Updates to WIM File Documentation and SHA-1 Usage in Lookup Tables

Question

Hello,

I have been reviewing the documentation related to the Windows Imaging Format (WIM) file and its internal structure, particularly the use of SHA-1 in the lookup table. I have a few questions that I’d like clarification on:

1. Are there any updates to the official documentation regarding WIM files, specifically about the internal workings of the lookup table and hashing algorithms? refer documentation - https://www.microsoft.com/en-us/download/details.aspx?id=13096
2. Could you elaborate on the significance of SHA-1 within the lookup table, such as why it was chosen and how it impacts ?
3. Is there support for any other hashing algorithms (e.g., SHA-256 or similar) in WIM files, and if so, is this documented anywhere for developers?

Looking forward to your insights.

Answer 1

Dear Muhammed Aslam2

Thank you for posting in the Microsoft community.

Updates and Answers on WIM Files and the Use of SHA-1 in Access Forms

1. Official Documentation Update on WIM Files The official WIM file format document you referenced (Download Windows Imaging File Format (WIM) from Official Microsoft Download Center is still the primary reference for detailing the structure of WIM files to date.

- Update: While this document provides a technical specification of the Windows image format, Microsoft does not provide a clear indication as to whether WIM file hash algorithms (such as SHA-1) have been superseded or new hash algorithms are supported.

- Specific Updates: Microsoft typically makes adjustments to the features supported by WIM files in version updates to the Windows Assessment and Deployment Kit (ADK). If new hash support has been added to a WIM file or the original structure has been changed, you can find the information in the release notes of the corresponding ADK version. Example: Windows ADK latest version.Download and install the Windows ADK | Microsoft Learn

- Existing Limitations: SHA-1 hashing is still a core component of WIM files, and there is no official statement of widespread support for SHA-256 or other hashing algorithms by Microsoft at this time.

About the Role of the SHA-1 Hash in WIM

SHA-1 has the following key roles in WIM files:

1. Performance and Deduplication.

WIM files are designed as a file storage format with deduplication.

The primary usage scenario for SHA-1 hashes in the WIM file format is to generate unique identifiers (hashes) for each block of file content to be used as keys in lookup tables. In a deduplication context, the role of SHA-1 is to:

Determine the uniqueness of each file content block.

Quickly retrieve existing blocks in the lookup table to avoid storing the same data over and over again.

SHA-1 is faster to compute than alternative algorithms of similar strength (e.g., SHA-256) and can be used to meet the performance requirements of deduplication according to the design goals.

2. Importance of lookup tables.

The lookup table of a WIM file is indexed with hash values, allowing WIM files to have efficient compression and decompression of file contents.

The SHA-1 hash value of each data chunk (“chunk”) is stored in the lookup table to record the uniqueness of the chunk and to support fast retrieval.

3. Historical background on the choice of SHA-1.

Standard at the time: WIM was originally implemented in the Windows Vista environment where SHA-1 was the industry default hash algorithm commonly used based on performance and security tradeoffs. o Impact: SHA-1 is now the industry default hash algorithm in Windows Vista.

Impact: SHA-1 is primarily used in WIM documents for hash lookups rather than cryptographic signature scenarios; therefore, its security standards, while widely discredited, still meet the design requirements for a small number of content validation uses where the risk of conflict is acceptable.

4.Security Risks and Implications.

With SHA-1 currently considered insufficiently secure (Google publicly demonstrated methods capable of finding SHA-1 hash conflicts in 2017), it is being phased out in cryptographic strength-related scenarios.

However, within WIM files, the primary role of SHA-1 is for deduplication optimization. Despite the potential for conflicts, it does not significantly affect file consistency in general WIM application scenarios.

Whether other hash algorithms are supported (e.g. SHA-256)

As of today, the following information is available about WIM files in terms of hash algorithm support:

1. Current status of SHA-256 support in WIM.

File-level validation: Windows Imaging and Configuration Designer (Windows ICD) and DISM tools used to process images currently still use SHA-1 internally for the default WIM format, and do not provide direct SHA-256 support.

If you wish to use a stronger checksum algorithm (e.g. SHA-256) to verify the integrity of the WIM file, this can be done with additional external tools (e.g. Verify-Signature or a custom tool), rather than relying on the format of the WIM file itself.

2. Reasons for this limitation.

The WIM file format has been designed to be mature, with strong coupling to SHA-1 built in. Adding support for new hashing algorithms while maintaining backward compatibility with all Windows support tools (e.g., DISM, Windows PE) would require significant adjustments to the tool chain and file format.

3. Potential Directions.

Microsoft is more likely to encourage developers to use the .esd file format (electronic software download image) to enhance data integrity than to support SHA-256 directly inside WIM files.

.esd is a variant of WIM designed for installation service optimization, and may also support modern cryptographic hashes more broadly in the future.

Possible alternatives

If you wish to enhance the hashing functionality of your WIM files for security or deduplication lookup, you can consider some of the following alternatives:

1. External validation.

After generating a WIM file, use a tool such as sha256sum or similar to independently generate SHA-256 checksums for the file contents and store them in an externally verified file.

2. Convert to ESD image.

If higher data integrity is required, you can use a DISM tool to convert the WIM to ESD format. Example:

dism /Export-Image /SourceImageFile:install.wim /SourceIndex:1 /DestinationImageFile:install.esd /Compress:recovery

Best Wish

Shawn.Z-MSFT | Microsoft Community Support Specialist