Block based hash optimized for disk images.

Disk images are typically sparse, containing unallocated areas read as zeros by the guest. The blkhash hash construction is optimized for computing checksums of sparse disk images.

This project provides the blksum command and the blkhash C library.

You can install the blkhash package on Fedora, Centos Stream, and RHEL from the blkhash copr repo:

dnf copr enable nsoffer/blkhash
dnf install blkash

This installs the blksum command and the libblkhash library.

If you want to use the libblkhash library please install the blkhash-devel package. Your application will depend on the blkhash-libs package.

The blksum command computes message digest for disk images, similar to standard tools like sha256sum.

Why use blksum?

• Much faster - up to 4 orders of magnitude faster compared with common tools such as sha256sum.
• Support qcow2 and raw image formats - images with same content but different format have the same checksum.
• Support multiple sources - file, block device, pipe, or NBD URL
• Configurable digest algorithms - using BLAKE3, SHA256 or other digest algorithms provided by openssl

Here is a graph showing blksum throughput in GiB/s using SHA256 digest algorithm:

• See blksum to learn more about the blksum command and how to use it.
• See blksum performance to learn more about blksum performance.

The blkhash C library implements the block based hash construction, using zero detection and multiple threads to speed up the computation.

The library provides 2 APIs:

• The simple API - easy to use API similar to other hashing libraries like openssl
• The async API - high performance API for very fast storage and large number of threads

Here is a graph showing blkhash throughput in GiB/s using SHA256 digest algorithm:

• See blkhash to learn more about the blkhash library and how to use it.
• See blkhash performance to learn more about blkhash performance.

For in-depth discussion of the blkhash construction and proof of security, please check the blkhash paper: Soffer, N. and Waisbard, E. (2024). An Efficient Hash Function Construction for Sparse Data. In Proceedings of the 21st International Conference on Security and Cryptography - SECRYPT; ISBN 978-989-758-709-2; ISSN 2184-7711, SciTePress, pages 698-703. DOI: 10.5220/0012764500003767.

The blkhash library and blksum command are developed on Linux, but should be portable to any platform where openssl is available. Some optimizations are implemented only for Linux.

The blkhash library requires:

• openssl for SHA256 and other digest algorithms
• blake3 for BLAKE3 digest algorithm

The blksum command requires:

• libnbd for NBD support
• qemu-nbd for qcow2 format support

See portability for more info.

Contribution is welcome!

We need help with:

• Testing with different storage, machines, and systems
• Reporting issues or ideas for new features
• Improving the documentation
• Packaging for more systems
• Porting to more systems
• Implementing new features or fixing bugs
• Integration with other projects

Please see CONTRIBUTING.md to learn how you can contribute to the project.

• The blkhash construction is based on ovirt-imageio blkhash module.

• The blksum command NBD support is powered by the libnbd library.

blkhashis licensed under the GNU Lesser General Public License version 2.1 or later. See the file LICENSES/LGPL-2.1-or-later.txt for details.