QRDM Threat Model and Security Guarantees

This document outlines the security properties, guarantees, and explicit non-guarantees of the QR Data Manager (QRDM).

What QRDM Guarantees

  1. Integrity and Corruption Detection

    • QRDM calculates a full SHA-256 cryptographic digest of the canonical source document and embeds it into every QR code’s metadata header.

    • The SHA-256 hash is printed in cleartext on the page header so inspectors can visually compare it against out-of-band hashes.

    • Decoders verify the full 256-bit hash upon reconstruction. If even a single byte differs or an uncorrectable corruption occurs, reconstruction is rejected.

  2. Resilience to Physical Degradation and Scan Loss

    • QRDM employs Reed-Solomon erasure coding (~20% redundancy by default). If up to 20% of QR codes are completely obscured, stained, cut off, or fail to scan, the document is mathematically reconstructed without data loss.

    • Each QR code also leverages standard QR-level Reed-Solomon error correction (Medium / Quartile) against surface scratches.

  3. Opt-In Privacy Defaults

    • By default (include_text=False), cleartext source content is never printed under QR codes.

    • Minimal-page mode (minimal_mode=True) strips timestamps, source metadata, and verbose captions, printing only the filename, truncated SHA-256 hash, and page numbers.

  4. Structured Verification (qrdm verify)

    • The verification subsystem provides non-destructive inspection without writing recovered payloads to disk.

    • Named validation reports indicate whether payloads decoded cleanly, hashes match, sequence counts are complete, and whether unexpected or foreign barcodes appear on the page.

  5. Strict Mode Enforcement

    • When strict mode is enabled, QRDM rejects payloads containing NUL bytes (0x00), payloads exceeding maximum byte limits, and documents matching binary signatures (such as ZIP, DOCX/XLSX/PPTX, OLE2, ELF, or Windows PE executables).

What QRDM Does NOT Guarantee

[!WARNING] QRDM DOES NOT ENCRYPT DATA. QRDM compresses data using zlib and serializes it using Protocol Buffers and Base85. This is encoding, not encryption. Any person, phone camera, or barcode reader can read the content.

If the data is confidential, encrypt the data before passing it to QRDM (for example, with GPG, age, or AES-GCM).

  1. No Cryptographic Authenticity or Non-Repudiation

    • QRDM verifies that the document matches the hash recorded in the QR metadata. However, QRDM does not cryptographically sign documents with a private PKI key.

    • An adversary with access to the source file can generate a new, valid QRDM PDF for a modified document with a valid corresponding SHA-256 hash. If tamper-evident provenance is needed, sign the payload cryptographically before encoding or sign the resulting PDF.

  2. Downstream Payload Safety

    • QRDM reconstructs text documents faithfully. It does not sanitize content against application-level vulnerabilities such as CSV formula injection (=cmd|...), SQL injection, or shell script injection.

    • Downstream consumers must apply standard defensive parsing to decoded content.

  3. Extreme Degradation Boundaries

    • While Reed-Solomon can recover from lost codes, decoding requires at least total_codes - num_ecc codes to be successfully read by the underlying scanner. Severe physical destruction beyond the configured error-correction budget will prevent document recovery.