DE55 carries chip data as BER-TLV (Tag-Length-Value), a sequence of tags. The most important ones:
| Tag | Meaning |
|---|---|
9F26 | Application Cryptogram (ARQC/TC/AAC) |
9F27 | Cryptogram Information Data (CID) |
9F36 | Application Transaction Counter (ATC) |
95 | Terminal Verification Results (TVR) |
9F34 | CVM Results |
82 | Application Interchange Profile (AIP) |
The parser tool on this site already decodes DE55 tag by tag and bit-decodes TVR (95), AIP (82), CID (9F27) and CVM (9F34) — the harder part is the logic behind those bits.
Using a key known only to the card and the issuer, the card computes a cryptographic MAC over key transaction data (amount, currency, unpredictable number, ATC, TVR …) to produce the ARQC. The issuer recomputes it with the same key to verify — a match is required for approval. Because it includes the ATC (incrementing) and a random number, every cryptogram is unique and cannot be replayed.
Decode the individual tags as you read: AIP 82 · TSI 9B · CID 9F27 · ATC 9F36 · IAD 9F10 · CVM List 8E
The rest covers: every TVR bit and "why it went online", how the CVM list picks the verification method (PIN/signature/no-CVM), offline data authentication (SDA/DDA/CDA), issuer scripts 71/72, and a full DE55 walkthrough on a real message.
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