gauss-key-card Open source Java Card applet th @codeKK AndroidOpen Source Website

gauss-key-card

Introduction: Open source Java Card applet that allows you to unlock, start, or lock a Tesla Model 3 just like the official key card

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Gauss Key Card is a Java Card applet that implements the minimal working subset of the Tesla Key Card Protocol. Supported Java Card implementations that load this application will be able to be paired with a compatible vehicle and subsequently unlock, start, or lock the vehicle in the same way you would with an official key card. (video)

If at this point you can't imagine why you might ever want to use this applet, then it is not for you.

Caveats

THIS APPLET IS INTENDED FOR EXPERIMENTAL USE ONLY. IT HAS NOT BEEN REVIEWED, CHECKED, APPROVED, OR ENDORSED BY TESLA. IT MAY BE BLOCKED BY TESLA AT ANY TIME.

BY USING THIS APPLET (OR INSTALLING IT TO A CARD FOR SOMEONE ELSE TO USE) YOU ASSUME ALL RESPONSIBILITY, RISK, AND LIABILITY FOR THE USE OR MISUSE OF THIS SOFTWARE. SEE LICENSE AGREEMENT FOR MORE INFORMATION.

There are some serious caveats that need to be well understood before using this applet:

Type-A vs. Type-B

ISO/IEC 14443 defines two types of contactless devices: Type-A and Type-B. There are javacards available in both varieties, and this applet will blissfully allow itself to be installed onto either type. However, Tesla vehicles currently ignore type-B cards, so make sure that your card is indeed a 14443 type-A card.

Maximum Frame Size

The maximum frame size (FSC) for the card must be 96 bytes or greater. This corresponds to a FSCI value of 6 or greater. Any card with an FSCI of 5 or less will not work.

Tesla vehicles will ignore the FSCI field of the ATS, which means that it will not attempt to break up larger frames if the indicated FSCI is small (<6). Specifically, the card MUST be able to properly handle receiving the authenticate APDU (86 bytes) in a single frame. If a card advertises an FSCI smaller than 6 then it is unlikely to be able to satisfy this requirement.

For example, smart cards with DESFire EV1 emulation support have an FSCI of 5, and will unfortunately choke if they receive a frame larger than 64 bytes. Such cards are not able to be used as Tesla Key Cards.

NOTE: In earlier versions of this document, this behavior was confused with the NFC UID length. It just happened to be the case that most of the 4-byte UID cards the author tested also had an FCI of 5. There is no limitation of the length of the UID on the card imposed by the vehicle.

A list of tested cards can be found here.

Software updates could make this applet useless

Official Tesla Key Cards have an attestation certificate, which means that Tesla could easily block non-Tesla key cards from pairing with vehicles at any point in the future via a software update. Keys already paired with a vehicle might continue to work in such a scenario, but there are no guarantees.

Since there is no publicly available comprehensive documentation for the Tesla Key Card Protocol, what is known was determined through observing the interactions between the vehicle and an official Tesla Key Card. Gauss Key Card supports all commands that are currently being used by the vehicle during pairing and authentication, but there are several additional commands that are not supported because there was not enough context to infer what their purpose might be. If Tesla at some point enables additional functionality or features which require those commands, existing Gauss Key Cards would be unable to participate.

Updating GaussKeyCard Applet Will Break Pairing

If you decide to upgrade the version of GaussKeyCard on your card to a new version, the pairing will be broken because a new internal private key will be generated for the card.

Security

To increase the difficulty of cloning, the private value of the ECDH key pair is generated on-card when the applet is installed and never leaves the secure element. No mechanism for externally supplying a private key with a known value is provided by this implementation.

Avoid buying Tesla Key Cards from anyone other than Tesla---the card may be have weak or known private keys. Authentic Tesla Key Cards should be safe to buy or use from anyone, but until the vehicle actually verifies attestation certificates there is no way to be sure. Until then you should always assume that whoever sold you a key card might also have the ability to unlock and start any vehicle you subsequently pair with the card.