{
  "id": 14023496,
  "title": "512 bytes at a time: reconstructing IBM i save-file checksums",
  "url": "https://urgent.news/2026/10/12/512-bytes-at-a-time-reconstructing-ibm-i-save-file-checksums",
  "topic": "tech",
  "section": "Tech",
  "published": "2026-10-12T18:12:46.000Z",
  "source": {
    "name": "Lobsters",
    "slug": "lobsters",
    "url": "https://blog.silentsignal.eu/2026/10/06/512-bytes-at-a-time-reconstructing-IBM-i-save-file-checksums/"
  },
  "original_language": "en",
  "account": "To reconstruct IBM i save-file checksums, one must consider two container-consistency checksum mechanisms used by IBM i save files, also known as SAVFs. Each 512-byte payload within a SAVF has a 16-byte trailer, and the logical stream carries a 16-byte global checksum in its final payload. Both mechanisms were reconstructed, modified, and repaired by creating a small SAVF containing a disposable program named USERB1/CALLAES. The original SAVF was transferred as a binary file for inspection byte by byte. After modifying three payload bytes and a stale global checksum, repairing the container changed only the global-checksum field and the final record trailer. The global algorithm's table and constants were found in IBM i Licensed Internal Code (LIC) installation media. The reconstructed format allowed removing one complete object from a two-object SAVF. The SAVF's deterministic, unkeyed checksum values provide internal consistency checks but do not authenticate the source or prevent modification.",
  "summary": null,
  "key_points": [],
  "editors_take": null,
  "illustration": null,
  "coverage": {
    "outlets": 1,
    "also_reported_by": []
  },
  "ai_generated": true,
  "disclaimer": "Summaries, key points and the editor’s take are written by software from other outlets’ reporting and may contain errors — always check the linked original."
}