC file content (32 bytes) with proper Type 4 Tag specification
  • Sets read access to ALL (0xE) - critical for iOS background detection
  • Sets write access to KEY_0 (0x0) - requires authentication
  • Uses PLAIN communication mode
  • Enables iOS to detect tag in background mode
• File Structure: Includes CCLEN, Mapping Version, MLe/MLc, and file control TLVs

6. UID Detection

• Function: Automatic during tag detection
• Purpose: Extract and display tag UID (Unique Identifier)
• Features:
  • Extracts UID from tag identifier
  • Displays in hex format (colon-separated)
  • Available for both ISO 7816 and MIFARE tag detections

Technical Details

• Library: Uses NfcDnaKit for ISO 7816 tag communication
• Encryption: AES-128 (16-byte keys)
• Tag Detection: Supports both ISO 7816 and MIFARE detection modes
• File System: Type 4 Tag file structure (CC file, NDEF file)
• Compatibility: Works with third-party tools (NXP TagWriter, TagInfo, etc.)

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Part 2: NTAG21X (NTAG213/215/216) Functions

The NTAG21X tag support is implemented using the NFCScanner class, which handles MIFARE Ultralight protocol communication.

1. Read Tag Information

• Function: beginReadingTagInfo()
• Purpose: Read comprehensive tag information and capabilities
• Features:
  • Reads serial number (UID) from pages 0-2
  • Reads Capability Container (CC) from page 3
  • Determines tag type (NTAG213/215/216) from NDEF memory size
  • Calculates actual total memory size (different from NDEF size)
  • Reads password protection pages (AUTH0, ACCESS, PWD, PACK)
  • Displays password protection status (enabled/disabled, active/configured)
  • Shows memory information (total bytes, pages, NDEF size)
• Information Displayed:
  • Serial number (hex format)
  • Tag type
  • Total memory size and pages
  • NDEF memory size
  • CC version and access
  • Password protection pages locations
  • Password protection status and configuration

2. Read Data

• Function: beginReading(password:)
• Purpose: Read NDEF data from the tag
• Features:
  • Automatically checks if authentication is needed based on ACCESS byte
  • Supports write-only protection mode (read without password)
  • Supports read/write protection mode (requires password)
  • Parses NDEF TLV structure (Tag-Length-Value format)
  • Extracts text or URI from NDEF records
  • Handles both short and long length formats
  • Reads dynamically based on CC size information
• Authentication: Only authenticates if ACCESS bit 7 is set (read protection enabled)

3. Write Data

• Function: beginWriting(password:textToWrite:)
• Purpose: Write NDEF data to the tag
• Features:
  • Always requires password authentication if password is set
  • Creates NDEF URI or text records from input string
  • Writes data in TLV format (0x03 tag, length, payload, 0xFE terminator)
  • Writes page-by-page (4 bytes per page) starting from page 4
  • Handles large messages by writing multiple pages
  • Uses native MIFARE write commands (0xA2)
• Data Format: TLV structure [0x03] [Length] [NDEF Data] [0xFE]

4. Set Password

• Function: beginSettingPassword(password:writeOnlyProtection:)
• Purpose: Set password protection on the tag
• Features:
  • Supports two protection modes:
    • Write Protected Only: Read access without password, write requires password
    • Read & Write Protected: Both read and write require password
  • Writes 4-byte password to PWD page
  • Sets AUTH0 page (specifies which page requires authentication, typically 0x04)
  • Configures ACCESS byte:
    • Bit 7 = 0: Write-only protection (read allowed without password)
    • Bit 7 = 1: Read/write protection (both require password)
  • Preserves existing PACK and RFUI values
  • Validates tag type and memory pages before setting
• Process:
  1. Verify tag type and password protection page locations
  2. Write password to PWD page
  3. Set AUTH0 to 0x04 (page 4 and above require authentication)
  4. Read current ACCESS page
  5. Set ACCESS byte based on protection mode
  6. Tag must be removed and re-presented for protection to take effect

5. Authentication

• Function: authenticateTag(miFareTag:session:)
• Purpose: Authenticate with password for protected operations
• Features:
  • Uses PWD_AUTH command (0x1B) followed by 4-byte password
  • Receives 2-byte PACK (Password Acknowledge) on success
  • Handles authentication errors (NAK responses)
  • Required for write operations if password is set
  • Required for read operations if read protection is enabled
• Command Format: [0x1B] [PWD(4 bytes)]

Technical Details

• Protocol: MIFARE Ultralight (ISO 14443-A)
• Tag Types Supported:
  • NTAG213: 180 bytes total (45 pages), NDEF: 144 bytes
  • NTAG215: 504 bytes total (126 pages), NDEF: 496 bytes
  • NTAG216: 888 bytes total (222 pages), NDEF: 872 bytes
• Password Protection Pages:
  • NTAG213: AUTH0=0x29, ACCESS=0x2A, PWD=0x2B, PACK=0x2C
  • NTAG215: AUTH0=0x83, ACCESS=0x84, PWD=0x85, PACK=0x86
  • NTAG216: AUTH0=0xE3, ACCESS=0xE4, PWD=0xE5, PACK=0xE6
• Password Format: 4 bytes (ASCII string, truncated/padded)
• NDEF Format: TLV structure with 0x03 tag for NDEF message

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Common Features

Error Handling

• Comprehensive error messages for all operations
• Connection loss detection and recovery guidance
• Authentication failure handling with helpful messages
• Tag type validation before operations

User Interface

• Real-time status updates during operations
• Alert messages for operation results
• Tag information display with detailed formatting
• Password protection mode selection (write-only vs read/write)

Compatibility

• NTAG4242: Compatible with NXP TagWriter, TagInfo, and other Type 4 Tag tools
• NTAG21X: Standard MIFARE Ultralight protocol, compatible with most NFC readers

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Notes

• NTAG4242 uses AES-128 encryption and requires 16-byte passwords
• NTAG21X uses 4-byte passwords and simpler authentication
• Both tag types support NDEF data format but use different storage structures
• Password protection must be configured before it takes effect (tag removal required for NTAG21X)
• iOS background detection requires proper CC file and NDEF file access configuration for NTAG4242