NDEF (NFC Data Exchange Format)
The NFC Data Exchange Format (NDEF) is a standardised data format that can be used to exchange information between any compatible NFC device and another NFC device or tag. The data format consists of NDEF Messages and NDEF Records. The standard is maintained by the NFC Forum and is freely available for consultation but requires accepting a license agreement to download.
The NDEF format is used to store and exchange information like URIs, plain text, etc., using a commonly understood format. NFC tags like Mifare Classic cards can be configured as NDEF tags, and data written to them by one NFC device (NDEF Records) can be understood and accessed by any other NDEF compatible device. NDEF messages can also be used to exchange data between two active NFC devices in "peer-to-peer" mode. By adhering to the NDEF data exchange format during communication, devices that would otherwise have no meaningful knowledge of each other or common language are able to share data in an organised, mutually understandable manner.
NDEF Messages
NDEF Messages are the basic "transportation" mechanism for NDEF records, with each message containing one or more NDEF Records.NDEF Records
NDEF Records contain a specific payload, and have the following structure that identifies the contents and size of the record:Bit 7 6 5 4 3 2 1 0 ------ ------ ------ ------ ------ ------ ------ ------ [ MB ] [ ME ] [ CF ] [ SR ] [ IL ] [ TNF ] [ TYPE LENGTH ] [ PAYLOAD LENGTH ] [ ID LENGTH ] [ RECORD TYPE ] [ ID ] [ PAYLOAD ]
Record Header (Byte 0)
The record header contains a number of important fields, including a 3-bit field that identifies the type of record that follows (the Type Name Format or TNF):TNF: Type Name Format Field
The Type Name Format or TNF Field of an NDEF record is a 3-bit value that describes the record type, and sets the expectation for the structure and content of the rest of the record. Possible record type names include:
TNF Value Record Type
--------- -----------------------------------------
0x00 Empty Record
Indicates no type, id, or payload is associated with this NDEF Record.
This record type is useful on newly formatted cards since every NDEF tag
must have at least one NDEF Record.
0x01 Well-Known Record
Indicates the type field uses the RTD type name format. This type name is used
to stored any record defined by a Record Type Definition (RTD), such as storing
RTD Text, RTD URIs, etc., and is one of the mostly frequently used and useful
record types.
0x02 MIME Media Record
Indicates the payload is an intermediate or final chunk of a chunked NDEF Record
0x03 Absolute URI Record
Indicates the type field contains a value that follows the absolute-URI BNF
construct defined by RFC 3986
0x04 External Record
Indicates the type field contains a value that follows the RTD external
name specification
0x05 Unknown Record
Indicates the payload type is unknown
0x06 Unchanged Record
Indicates the payload is an intermediate or final chunk of a chunked NDEF Record
IL: ID LENGTH Field
The IL flag indicates if the ID Length Field is preent or not. If this is set to 0, then the ID Length Field is ommitted in the record.
SR: Short Record Bit
The SR flag is set to one if the PAYLOAD LENGTH field is 1 byte (8 bits/0-255) or less. This allows for more compact records.
CF: Chunk Flag
The CF flag indicates if this is the first record chunk or a middle record chunk.
ME: Message End
The ME flag indicates if this is the last record in the message.
MB: Message Begin
The MB flag indicates if this is the start of an NDEF message.
Type Length
Indicates the length (in bytes) of the Record Type field. This value is always zero for certain types of records defined with the TNF Field described above.Payload Length
Indicates the length (in bytes) of the record payload. If the SR field (described above) is set to 1 in the record header, this value will be one byte long (for a payload length from 0-255 bytes). If the SR field is set to 0, this value will be a 32-bit value occupying 4 bytes.ID Length
Indicates the length in bytes of the ID field. This field is present only if the IL flag (described above) is set to 1 in the record header.Record Type
This value describes the 'type' of record that follows. The values of the type field must corresponse to the value entered in the TNF bits of the record header.Record ID
The value of the ID field if an ID is included (the IL bit in the record header is set to 1). If the IL bit is set to 0, this field is ommitted.Payload
The record payload, which will be exactly the number of bytes described in the Payload Length field earlier.Well-Known Records (TNF Record Type 0x01)
Probably the most useful record type is the "NFC Forum Well-Known Type" (TNF Type 0x01). Record types that adhere to the "Well-Defined" type are each described by something called an RTD or Record Type Definition. Some of the current Well-Defined RTDs are:URI Records (0x55/'U')
The "Well Known Type" for a URI record is 0x55 ('U'), and this record type can be used to store a variety of useful information such as telephone numbers (tel:), website addresses, links to FTP file locations, etc.URI Records are defined in the document "URI Record Type Definition" from the NFC Forum, and it has the following structure:
Name Offset Size Description ---- ------ ---- ----------- Identifier Code 0 1 byte See table below URI Field 1 N bytes The rest of the URI (depending on byte 0 above)
The URI Identifier Code is use to shorten the URI length, and can have any of the following values:
Value Protocol ----- -------- 0x00 No prepending is done ... the entire URI is contained in the URI Field 0x01 http://www. 0x02 https://www. 0x03 http:// 0x04 https:// 0x05 tel: 0x06 mailto: 0x07 ftp://anonymous:anonymous@ 0x08 ftp://ftp. 0x09 ftps:// 0x0A sftp:// 0x0B smb:// 0x0C nfs:// 0x0D ftp:// 0x0E dav:// 0x0F news: 0x10 telnet:// 0x11 imap: 0x12 rtsp:// 0x13 urn: 0x14 pop: 0x15 sip: 0x16 sips: 0x17 tftp: 0x18 btspp:// 0x19 btl2cap:// 0x1A btgoep:// 0x1B tcpobex:// 0x1C irdaobex:// 0x1D file:// 0x1E urn:epc:id: 0x1F urn:epc:tag: 0x20 urn:epc:pat: 0x21 urn:epc:raw: 0x22 urn:epc: 0x23 urn:nfc:
Following the URI Identifier Code is the URI Field. This field provides the URI as per RFC 3987 and contains the rest of the URI after the value corresponding to the URI Identifier is prepended (unless the URI ID is 0x00, in which case the complete URI will be contained in the URI Field).
Example NDEF Records
Well Known Records
URI Record
An example of a URI record is shown in "Memory Dump of a Mifare Classic 1K Card with an NDEF Record" below.
Text Record
To Do
Smartposter Record
To Do
Absolute URI Record
To Do
Using Mifare Classic Cards as an NDEF Tag
Mifare Classic 1K and 4K cards can be configured as NFC Forum compatible NDEF tags, but they must be organised in a certain manner to do so. The requirements to make a Mifare Classic card "NFC Forum compliant" are described in the following App Note from NXP:AN1304 - NFC Type MIFARE Classic Tag Operation
While the App Note above is the authoritative source on the matter, the following notes may also offer a quick overview of the key concepts involved in using Mifare Classic cards as NFC Forum compatible 'NDEF' tags:
Mifare Application Directory (MAD)
In order to form a relationship between the sector-based memory of a Mifare Classic card and the individual NDEF records, the Mifare Application Directory (MAD) structure is used. The MAD indicates which sector(s) contains which NDEF record. The definitive source of information on the Mifare Application Directory is the following application note:AN10787 - MIFARE Application Directory (MAD)
For reference sake, the two types of MADs (depending on the size of the card in question) are defined below:
Mifare Application Directory 1 (MAD1)
MAD1 can be used in any Mifare Classic card regardless of the size of the EEPROM, although if it is used with cards larger than 1KB only the first 1KB of memory will be accessible for NDEF records.The MAD1 is stored in the Manufacturer Sector (Sector 0x00) on the Mifare Classic card.
Mifare Application Directory 2 (MAD2)
MAD2 can only be used on Mifare Classic cards with more than 1KB of storage (Mifare Classic 4K cards, etc.). It is NOT compatible with cards containing only 1KB of memory!The MAD2 is stored in sectors 0x00 (the Manufacturer Sector) and 0x10.
MAD Sector Access
The sectors containing the MAD1 (0x00) and MAD2 (0x00 and 0x10) are protected with a KEY A and KEY B (if you're not familiar with this concept, consult the Mifare Classic summary elsewhere in the PN532/NFC wiki). To ensure that these sectors can be read by any application, the following common KEY A should always be used:Public KEY A of MAD Sectors --------------------------------------------------- BYTE 0 BYTE 1 BYTE 2 BYTE 3 BYTE 4 BYTE 5 0xA0 0xA1 0xA2 0xA3 0xA4 0xA5
The MAD sector may optionally be write-protected using KEY B if you wish to limit the ability of customers to modify the card contents. The public KEY A will ensure that they can always read the data.
Storing NDEF Messages in Mifare Sectors
NDEF messages/records may be stored in any sector of the Mifare card, other than the sector(s) use by the MAD or sectors beyond the 1K range if a MAD1 table is used.When a sector is used to store NDEF records, it is referred to as an NFC Sector. As with the MAD Sector(s) described above, these sectors must always be accessible in at least read-only mode, and as such a common public KEY A also exists for NFC Sectors, though it is not the same KEY A used in the MAD sector(s):
Public KEY A of NFC Sectors --------------------------------------------------- BYTE 0 BYTE 1 BYTE 2 BYTE 3 BYTE 4 BYTE 5 0xD3 0xF7 0xD3 0xF7 0xD3 0xF7
In order to store an NDEF Message on the Mifare Classic card, the message needs to be wrapped inside something called a TLV Block. The basic structure of a TLV Block is described below.
TLV Blocks
TLV is an abbreviation for three different fields: T for Tag Field, L for Length Field and V for Value Field. A TLV Block consist of one or more bytes, depending on which of these three fields is present. Note that the TLV Block will always be at least one byte since the T Field is mandatory in every case.Tag Field
The Tag Field (or T Field) is the only mandatory field, and uses a single-byte to identify the type of TLV block accordingly to a pre-determined table of values:
TLV Block Types Block Type Value Description ------------- ----- -------------------------------------- NULL 0x00 These blocks should be ignored NDEF Message 0x03 Block contains an NDEF message Proprietary 0xFD Block contains proprietary information Terminator 0xFE Last TLV block in the data area
Length Field
The Length Field (or L Field) contains the size (in bytes) of the value field. The Length Field can be organised in two different ways, using either one or three bytes.
The one byte format simple contains a single byte value from 0x00..0xFF.
The three byte format consists of the following format:
The Length Field (or L Field) contains the size (in bytes) of the value field. The Length Field can be organised in two different ways, using either one or three bytes.
The one byte format simple contains a single byte value from 0x00..0xFF.
The three byte format consists of the following format:
Byte 0: Always 0xFF to indicate that we are using the three byte format Byte 1..2: Can be a value between 0x00FF and 0xFFFE
Both the one byte and three byte format must be supported for NFC Forum and NDEF compatability.
Value Field
The Value Field (or V Field) is only present if the Length Field (described above) is present and not equal to 0x00. If the Length Field is not equal to 0, the Value Fields will contain N bytes of data in the format indicated by the T Field above.
The value field is where the payload (an NDEF Message, for example) is stored.
Terminator TLV
The Terminator TLV is the last TLV block in the data area, and consist of a single byte: 0x0FE (see the TLV Block Type table above). This TLV Block in mandatory.
The Value Field (or V Field) is only present if the Length Field (described above) is present and not equal to 0x00. If the Length Field is not equal to 0, the Value Fields will contain N bytes of data in the format indicated by the T Field above.
The value field is where the payload (an NDEF Message, for example) is stored.
Terminator TLV
The Terminator TLV is the last TLV block in the data area, and consist of a single byte: 0x0FE (see the TLV Block Type table above). This TLV Block in mandatory.
Memory Dump of a Mifare Classic 1K Card with an NDEF Record
[ Start of Memory Dump ] ------------------------Sector 0------------------------- Block 0 3E 39 AB 7F D3 88 04 00 47 41 16 57 4D 10 34 08 >9«Ó?..GA.WM.4. Block 1 14 01 03 E1 03 E1 03 E1 03 E1 03 E1 03 E1 03 E1 ...á.á.á.á.á.á.á Block 2 03 E1 03 E1 03 E1 03 E1 03 E1 03 E1 03 E1 03 E1 .á.á.á.á.á.á.á.á Block 3 00 00 00 00 00 00 78 77 88 C1 00 00 00 00 00 00 ......xw?Á...... ------------------------Sector 1------------------------- Block 4 00 00 03 11 D1 01 0D 55 01 61 64 61 66 72 75 69 ....Ñ..U.adafrui Block 5 74 2E 63 6F 6D FE 00 00 00 00 00 00 00 00 00 00 t.comþ.......... Block 6 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 7 00 00 00 00 00 00 7F 07 88 40 00 00 00 00 00 00 .......?@...... ------------------------Sector 2------------------------- Block 8 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 9 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 10 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 11 00 00 00 00 00 00 7F 07 88 40 00 00 00 00 00 00 .......?@...... ------------------------Sector 3------------------------- Block 12 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 13 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 14 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 15 00 00 00 00 00 00 7F 07 88 40 00 00 00 00 00 00 .......?@...... ------------------------Sector 4------------------------- Block 16 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 17 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 18 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 19 00 00 00 00 00 00 7F 07 88 40 00 00 00 00 00 00 .......?@...... ------------------------Sector 5------------------------- Block 20 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 21 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 22 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 23 00 00 00 00 00 00 7F 07 88 40 00 00 00 00 00 00 .......?@...... ------------------------Sector 6------------------------- Block 24 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 25 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 26 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 27 00 00 00 00 00 00 7F 07 88 40 00 00 00 00 00 00 .......?@...... ------------------------Sector 7------------------------- Block 28 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 29 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 30 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 31 00 00 00 00 00 00 7F 07 88 40 00 00 00 00 00 00 .......?@...... ------------------------Sector 8------------------------- Block 32 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 33 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 34 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 35 00 00 00 00 00 00 7F 07 88 40 00 00 00 00 00 00 .......?@...... ------------------------Sector 9------------------------- Block 36 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 37 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 38 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 39 00 00 00 00 00 00 7F 07 88 40 00 00 00 00 00 00 .......?@...... ------------------------Sector 10------------------------- Block 40 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 41 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 42 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 43 00 00 00 00 00 00 7F 07 88 40 00 00 00 00 00 00 .......?@...... ------------------------Sector 11------------------------- Block 44 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 45 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 46 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 47 00 00 00 00 00 00 7F 07 88 40 00 00 00 00 00 00 .......?@...... ------------------------Sector 12------------------------- Block 48 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 49 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 50 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 51 00 00 00 00 00 00 7F 07 88 40 00 00 00 00 00 00 .......?@...... ------------------------Sector 13------------------------- Block 52 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 53 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 54 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 55 00 00 00 00 00 00 7F 07 88 40 00 00 00 00 00 00 .......?@...... ------------------------Sector 14------------------------- Block 56 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 57 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 58 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 59 00 00 00 00 00 00 7F 07 88 40 00 00 00 00 00 00 .......?@...... ------------------------Sector 15------------------------- Block 60 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 61 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 62 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ Block 63 00 00 00 00 00 00 7F 07 88 40 00 00 00 00 00 00 .......?@...... [ End of Memory Dump ]
NDEF Records
The above example contains two records, both located in sector 1 (sector 0 contains the MAD).Record 1
The first record on the card can be identified by looking at the first byte of block 4 in sector 1.
Block 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 Char Value ----- ----------------------------------------------- ------------ 04 00 00 ..
Every record on the Mifare card starts with the TLV Block (described above), and the first byte of the TLV Block (the Tag Field) indicates that this is a NULL Block type (value 0x00). The second byte is the Length Field, and is 0. Since there is no payload for this record (Length = 0), the third byte of the TLV block is not present (the Value Field).
This record was likely inserted when the card was first formatted to ensure that at least one record is present.
This record was likely inserted when the card was first formatted to ensure that at least one record is present.
Record 2
The second record on the card starts at byte 0x02 of block 4 and continues into block 5.
The second record on the card starts at byte 0x02 of block 4 and continues into block 5.
Block 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 Char Value ----- ----------------------------------------------- ------------ 04 03 11 D1 01 0D 55 01 61 64 61 66 72 75 69 Ñ..U.adafrui 05 74 2E 63 6F 6D t.com
Starting with the TLV Block data in the first two bytes, we can determine the following:
Byte(s) Value Description ------- ----- ----------- 04:02 0x03 Field Type (0x03 = NDEF Message) 04:03 0x11 Length Field (17 bytes)
This indicates to us that the record contains an NDEF Message (value 0x03), and that the message is 17 bytes long (0x11 in hexadecimal = 17 in decimal value). This means that our NDEF message is contained in the next 17 bytes (04:04..05:04). The NDEF record can then be analysed as follows:
Byte(s) Value Description
------- ----- -----------
04:04 0xD1 This byte is the **NDEF Record Header**, and indicates that this is
an NFC Forum Well Known Record (0x01 in the first 3 bits),
and that this is the first and last record (MB=1, ME=1),
and that this is a short record (SR = 1) meaning the payload
length is less than or equal to 255 chars (len=one byte).
TNF = 0x01 (NFC Forum Well Known Type)
IL = 0 (No ID present, meaning there is no ID Length or ID Field either)
SR = 1 (Short Record)
CF = 0 (Record is not 'chunked')
ME = 1 (End of message)
MB = 1 (Beginning of message)
04:05 0x01 This byte is the **Type Length** for the Record Type Indicator
(see above for more information), which is 1 byte (0x55/'U' below)
04:06 0x0D This is the payload length (13 bytes)
04:07 0x55 Record Type Indicator (0x55 or 'U' = URI Record)
04:08 0x01 This is the **start of the record payload**, which contains the
URI Identifier ("http://www.") since this is a URI Well-Defined
Record Type (see Well-Defined Records above). This will be
prepended to the rest of the URI that follows in the rest of the
message payload
04:09..05:04 ... The remainder of the URI ("adafruit.com"), which combined with the
pre-pended value from byte 04:08 yields: http://www.adafruit.com
TLV Terminator
Block 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 Char Value ----- ----------------------------------------------- ------------ 05 FE þ
The final byte (block 5, byte 5), with the value 0xFE, is the TLV Terminator and indicates that this is the end of the TLV Block.
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