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14. Transaction Log Structure

Design​

The card stores a fixed-capacity ring buffer of transaction log entries. Storage is constrained by card capacity; the buffer holds approximately 7 entries on NTAG215.

Each entry records:

  • The time elapsed since session start (as a delta, not an absolute timestamp)
  • The transaction amount
  • The balance after the transaction
  • The transaction type and operational flags
  • A truncated hash linking this entry to the previous one

Ring buffer behaviour​

  • When the buffer is full, the oldest entry is overwritten.
  • The chain continues from the surviving entries — older entries that have been overwritten are no longer individually verifiable, but the surviving chain is still intact.
  • The trailer rootHash always points to the hash of the most recent entry (the chain head).

Security guarantees​

  • Any modification to a surviving entry invalidates its hash and all subsequent hashes.
  • Overwriting the oldest slot is only valid when the ring buffer advances normally — an out-of-order overwrite would break the chain.

Exact field sizes, flag definitions, and chain anchor computation: Tech Specs §14 Transaction Log Format.