Google Smart Tap

Overview

Google Smart Tap is a proprietary NFC protocol that can be used for sending data from a mobile device to an NFC terminal.

Data is conveyed from the device to the terminal in encrypted form, using keys derived during the channel negotiation phase. At the moment of negotiation, the reader sends a device its collector id, key version, and a signature of derived data signed using the collector key, thus proving to the device that the reader is allowed to get the information.

Only one pass (object) could be conveyed during a single tap (single read).
If more than one pass is eligible for redemption, a selection carousel will appear and a user will be prompted to tap again.

Version 2.1 was current at the time of writing.

Application identifiers

Smart Tap can be activated using multiple application ids (AID):

Universal VAS AID (hex of encoded OSE.VAS.01), also used by Apple VAS.
```
4f53452e5641532e3031
```
Smart Tap 1 (Deprecated, does not work anymore)
```
a000000476d0000101
```
Smart Tap 2
```
a000000476d0000111
```

The ususal implementation for most readers is to select OSE.VAS.01 in order to detect what wallet provider is available on device (stored in TLV tag 50), if "AndroidPay" is the value, then we have a device with Google Wallet, and Smart Tap 2 can be reselected if required.
As of version 2.1 device nonce and key is returned in OSE, so a separate selection of Smart Tap is not needed.

Command overview

SmartTap-exclusive commands and responses use multi-layer nested NDEF messages and records for conveying information. As of version 2.1 following commands are available:

Command name	CLA	INS	P1	DATA	Response data	NOTES
SELECT VAS APPLET	00	A4	04	VAS AID	BER-TLV	Optional. Should be implemented if you want to support other wallets with value-added services, as this AID allows to find out what implementation is used without bruteforce
SELECT SMART TAP APPLET	00	A4	04	Smart Tap VX AID	NDEF message	Optional if SELECT VAS APPLET has been used.
NEGOTIATE SECURE CHANNEL	90	53	00	NDEF message with nested NDEF message	NDEF message with nested NDEF
GET DATA	90	50	00	NDEF message with nested NDEF message	Part of NDEF message with encrypted and/or compressed nested NDEF	Can be used only after channel negotiation
GET MORE DATA	90	C0	00	No data (V 2.1)	Part of NDEF message with encrypted and/or compressed nested NDEF	Can be used if GET DATA response sw is 9100
PUSH DATA	90	52	00	NDEF message with nested NDEF	NDEF message with nested NDEF	Can be used before of after data read. Secure channel not needed. Exact use case of this command is not known. Possible use is to push signup URL but this feature seems to be disabled.

Commands are executed as follows:

SELECT VAS APPLET:
Optional. May be the first command in a read flow. Reader transmits universal VAS AID; Device response with wallet implementation name in TLV tag 50.
If value is 416e64726f6964506179, which is a value of AndroidPay string in ASCII-encoded form, we have a device that supports SmartTap.
Device returns supported versions, other info. For, newer smart tap implementations it returns device nonce and device ephemeral key.
SELECT SMART TAP APPLET:
Optional. May be the first command in a read flow, or a fallback if SELECT VAS APPLET returned no device nonce or device ephemeral key; Device returns version support range, and a device nonce.
NEGOTIATE SECURE CHANNEL:
Reader generates a nonce, ephemeral public key. Reader generates a signature over concatenation of reader nonce, device nonce, collector id, and reader ephemeral public ke using a private key of collector.
It then transmits session-related information together with the signature to the end device, proving to it that the reader is owned by a particular pass collector. Device responds with ephemeral public key, session information. Reader then uses all collected information in order to calculate session keys that will be used to encrypt and decrypt data.
GET DATA:
Reader transmits its configuration, capabilityies, session information. Device responds with full or partial NDEF data with encrypted nested data.
GET MORE DATA:
If a response to previous GET DATA or GET MORE DATA returned status word 91 00, then more data has to be read. Reader transmits this command as many times as needed, until a device responds any response rather than 91 00.
PUSH DATA:
This command is not known to be used by any IRL readers. It was intended to be used by readers in order to push some information, like sign-up urls, pass data updates, pass addition, etc.
None of the possible parameter combinations seem to have any visible effects, so theres a big chance that its a leftover of unfinished or cut functionality. Reader transmits session info, list of service statuses (changes to the passes, usage history, URL signups), total transaction info (ability to send receipts). Device responds with an acknowledgment and session info.

Entities, constants and data format

SmartTap protocol uses NDEF records for transmitting data, sometimes in nested fashion.
Before closer look at communication, it is important to undersand data representation used during communication.

Data format

Some records may contain a data format identifier in the first byte of payload

Following data formats are known:

Name	Field
UNSPECIFIED	0x00
ASCII	0x01
UTF_8	0x02
UTF_16	0x03
BINARY	0x04
BCD	0x05

Most common used type is BINARY, but even it is not used for all payloads.
There seems to be no particular pattern as for where data format identifier is mandatory, so its use will be mentioned for each record type.

Record types

Type in SmartTap defines what kind of object/data does the record hold. Each type is denoted by a specific short string. Depending on TNF, type value may be populated into either type or id fields: Type field follows following rules when being populated into NDEF Records

TNF	Field
WELL_KNOWN(0x01)	id
EXTERNAL(0x04)	type

Older SmartTap versions required id field to be used instead to populate type, but new ones recognize only the described rules.

Records

Following types exist in SmartTap, but not all of them are used:

Name	Type	Payload
HANDSET_NONCE	mdn	BINARY format flag + 32 byte long nonce
SESSION	ses	8 byte long id, 1 byte long sequence counter, 1 byte long status
NEGOTIATE_REQUEST	ngr	2 byte long version, nested NDEF message with `ses` and `cpr` records
NEGOTIATE_RESPONSE	nrs	Nested NDEF message with `ses` and `dpk` records
CRYPTO_PARAMS	cpr	32 byte long reader nonce, 1 byte long auth flag, 33 byte long reader ephemeral public key, 4 byte long key version, NDEF message with `sig` and `cld` records
SIGNATURE	sig	BINARY format flag + 72 byte long signature data encoded in ASN1 as Dss-Sig-Value
SERVICE_REQUEST	srq	2 byte long version, nested NDEF message with `ses`, `mer`, `slr`, `pcr` records
ADDITIONAL_SERVICE_REQUEST	asr	Was used to get data continuation. Unused in SmartTap 2.0
SERVICE_RESPONSE	srs	Contains nested `ses` and `reb` records
MERCHANT	mer	Nested NDEF message with mandatory `cld` and optional `lid`, `tid`, `mnr`, `mcr` records
COLLECTOR_ID_V0	mid
COLLECTOR_ID	cld	4 byte long big endian representation of collector id number
LOCATION_ID	lid
TERMINAL_ID	tid
MERCHANT_NAME	mnr
MERCHANT_CATEGORY	mcr
SERVICE_LIST	slr	Nested NDEF message with `str` record
SERVICE_TYPE_REQUEST	str	List of 1 byte long service objects types to request
HANDSET_EPHERMAL_PUBLIC_KEY	dpk	33-byte long public ephemeral EC key
ENCRYPTED_SERVICE_VALUE	enc
SERVICE_VALUE	asv	Contains `i` record. Depending on object type, may contain one of `cus`, `et`, `fl`, `gc`, `ly`, `of`, `pl`, `tr`, `gr` records.
SERVICE_ID	sid
OBJECT_ID	oid	BINARY format flag + 8 byte identifier
RECORD_BUNDLE	reb	First byte is response flag, other bytes is encrypted and/or compressed data depending on flag, containing `asv` object records
CUSTOMER	cus	May be contained in unpacked record bundle. Contains
CUSTOMER_ID	cid	BINARY format flag + 16 byte identifier
CUSTOMER_LANGUAGE	cpl	NDEF-encoded string with language code
CUSTOMER_TAP_ID	cut	BINARY format flag + 16 byte identifier
EVENT	et	Object
FLIGHT	fl	Object
GIFT_CARD	gc	Object
LOYALTY	ly	Object
OFFER	of	Object
PLC	pl	Object
TRANSIT	tr	Object
GENERIC	gr	Object
ISSUER	i	BINARY format flag + 5 byte identifier
SERVICE_NUMBER	n	UNSPECIFIED format flag + variable length value of a particular service. In essence, this is the pass data.
TRANSACTION_COUNTER	tcr
PIN	p
EXPIRATION_DATE	ex
CVC	c1
POS_CAPABILITIES	pcr	Contains 5 byte long flag POS CAPABILITIES mask
PUSH_SERVICE_REQUEST	spr	Nested NDEF message with mandatory `ses` and optional, `bpr`, `nsr`, `ssr` records
PUSH_SERVICE_RESPONSE	psr	Nested NDEF message with `ses` record
SERVICE_STATUS	ssr	Nested NDEF message with `oid`, `sug` and `sup` records
SERVICE_USAGE	sug	Nested NDEF message with `sut` and `sud` records
SERVICE_USAGE_TITLE	sut	NDEF encoded text
SERVICE_USAGE_DESCRIPTION	sud	NDEF encoded text
SERVICE_UPDATE	sup	1 byte long service operation code concatenated with value
NEW_SERVICE	nsr	Nested NDEF message with `nst` and `nsu` records
NEW_SERVICE_TITLE	nst	NDEF encoded text
NEW_SERVICE_URI	nsu	NDEF encoded URI
BASKET_PRICE	bpr	Nested NDEF message with `mon` and `ccd` records
BASKET_PRICE_AMOUNT	mon	Numeric price value
BASKET_PRICE_CURRENCY	ccd	NDEF encoded text currency code

Records marked as Object in value are transmitted in GET DATA response.
Inner fields of objects depend on object type, but usually, each object contains at least i and n records in nested NDEF message.

Statuses

Status field is returned inside of the session record as the last byte.

Name	Value
UNKNOWN	0x00
OK	0x01
NDEF_FORMAT_INVALID	0x02
UNSUPPORTED_VERSION	0x03
INVALID_SEQUENCE_NUMBER	0x04
UNKNOWN_MERCHANT	0x05
MERCHANT_INFO_MISSING	0x06
SERVICE_DATA_MISSING	0x07
RESEND_REQUEST	0x08
DATA_NOT_AVAILABLE_YET	0x09

Terminal capabilities

Used in side POS capabilities record. Capabilities are listed like bytes in order from left to right (aka reverse order)

System capabilities:

Name	Value	Effect
SYSTEM_STANDALONE	0x01
SYSTEM_SEMI_INTEGRATED	0x02
SYSTEM_UNATTENDED	0x04
SYSTEM_ONLINE	0x08
SYSTEM_OFFLINE	0x10
SYSTEM_MMP	0x20
SYSTEM_ZLIB_SUPPORTED	0x40	Pass data will be compressed. Adviced to use

User interface capabilities. No known effects:

Name	Value	Effect
UI_PRINTER	0x01
UI_PRINTER_GRAPHICS	0x02
UI_DISPLAY	0x04
UI_IMAGES	0x08
UI_AUDIO	0x10
UI_ANIMATION	0x20
UI_VIDEO	0x40

Checkout capabilities. No known effects:

Name	Value	Effect
CHECKOUT_SUPPORT_PAYMENT	0x01
CHECKOUT_SUPPORT_DIGITAL_RECEIPT	0x02
CHECKOUT_SUPPORT_SERVICE_ISSUANCE	0x04
CHECKOUT_SUPPORT_OTA_POS_DATA	0x08

CVM capabilities. No known effects:

Name	Value	Effect
CVM_ONLINE_PIN	0x01
CVM_CD_PIN	0x02
CVM_SIGNATURE	0x04
CVM_NOCVM	0x08
CVM_DEVICE_GENERATED_CODE	0x10
CVM_SP_GENERATED_CODE	0x20
CVM_ID_CAPTURE	0x40
CVM_BIOMETRIC	0x80

VAS Type capabilities. Set proper type for proper UI interaction:

Name	Value	Effect
TAP_PASS_ONLY	0x01
TAP_PAYMENT_ONLY	0x02
TAP_PASS_AND_PAYMENT	0x04
TAP_PASS_OVER_PAYMENT	0x08

Request object type

Used in service request list:

Name	Value
ALL	0x00
ALL_EXCEPT_PPSE	0x01
PPSE	0x02
LOYALTY	0x03
OFFER	0x04
GIFT_CARD	0x05
PRIVATE_LABEL_CARD	0x06
EVENT_TICKET	0x07
FLIGHT	0x08
TRANSIT	0x09
CLOUD_BASED_WALLET	0x10
MOBILE_MARKETING_PLATFORM	0x11
GENERIC	0x12
WALLET_CUSTOMER	0x40

Push Data related options

PUSH DATA seems to be cut or limited in newer versions of SmartTap. This info is provided solely for reference, it has no use IRL.

Push service type

Name	Value
UNSPECIFIED	0x00
VALUABLE	0x01
RECEIPT	0x02
SURVEY	0x03
GOODS	0x04
SIGNUP	0x05

Service status usage

Name	Value
UNDEFINED	0x00
SUCCESS	0x01
INVALID_FORMAT	0x02
INVALID_VALUE	0x03
UNKNOWN	0xff

Service update operation

Name	Value
NO_OP	0x00
REMOVE	0x01
SET_BALANCE	0x02
ADD_BALANCE	0x03
SUBTRACT_BALANCE	0x04
FREE	0x05
UNKNOWN	0xFF

Command and response data format

Status words

Following status words may be met during proper communication

SW1	SW2	Meaning
90	00	Ok
90	91	More data pending
90	01	No passes
94	06	Too many requests
93	02	User has to choose pass

SELECT VAS APPLET

Request:

CLA	INS	P1	P2	DATA	LE
00	A4	04	00	`4f53452e5641532e3031`	00

Data contains an ASCII encoded form of "OSE.VAS.01" string;

Response

SW1	SW2	DATA
90	00	Dynamic. Data format below. 135 bytes long

Response data example:

Payload:
- 6f8184500a416e64726f6964506179c0020001c108cc00000000008080c22056d2ec8f857f0049aa54f1ca1de2791b5693a7014e6e4565d5644b1c2a305136c32103dfee38dbdb68a607383ad622640b180cc7e27d796b4e788c40e5d994291c71fca523bf0c20611e4f09a000000476d0000111870101730edf6d020000df4d020001df620103

TLV decoded:

TLV:
  6f[8184]: # File Control Information Template
      50[0a]: # Application Label
         416e64726f6964506179 # ASCII form of "AndroidPay"
      c0[02]: # Application version
         0001
      c1[08]: # Unknown. Feature flags?
         cc00000000008080
      c2[20]: # Mobile device Nonce
         56d2ec8f857f0049aa54f1ca1de2791b5693a7014e6e4565d5644b1c2a305136
      c3[21]: # Mobile device ephemeral key
         03dfee38dbdb68a607383ad622640b180cc7e27d796b4e788c40e5d994291c71fc
      a5[23]: 
         bf0c[20]:
            61[1e]:
               4f[09]: # Application ID
                  a000000476d0000111
               87[01]: # Priority
                  01
               73[0e]: # Proprietary data
                  df6d[02]: # Minimum version
                     0000
                  df4d[02]: # Maximum version
                     0001
                  df62[01]: # Unknown. Format flags?
                     03

Tags c1, c2, c3, a5, 73 may be missing depending on device software version. Be aware of this fact.

SELECT SMART TAP APPLET

Request:

CLA	INS	P1	P2	DATA	LE
00	A4	04	00	`a000000476d0000111`	00

Response

SW1	SW2	DATA
90	00	Dynamic. Data format below. 47 bytes long

Response data example:

Payload:
- 00000001dc0321036d646e6d646e0456d2ec8f857f0049aa54f1ca1de2791b5693a7014e6e4565d5644b1c2a305136

Decoded:

min_version=0000
max_version=0001
message=NDEFMessage(
   NDEFRecord(
      tnf=EXTERNAL(04), 
      type=bytearray(b'mdn'), 
      id=bytearray(b'mdn'), 
      payload=0x0456d2ec8f857f0049aa54f1ca1de2791b5693a7014e6e4565d5644b1c2a305136
   )
)

NEGOTIATE SECURE CHANNEL

Request:

CLA	INS	P1	P2	DATA	LE
90	53	00	00	Dynamic. Data format below.	00

Command data example:

Payload:
- d403b86e6772000194030a7365736b159a80fc8283fd00015403a0637072a8aa2bae1ba891783d8c5be8a95bf2f9e5bb90fd9d197f8b2b1a84d9cc80427501027b2e12f1a1a542084b4d01b8799380fa4cb77e530ba2305b0bf2b3e4b474fe7d0000000194034973696704304602210086e43dc483b22e51aa177ae8112ed83d399a58b41d6d8cbe900cde03c4524da5022100e94b6a919c9e097568f4efa9a7123b86b97ee44342593f8a77fc9e12e3f95ae4540305636c640401020304

Decoded:

message=NDEFMessage(
   NDEFRecord(
      tnf=EXTERNAL(04),
      type=b'ngr',
      id=b'',
      payload=[
         Version(00:01), 
         NDEFMessage(
            NDEFRecord(
               tnf=EXTERNAL(04),
               type=b'ses',
               id=b'',
               payload=[
                  0x6b159a80fc8283fd,
                  0x00, 
                  OK(01)
               ]
            ), 
            NDEFRecord(
               tnf=EXTERNAL(04),
               type=b'cpr',
               id=b'',
               payload=[
                  0xa8aa2bae1ba891783d8c5be8a95bf2f9e5bb90fd9d197f8b2b1a84d9cc804275, 
                  0x01, 
                  0x027b2e12f1a1a542084b4d01b8799380fa4cb77e530ba2305b0bf2b3e4b474fe7d,
                  0x00000001,
                  NDEFMessage(
                     NDEFRecord(
                        tnf=EXTERNAL(04),
                        type=b'sig',
                        id=b'',
                        payload=[
                           BINARY(04), 
                           0x304602210086e43dc483b22e51aa177ae8112ed83d399a58b41d6d8cbe900cde03c4524d
                              a5022100e94b6a919c9e097568f4efa9a7123b86b97ee44342593f8a77fc9e12e3f95ae4
                        ]
                     ), 
                     NDEFRecord(
                        tnf=EXTERNAL(04),
                        type=b'cld',
                        id=b'',
                        payload=[
                           BINARY(04), 
                           0x01020304
                        ]
                     )
                  )
               ]
            )
         )
      ]
   )
)

Response

SW1	SW2	DATA
90	00	Dynamic. Data format below. 61 bytes long

Response data example:

Payload:
- d403376e727394030a7365736b159a80fc8283fd010154032164706b03dfee38dbdb68a607383ad622640b180cc7e27d796b4e788c40e5d994291c71fc

Decoded:

NDEFMessage(
   NDEFRecord(
      tnf=EXTERNAL(04), 
      type=bytearray(b'nrs'),
      id=bytearray(b''),
      payload=NDEFMessage(
         NDEFRecord(
            tnf=EXTERNAL(04),
            type=b'ses',
            id=b'',
            payload=[
               0x6b159a80fc8283fd,
               0x01,
               0x01
            ]
         ),
         NDEFRecord(
            tnf=EXTERNAL(04),
            type=b'dpk',
            id=b'',
            payload=[
               0x03dfee38dbdb68a607383ad622640b180cc7e27d796b4e788c40e5d994291c71fc
            ]
         )
      )
   )
)

GET DATA

Request:

CLA	INS	P1	P2	DATA	LE
90	50	00	00	Dynamic. Data format below.	00

Command data example:

Payload:
- d4033b737271000194030a7365736b159a80fc8283fd010114030b6d6572d40305636c640401020304140307736c72d40301737472005403057063724100000004

Decoded:

message=NDEFMessage(
   NDEFRecord(
      tnf=EXTERNAL(04), 
      type=b'srq', 
      id=b'', 
      payload=[
         Version(00:01), 
         NDEFMessage(
            NDEFRecord(
               tnf=EXTERNAL(04), 
               type=b'ses', 
               id=b'', 
               payload=[
                  0x6b159a80fc8283fd,
                  0x01,
                  OK(01)
               ]
            ),
            NDEFRecord(
               tnf=EXTERNAL(04),
               type=b'mer',
               id=b'',
               payload=NDEFMessage(
                  NDEFRecord(
                     tnf=EXTERNAL(04),
                     type=b'cld',
                     id=b'',
                     payload=[
                        BINARY(04),
                        0x01020304
                     ]
                  )
               )
            ), 
            NDEFRecord(
               tnf=EXTERNAL(04),
               type=b'slr',
               id=b'',
               payload=NDEFMessage(
                  NDEFRecord(
                     tnf=EXTERNAL(04),
                     type=b'str',
                     id=b''
                     payload=0x00
                  )
               )
            ), 
            NDEFRecord(
               tnf=EXTERNAL(04),
               type=b'pcr',
               id=b'',
               payload=[SYSTEM_ZLIB_SUPPORTED(40) + SYSTEM_STANDALONE(01), 0x00, 0x00, 0x00, TAP_PASS_AND_PAYMENT(04)]
            )
         )
      ]
   )
)

Response:

SW1	SW2	DATA
XX	XX	Dynamic. Data format below.

Status word differs if:

Pass data is available;
Pass data is available, but more data is pending;
Pass data is unavailable;
User has to select a pass

Response may contain a part or a full NDEF message with nested encrypted and or compressed nested NDEF message

Full* response data example:

Payload:
- d403ce73727394030a7365736b159a80fc8283fd02015403b8726562031c5b7d2e9f0f6937a7c043dc6913dd976002ce5cf179dedbcaaab629442a89a72b6a92caa26e808903a6d02207fd2d42702161d21c76f0a2a135bd8d45ac796786787e268bcc7498d17eccab47ad98be601a3470618fcd9790b40e3c324fc01ec29a98d1cb784fd4390ea1b045437cfdfa121447854c5ae5375c3f721a7a9f80b7da35868e063f18c545f07d934e4cc557fc93ee52b6231c95ef2d2d1de6be82be0d76e8d555dc41cc4894e44af98c68e4245e5bb6cecc

Decoded:

 message=NDEFMessage(
    NDEFRecord(
       tnf=EXTERNAL(04),
       type=b'srs',
       id=b'',
       payload=NDEFMessage(
          NDEFRecord(
             tnf=EXTERNAL(04), 
             type=b'ses', 
             id=b'', 
             payload=[
                0x6b159a80fc8283fd,
                0x02,
                OK(01)
             ]
          ),
          NDEFRecord(
             tnf=EXTERNAL(04), 
             type=b'reb', 
             id=b'', 
             payload=0x031c5b7d2e9f0f6937a7c043dc6913dd976002ce5cf179dedbcaaab629442a89a72b6a92caa26e808903a6d02207fd2d42702161d21c76f0a2a135bd8d45ac796786787e268bcc7498d17eccab47ad98be601a3470618fcd9790b40e3c324fc01ec29a98d1cb784fd4390ea1b045437cfdfa121447854c5ae5375c3f721a7a9f80b7da35868e063f18c545f07d934e4cc557fc93ee52b6231c95ef2d2d1de6be82be0d76e8d555dc41cc4894e44af98c68e4245e5bb6cecc
          )
       )
    )
 )

GET MORE DATA

Request:

CLA	INS	P1	P2	DATA	LE
90	c0	00	00	None	00

Response:

Same format and rules as in GET DATA

PUSH DATA

This command has no effect and seems to be a leftover from unfinished/cut functionality.
Information is provided for reference only

Request:

CLA	INS	P1	P2	DATA	LE
90	52	00	00	Dynamic. Data format below.	00

Command data example:

Payload:
- d403b9737072000194030a7365736b159a80fc8283fd02011403126270729403016d6f6e0059010303546363645553441403246e73720599010703546e737402656e5041535359010c03556e7375046578616d706c652e636f6d54035f7373729403096f696404010203dc3be19e4814034373756701990116035473757402656e534552564943455f55534147455f5449544c4559011c035473756402656e534552564943455f55534147455f4445534352495054494f4e54030173757005
Decoded:

message=NDEFMessage(
   NDEFRecord(
      tnf=EXTERNAL(04),
      type=b'spr',
      id=b'',
      payload=[
         Version(00:01),
         NDEFMessage(
            NDEFRecord(
               tnf=EXTERNAL(04),
               type=b'ses',
               id=b'',
               payload=[0x6b159a80fc8283fd, 0x02, OK(01)]
            ), 
            NDEFRecord(
               tnf=EXTERNAL(04),
               type=b'bpr',
               id=b'',
               payload=NDEFMessage(
                  NDEFRecord(
                     tnf=EXTERNAL(04),
                     type=b'mon',
                     id=b'',
                     payload=0x00
                  ), 
                  NDEFRecord(
                     tnf=WELL_KNOWN(01),
                     type=b'T',
                     id=b'ccd',
                     payload=b'USD'
                  )
               ) 
            ), 
            NDEFRecord(
               tnf=EXTERNAL(04), 
               type=b'nsr', 
               id=b'', 
               payload=[
                  SIGNUP(05), 
                  NDEFMessage(
                     NDEFRecord(
                        tnf=WELL_KNOWN(01),
                        type=b'T',
                        id=b'nst',
                        payload=0x02656e50415353
                     ), 
                     NDEFRecord(
                        tnf=WELL_KNOWN(01),
                        type=b'U',
                        id=b'nsu',
                        payload=0x046578616d706c652e636f6d
                     )
                  )
               ]
            ), 
            NDEFRecord(
               tnf=EXTERNAL(04),
               type=b'ssr',
               id=b'',
               payload=NDEFMessage(
                  NDEFRecord(
                     tnf=EXTERNAL(04),
                     type=b'oid',
                     id=b'',
                     payload=[BINARY(04), 0x01, 0x02, 0x03, 0xdc, 0x3b, 0xe1, 0x9e, 0x48]
                  ), 
                  NDEFRecord(
                     tnf=EXTERNAL(04),
                     type=b'sug',
                     id=b'',
                     payload=[
                        SUCCESS(01), 
                        NDEFMessage(
                           NDEFRecord(
                              tnf=WELL_KNOWN(01),
                              type=b'T',
                              id=b'sut',
                              payload=02656e534552564943455f55534147455f5449544c45
                           ), 
                           NDEFRecord(
                              tnf=WELL_KNOWN(01),
                              type=b'T',
                              id=b'sud',
                              payload=02656e534552564943455f55534147455f4445534352495054494f4e
                           )
                        )
                     ]
                  ), 
                  NDEFRecord(
                     tnf=EXTERNAL(04),
                     type=b'sup',
                     id=b'',
                     payload=[FREE(05)]
                  )
               )
            )
         )
      ]
   )
)

Response

SW1	SW2	DATA
90	00	Dynamic. Data format below. 61 bytes long

Response data example:

Payload:
- d40310707372d4030a7365736b159a80fc8283fd0301

Decoded:

message=NDEFMessage(
   NDEFRecord(
      tnf=EXTERNAL(04),
      type=b'psr',
      id=b'',
      payload=NDEFMessage(
         NDEFRecord(
            tnf=EXTERNAL(04),
            type=b'ses',
            id=b'',
            payload=0x6b159a80fc8283fd0301
         )
      )
   )
)

Cryptography

NEGOTIATE SECURE CHANNEL Signature

During the NEGOTIATE SECURE CHANNEL command, reader has to prove to the device that it is allowed to retreive particular objects (read as passes).

To generate a proof, device uses a collector private key in order to sign following data retreived prior during communication:

Order	Name	Length	Example
1	reader_nonce	32	7131b05f5cfbd94feae19204d59d4ee5a4ce8172462e3f4577426040916e5b48
2	device_nonce	32	00f363e09bd98d971bda253bb5e001e554d5255b6adf0713c8bfc7eea4e3957f
3	collector_id	4	01020304
4	reader_ephemeral_public_key_bytes	33	03c3d36bf9509924f159e9b5f02cb3d479d2fde4dedde1a8054fd5018286b2e6f8

When concatenated, the byte string to sign would be:

data_to_sign = 7131b05f5cfbd94feae19204d59d4ee5a4ce8172462e3f4577426040916e5b4800f363e09bd98d971bda253bb5e001e554d5255b6adf0713c8bfc7eea4e3957f0102030403c3d36bf9509924f159e9b5f02cb3d479d2fde4dedde1a8054fd5018286b2e6f8`

Then, using reader long term private key (the version of which is defined inside cpr record), device generates a 72-byte long ASN1 encoded Dss-Sig-Value signature:

signature = 3046022100cc4414b542a2fc42d41a29da56e897cb38593380fe529d473f24b8c450f422c7022100f0160f981dd28ec2842f8ed5e9adc533b685258987fd602815caf88aa08f8ddd

Keep values of data_to_sign and signature in mind, as they'll be used later for session key generation.

GET DATA Encryption

To decrypt GET DATA response payload, we have to establish encryption keys.

Shared key is generated using ECDH exchange of ephemeral device public and reader private keys:

shared_key = ECDH(reader_ephemeral_private_key, device_ephemeral_public_key)

Then, using the HKDF, data is generated with following parameters

Name	Value
algorithm	SHA256
length	48
salt	device_ephemeral_public_key_bytes
shared_info	data_to_sign + signature
key_material	shared_key

As a result, we get 48 bytes of keying material derived_ephemeral_key.

First 16 bytes are used as an AES encryption key:
aes_key=derived_ephemeral_key[:16]

Next 32 bytes are used for HMAC key:
hmac_key=derived_ephemeral_key[16:]

Those keys are used to verify and decrypt GET DATA responses: Encryption/Decryption uses AES CTR algorithm.

IV is provided in first 12 bytes of payload:
iv = payload[:12]
Ciphertext is the middle bytes starting from 12 and ending with -32 from the end:
ciphertext = payload[12:-32]
HMAC is provided in last 32 bytes of payload:
hmac = payload[-32:]

HMAC has to be verified over whole IV + Ciphertext value. For decryption, IV is appended with zero bytes until it's length becomes 16.

Following Python pseudocode (with cryptography library) decribes the decryption proccess:


from cryptography.hazmat.primitives import hashes
from cryptography.hazmat.primitives.asymmetric import ec
from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
from cryptography.hazmat.primitives.hmac import HMAC
from cryptography.hazmat.primitives.kdf.hkdf import HKDF

iv = payload[:12]
ciphertext = payload[12:-32]
hmac = payload[-32:]

h = HMAC(hmac_key, hashes.SHA256())
h.update(iv + ciphertext)
hmac_to_verify = h.finalize()

cipher = Cipher(algorithms.AES(aes_key), modes.CTR(iv + (b'\x00' * (16 - len(iv)))))
decryptor = cipher.decryptor()
decrypted_data = decryptor.update(ciphertext) + decryptor.finalize()
payload = decrypted_data

Other

Compression

If ZLIB support has been toggled on in POS capabilities, inner decrypted payload will be compressed with ZLIB. To uncompress, use libraries or methods depending on your programming language of choice.

Notes

If you find any mistakes/typos or have extra information to add, feel free to raise an issue or create a pull request.
Information provided in this repository is intended for educational, research, and personal use. Its use in any other way is not encouraged.
Beware that SmartTap, just like any other proprietary technology, might be a subject to legal protections depending on jurisdiction. A mere fact of it being reverse-engineered does not always mean that it can be used in a commercial product as-is without causing an infringement. For use in commercial applications, you should contact Google through official channels in order to get approval.
Reverse-engineering efforts were started way before Google published an open-source implementation example at the end of 2022, which made this project somewhat obsolete.
There are some aspects still left uncovered (such as data format, parameters, extra commands), goal of this repo would be to describe blind spots in more detail in the near future ©, plus to provide examples, such as communication logs.

References

Google resources:
- Google Smart Tap;
- Smart Tap overview;
- Smart Tap communication flow;
- Smart Tap example project. Note that it does not implement the full protocol, for instance "Get more data" and "Push data" commands are missing;
Analysed applications:
- Google Play services;
- Google Wallet.
Software analysis tools:
- Jadx.

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Google Smart Tap

Overview

Application identifiers

Command overview

Entities, constants and data format

Data format

Record types

Records

Statuses

Terminal capabilities

Request object type

Push Data related options

Push service type

Service status usage

Service update operation

Command and response data format

Status words

SELECT VAS APPLET

Request:

Response

SELECT SMART TAP APPLET

Request:

Response

NEGOTIATE SECURE CHANNEL

Request:

Response

GET DATA

Request:

Response:

GET MORE DATA

Request:

Response:

PUSH DATA

Request:

Response

Cryptography

NEGOTIATE SECURE CHANNEL Signature

GET DATA Encryption

Other

Compression

Notes

References

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