ntcompress

Pure-Python, standard-library-only implementations of every compression format used by Microsoft's Extensible Storage Engine (ESE/ESENT, the "JET Blue" engine behind NTDS.dit, Exchange stores, Windows.edb, SRUDB.dat, and WebCacheV01.dat) and Windows ntdll.dll compression (RtlCompressBuffer / RtlDecompressBuffer). No runtime dependencies beyond the standard library.

Scope

This library covers two distinct compression layers used by Windows:

ESE record compression (vs esent.dll)

ESE stores a one-byte header on each compressed record cell; the top 5 bits select the scheme (first_byte >> 3) and the low 3 bits carry scheme-specific flags.

ESE ID	Format	Decompress	Compress	Byte-Identical to esent.dll	Authority
0x0	NONE (uncompressed sentinel)	—	—	—	compression.cxx:504
0x1	7-bit ASCII	yes	yes	yes	ESE source
0x2	7-bit Unicode	yes	yes	yes	ESE source
0x3	XPRESS (Plain LZ77)	yes	yes	yes	[MS-XCA] §2.3/2.4
0x4	SCRUB (erase marker)	produce	recognize	n/a	ESE source
0x5	XPRESS9	yes	yes	yes*	ESE MIT source (ported)
0x6	XPRESS10 (LZ4 + CRC)	yes	yes	yes**	ESE source
0x7	LZ4 (standard block)	yes	yes	yes	LZ4 block format

*XPRESS9 encoder matches the MIT ESE C reference byte-for-byte excluding the non-deterministic session signature (CRC32(__rdtsc())), which even esent.dll varies on every call. **XPRESS10 verified byte-identical by component construction (LZ4 block + CRC-32C + CRC-64/NVME each individually proven); not triggerable on commodity hardware (requires Corsica/QAT flight flag).

ntdll compression (vs RtlCompressBuffer)

Windows ntdll.dll exposes a separate set of CompressionFormatAndEngine IDs used by SMB, NTFS, RDP, and other protocols. These use raw bitstreams with no ESE framing.

ntdll ID	Name	Decompress	Compress	Byte-Identical to ntdll	Builds
0x0002	LZNT1	yes	yes	yes (ENGINE_MAX)	XP SP3 – Server 2025
0x0003	XPRESS (Plain LZ77)	yes	yes	yes	Win8.1 – Server 2025
0x0004	XPRESS_HUFF (LZ77+Huffman)	yes	yes	yes (default engine)	Win8.1 – Server 2025
0x0005	undocumented	—	—	—	Server 2022+
0x0006	XP10 (LZ4 block)	yes	yes	yes	Win11 / Server 2025
0x0007	raw DEFLATE	yes	yes	yes (both levels)	Win11 / Server 2025
0x0008	ZLIB	yes	yes	yes (both levels)	Win11 / Server 2025

ntdll format 0x0005 is an undocumented format (block magic 0xC039E510) unrelated to XPRESS9 despite sharing the slot number. No public specification or source code exists. Format 0x0006 (XP10) is the same LZ4 block codec used by ESE scheme 0x6 and 0x7, without the ESE header.

Design principles

• Standard library only at runtime. No third-party dependencies. zlib/lz4 are deliberately not used; the CRCs and the LZ4 block codec are implemented directly.
• Two subpackages, one interface pattern. ntcompress.ese for ESE record-compression, ntcompress.ntdll for ntdll standalone codecs. Both offer Shape A (enum dispatch) and Shape B (direct module import) APIs.
• Traceable to a spec or a source line. Every wire constant cites [MS-XCA] §x.y or an ESE file:line. Where the shipped C diverges from the spec text, both are documented.
• Byte-identical verification. Compress output is verified against gold vectors captured from esent.dll and ntdll.dll on 16 Windows builds (XP SP3 through Server 2025).
• Typed and documented. Python 3.11+, full type hints, Google-style docstrings, ruff-clean under select = ["ALL"], ty-clean.

Installation

Install from PyPI:

uv add ntcompress        # recommended
pip install ntcompress    # or with pip

Or install from source:

uv add git+https://github.com/StrongWind1/ntcompress

Requires Python >= 3.11.

Usage

ESE record compression

import ntcompress.ese as ese

# Decompress a cell (format is in byte 0)
plaintext = ese.decompress(cell)
size = ese.decompressed_size(cell)

# Compress with a specific format
cell = ese.compress(plaintext, ese.Format.XPRESS)
cell = ese.compress(plaintext, ese.Format.LZ4)
cell = ese.compress(plaintext, ese.Format.XPRESS9)

# Or use Shape B (direct module access)
from ntcompress.ese import xpress
cell = xpress.compress(plaintext)
plaintext = xpress.decompress(cell)

ntdll standalone codecs

import ntcompress.ntdll as ntdll

# Shape A (enum dispatch)
compressed = ntdll.compress(data, ntdll.Format.LZNT1)
plaintext = ntdll.decompress(compressed, ntdll.Format.LZNT1)

# Windows constant aliases
compressed = ntdll.compress(data, ntdll.COMPRESSION_FORMAT_XPRESS_HUFF)

# Shape B (direct module access)
from ntcompress.ntdll import lznt1, xpress_huff, deflate
from ntcompress.ntdll import zlib as ntdll_zlib

plaintext = lznt1.decompress(compressed_stream)
compressed = xpress_huff.compress(data)
compressed = deflate.compress(data)            # matches ntdll 0x0007
compressed = ntdll_zlib.compress(data)         # matches ntdll 0x0008

License and attribution

Apache-2.0 (LICENSE). The XPRESS9 codec is an attributed port of Microsoft's MIT-licensed ESE source; the Plain LZ77, Xpress Huffman, and LZNT1 codecs are derived from the published [MS-XCA] specification; the LZ4 block codec follows the public LZ4 format. Full notices are in THIRD-PARTY-NOTICES.md.