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6. Log Chain Model

Purpose​

The log chain provides a tamper-evident record of all value changes on a card. Each entry is cryptographically linked to the entry before it, anchored at session start, so any modification to any entry — or its position in the sequence — breaks the chain and is detectable on the next read.

Chaining model​

  • Each log entry includes a hash computed over its own data and the hash of the previous entry.
  • The chain starts at session open time, providing an anchor that cannot be forged without knowing the session key.
  • The trailer stores the hash of the most recent entry (the chain head). This binds the full log sequence to the HMAC-protected trailer.

Security guarantees​

  • Modifying a single log entry invalidates its hash and every hash after it.
  • Partial tampering is detectable — the chain does not need to be fully traversed to find inconsistency.
  • The rootHash in the trailer ties the log chain to the overall card authentication.

Exact hash function, field layout, and chain anchor computation: Tech Specs §14 Transaction Log Format and Tech Specs §5 Tamper Detection & Validation.