Backend server for Secure Direct Messaging (SDM)

An example of Flask application which can decrypt data contained in NDEF "mirrors" and validate their AES-CMAC cryptographic signature. Implemented according to AN12196 "NTAG 424 DNA and NTAG 424 DNA TagTamper features and hints".

Pull requests welcome.

Note: NTAG — is a trademark of NXP B.V.

Note: This GitHub project is not affiliated with NXP B.V. in any way. Product names are mentioned here in order to inform about compatibility.

How to test?

Manual installation

1. Clone the repository

   apt install -y git
   git clone https://github.com/icedevml/sdm-backend.git
   cd sdm-backend
   

2. Setup the virtualenv

   apt install -y python3 python3-pip python3-venv
   python3 -m venv venv
   source venv/bin/activate
   

3. Install the required dependencies and copy example config:

   pip3 install -r requirements.txt
   cp config.dist.py config.py
   

4. Run Flask development server:

   python3 app.py --host 0.0.0.0 --port 5000
   

5. Visit localhost:5000 and check out the examples.

Using Docker

1. Run

   docker run -p 5000:80 icedevml/sdm-backend
   

2. Visit localhost:5000 and check out the examples.

How to setup SDM?

Use NXP's TagWriter application for Android. When writing an URL record, choose "Configure mirroring options". Refer to the tag's datasheet to understand particular options/flags.

Supported cases

PICCData Encrypted mirroring (CMACInputOffset == CMACOffset)

Example:

http://myserver.example/tag?picc_data=EF963FF7828658A599F3041510671E88&cmac=94EED9EE65337086

Proposed SDM Settings for TagWriter:

• Enable SDM Mirroring (SDM Meta Read Access Right: 00)
• Enable UID Mirroring
• Enable Counter Mirroring (SDM Counter Retrieval Key: 00)
• Enable Read Counter Limit
• Enable Encrypted File Data Mirroring

Input URL:

http://myserver.example/tag?picc_data=00000000000000000000000000000000&cmac=0000000000000000

PICCDataOffset:

http://myserver.example/tag?picc_data=00000000000000000000000000000000&cmac=0000000000000000
                                      ^ PICCDataOffset

i.e.: in TagWriter, set the cursor between = and 0 when setting offset.

SDMMACInputOffset/SDMMACOffset:

http://myserver.example/tag?picc_data=00000000000000000000000000000000&cmac=0000000000000000
                                                                            ^ SDMMACInputOffset/SDMMACOffset

SDMENCFileData mirror with PICCData Encrypted mirroring (must satisfy: CMACInputOffset != CMACOffset && SDMMACInputOffset == ENCDataOffset)

Example:

http://myserver.example/tag?picc_data=FD91EC264309878BE6345CBE53BADF40&enc=CEE9A53E3E463EF1F459635736738962&cmac=ECC1E7F6C6C73BF6

Proposed SDM Settings for TagWriter:

• Enable SDM Mirroring (SDM Meta Read Access Right: 00)
• Enable UID Mirroring
• Enable Counter Mirroring (SDM Counter Retrieval Key: 00)
• Enable Read Counter Limit
• Enable Encrypted File Data Mirroring (Encryption data Length: 32)
• PICC Data Offset: set cursor between picc_data= and 0 then click Set PICC Data Offset
• Enc Data Offset: set cursor between enc= and 0 then click Set Enc File Mirroring Offset
• SDM MAC Input Offset: set cursor between enc= and 0 then click Set Offset (upper button)
• SDM MAC Offset: set cursor between cmac= and 0 then click Set Offset (lower button)

TagTamper Status mirror

In this case, TT Status Offset should be equal Enc Data Offset.

Example:

http://myserver.example/tagtt?picc_data=FDD387BF32A33A7C40CF259675B3A1E2&enc=EA050C282D8E9043E28F7A171464D697&cmac=758110182134ECE9

First two letters of File data (UTF-8) will describe TagTamper Status (C - loop closed, O - loop open, I - TagTamper not enabled yet).

Further usage

1. Edit config.py to adjust the decryption keys.
2. Setup nginx (with obligatory SSL encryption).
3. Configure the application to run with uwsgi (example tutorial).

Dealing with LRP cipher

In general, SDMs generated with LRP cipher are not supported by this code. See icedevml/ntag424-ev2-crypto for the implementation of LRP primitive. In test_lrp_sdm.py file, there is a short example of SDM message decryption with LRP primitives.

Contact

Feel free to reach me at ml@icedev.pl if you have any questions concerning this topic.