Venti (software)

Venti is a network storage system that permanently stores data blocks. A 160-bit SHA-1 hash of the data (called score by Venti) acts as the address of the data. This enforces a write-once policy since no other data block can be found with the same address: the addresses of multiple writes of the same data are identical, so it is highly likely that duplicate data is easily identified and the data block is stored only once. Data blocks cannot be removed, making it ideal for permanent or backup storage. Venti is typically used with Fossil to provide a file system with permanent snapshots.

Venti is a network storage system that permanently stores data blocks. A 160-bit SHA-1 hash of the data (called score by Venti) acts as the address of the data. This enforces a write-once policy since no other data block can be found with the same address: the addresses of multiple writes of the same data are identical, so it is highly likely that duplicate data is easily identified and the data block is stored only once. Data blocks cannot be removed, making it ideal for permanent or backup storage. Venti is typically used with Fossil to provide a file system with permanent snapshots. == History== Venti was designed and implemented by Sean Quinlan and Sean Dorward at Bell Labs. It appeared in the Plan 9 distribution in 2002. Development has been continued by Russ Cox who has reimplemented most of the server, written a library for creating datastructures (files, directories and meta-data) to store in Venti and implemented optimizations. Venti is available both in the Plan 9 distribution and for many Unix-like operating systems as part of Plan 9 from User Space. Venti is included as part of Inferno with accompanying modules for access. There is a Go set of programs to build your own Venti servers. Included are examples using different kinds of backend storage. == Hash collisions== A basic principle of information theory is the pigeonhole principle, which states that if set A contains more values than set B, then for any function that maps A to B there will be members of B that are associated with more than one member of set A. In the case of Venti, the set of possible SHA-1 hashes is obviously smaller than the set of all possible blocks that could be stored in the filesystem, and thus a hash collision is possible. The risk of accidental hash collision in a 160-bit hash is very small, even for exabytes of data. Historically, however, many hash functions become increasingly vulnerable to malicious hash collisions due to both cryptographic and computational advances. Venti does not address the issue of hash collisions; as of this time, it is still computationally infeasible to find collisions in SHA-1, but it may become necessary for Venti to switch to a different hash function at some point in the future. On 23 February 2017, Google announced the SHAttered attack, in which they generated two different PDF files with the same SHA-1 hash in roughly 2 63.1 SHA-1 evaluations.

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