Making sense of EMV card data – how to decode the TLV data format

At the Payment Village in DEFCON 28, I presented a talk about my research in payment system security. While my talks have in the past covered high-level issues or particular security vulnerabilities, for this presentation, I went into depth about the TLV (tag-length-value) data format that anyone researching payment security is going to have to deal with. This format is used for Chip and PIN cards, as specified by the EMV standard, and is present in related standards like contactless and mobile payments. The TLV format used in EMV is also closely related to the ASN.1 format used in HTTPS certificates. There are automated decoders for TLV (the one I wrote is available on EMVLab), but for the purposes of debugging, testing and handling corrupt or incomplete data, it’s sometimes necessary to get your hands dirty and understand the format yourself. In this talk, I show how this can be done.

Rather than the usual PowerPoint, I tried something different for this talk. The slides are an interactive RISE show based on a Juptyer notebook, demonstrating a Python library I wrote to show TLV data-structure decoding. Everything is in my talk’s GitHub repository, and you can experiment with the notebook and view the slides without installing any software through its Binder. I have an accompanying Sway notebook with the reference guides I relied upon for the talk. Do have a try with this material, and I’d welcome your comments on how well (or badly) this approach works.

The DEFCON Payment Village is running again this year in August. If you’ve got something you would like to share with the community, the call for papers is open until 15 July 2021.

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