ReFS Reference

Integrity Streams

An integrity stream is a per-file, opt-in feature that protects a file’s data with per-block checksums, so the driver can detect — and, on a mirror or parity storage space, repair — silent corruption or out-of-band tampering of file content. For an analyst the two questions that matter are: which files were checksum-protected, and where on disk that fact is recorded. Both reduce to a single bit — FILE_ATTRIBUTE_INTEGRITY_STREAM (0x8000) in the file’s Win32 attribute word — plus a per-block checksum store. This page explains the mechanism, the one authoritative on-disk marker, and the several places an analyst is tempted to look that hold no per-file integrity signal.

Per-file opt-in, not per-volume

Integrity is enabled per object, never volume-wide. A file becomes an integrity stream either at format time (format /fs:ReFS /i enable, i.e. -SetIntegrityStreams 1), per object (Set-FileIntegrity), or by inheriting the setting from its parent directory at create time. Once a file is marked, the driver keeps a checksum for every data block; on read, a stored-vs-recomputed mismatch is surfaced as corruption and is self-healed wherever a redundant copy exists.

Because the setting is inherited at create time rather than applied dynamically, the flag is a per-file historical fact, not a live volume property. Disabling a directory’s integrity does not clear the flag on files already created underneath it, so the only reliable way to know a file’s status is to read that file’s own attribute word — never infer it from the directory or the volume.

The one authoritative marker

The on-disk marker is established by the driver function RefsSetIntegrity, which sets or clears bit 15 (0x8000) at SCB+0x98 in the stream control block. That bit is then reflected into the file’s Win32 attribute word at $SI+0x20 as FILE_ATTRIBUTE_INTEGRITY_STREAM (0x8000). The attribute bit in $STANDARD_INFORMATION is the only authoritative on-disk signal that a given file is an integrity stream:

RefsSetIntegrity ──sets──▶ SCB+0x98 bit15 (0x8000)
                              │ reflected to
                              ▼
                 $SI+0x20 file_attrs |= 0x8000   ← THE marker
                 (FILE_ATTRIBUTE_INTEGRITY_STREAM)

The decisive test for “was this file checksum-protected?” is therefore $SI+0x20 & 0x8000. A file enabled this way carries the bit alongside whatever other Win32 attributes it has — an integrity-marked test file, for example, reads 0x8020 (integrity 0x8000 ORed with archive 0x20).

Where the checksum machinery lives

Two distinct stores hold the actual checksum data, and neither is the per-file roster:

  • Per-block data checksums. With integrity enabled, the driver computes CRC32 per data block — note CRC32, not the CRC64 used for metadata page references. The selected checksum type is also reflected in the file’s $DATA stream summary: the stream-flags u32 at val+0x38 uses its low byte to select the algorithm (0x02 = CRC, 0x04 = SHA-256), with bit 0x10000 = integrity. This selector tracks the volume’s checksum configuration (0x02 on None/CRC64 volumes, 0x04 on SHA-256 volumes), so it follows the format choice rather than identifying individual files — see $DATA for the stream-summary layout.
  • Integrity State Table (root #11, schema 0xe080). A volume-level B+-tree (CmsIntegrityState) that tracks integrity-stream coverage by LCN range, reusing the same row format as the allocator tables. It is present on every volume regardless of whether any file uses integrity streams, and on baseline images carries exactly one row spanning the whole volume. See Integrity State Table for the byte layout. It is not a per-file lookup — per-file status lives only in the 0x8000 attribute bit.

The block checksums themselves are reached through the ordinary page-reference / Merkle machinery, but note the orthogonality: the page-reference checksum-type byte (cktype 0x01/0x02/0x04) governs metadata verification and is entirely separate from whether a file’s data is an integrity stream. See Checksum Architecture for the metadata side of the same coin.

Two places that hold no per-file integrity signal

Both of the structures below look like plausible per-file integrity rosters and are not.

  • $SI+0x24 (internal flags) carries no integrity bit. Its bit 0 (0x01) is the delete-disposition / EFS transient state, derived from FCB+0x08 bit 27 (set in the driver’s DeleteDirectoryOnDisk path). It is not an integrity flag. The proof that the integrity bit cannot appear in this field is structural — RefsComputeStandardInformationInternalFromFcb builds $SI+0x24 from FCB+0x08 bits only and never reads SCB+0x98, where the integrity bit lives. On disk, an integrity-enabled volume with ~6,000 integrity files shows that bit set on zero of them. Use $SI+0x20 bit 0x8000, never $SI+0x24.
  • The Integrity State Table is not a per-file roster. Root #11 is volume-level and invariant — the same single whole-volume row whether or not any file is an integrity stream, and across CRC64 versus SHA-256 metadata configurations. Reading it expecting a list of protected files yields nothing.

Forensic consequences

The integrity marker hands the analyst two distinct capabilities. First, it is the authoritative per-file answer to “was this checksum-protected?” — a single bit, read per file. Second, because an integrity stream carries a per-block CRC32 over its content, a stored-vs-recomputed mismatch on an integrity-marked file is strong evidence the data was altered out-of-band: an offline edit, bit-rot, or a write that bypassed the driver. On a non-integrity file there is no per-block data checksum at all, so content tampering leaves no equivalent on-disk trace — the presence of the 0x8000 marker is what makes after-the-fact tamper detection possible in the first place.

Version and state differences

The FILE_ATTRIBUTE_INTEGRITY_STREAM (0x8000) marker and the Integrity State Table (root #11) are present across all studied versions, from Win10 v3.4 through Insider. The marker test is therefore version-agnostic.

By contrast, the $SI+0x24 internal-flags word — the field one might wrongly key an integrity test on — is zero on v3.4 through v3.10 and only becomes populated on v3.14+, so a test built on it would produce no signal at all on older volumes regardless of correctness. The $SI+0x20 bit 0x8000 marker works on every version.

Finally, the per-file data-block integrity checksum is CRC32 and is independent of the volume metadata checksum type (None / CRC64 / SHA-256). The metadata type is fixed at format and reported through VBR offset 0x2A and the checkpoint flags (see Checksum Architecture); the per-file data CRC32 is a separate mechanism layered on top.

Reading the marker with the tools

The integrity marker is read directly from the $SI attribute word surfaced by the file-enumeration path of forefst.py / refsanalysis.py — test file_attrs & 0x8000. Do not confuse this with forefst.py integrity --checksums, which exercises the page-reference metadata checksum machinery (CRC64, not the per-file data CRC32) and verifies root-table metadata — a separate mechanism from per-file integrity streams.

Cross-references

  • Integrity State Table — root #11 byte layout, the volume-level tracker (not a per-file roster)
  • $STANDARD_INFORMATION — the $SI+0x20 attribute word holding the 0x8000 marker, and the corrected $SI+0x24 internal flags that do not
  • $DATA — the stream summary whose val+0x38 flags carry the checksum-type / integrity selector
  • Checksum Architecture — the metadata CRC64 / SHA-256 page references, distinct from per-file data CRC32
  • Page References — the Merkle-tree checksum carriers and the cktype byte
  • Allocators — the row format the Integrity State Table reuses
  • Redundancy — the mirror / parity self-healing that integrity checksums enable

Evidence

The marker mechanism is confirmed in the driver: RefsSetIntegrity sets/clears SCB+0x98 bit 0x8000 and reflects it to $SI+0x20, and RefsComputeStandardInformationInternalFromFcb builds $SI+0x24 from FCB+0x08 bits only — the static proof that integrity can never appear in the internal-flags field. The CmsIntegrityState class backs the Integrity State Table: Initialize reads root #11 at mount, SetClearIntegrityState sets/clears a stream’s state, and GetIntegrityStateTable / GetRangeBitmap read the per-range checksum bitmap. On the raw-disk corpus, the 0x8000 attribute bit was set on every integrity file across the two v3.14 integrity images (the only integrity images in the corpus), while the $SI+0x24 bit 0 was set on none of them, and the Integrity State Table’s single whole-volume row was invariant across checksum configurations.