Nix Hash

Cryptographic hashes play an essential role in a lot of places in the Nix ecosystem. When using a hash somewhere, two criteria are essential to do so properly: the algorithm used and the encoding (and, to some extent, what is hashed).

Supported algorithms are md5, sha1, sha256, sha512. The first two are deprecated and should not be used anymore, but you may still stumble upon them in existing code.

A hash – which is simply a sequence of bytes – is usually encoded in order to be representable as string. Common encodings are base16 (commonly called "hex"), base32 and base64. Note that the base32 is a custom one that is not documented nor standardized in any way! If possible, use the provided hashing tools to convert hashes to it (see below). base32 is used by Nix in a lot of places because it is shorter than hex but can still safely be part of a file path (as it contains no slashes).

Usage

Many derivations are so-called fixed-output derivations, meaning that you need to know and specify the hash of the output in advance. As an example, let's look at nixpkgs function fetchurl:

src = fetchurl {
  url = "https://example.org/downloads/source-code.zip";
  hash = "sha256-IdU23rswdtT26QRL2e8VyMWLKfnL1K1AawWDEKVl3rw=";
};

The format of the hash follows the SRI (Subresource Integrity) specification.

Updating Packages

Using TOFU to get the new hash

What exactly is hashed

Some content can either be hashed "flat" or "recursively". "flat" (sometimes also called "file") is simply taking the hash of the file, byte by byte, and will give you the same result as for example `sha256sum -b myfile.zip`. "recursive" (or sometimes "path") hashing takes multiple files, path names and metadata (attributes) into consideration. It works by NARing the input before hashing.

For fetchurl, the option to switch between both is called recursiveHash and defaults to false.

fetchzip on the other hand will download the file, unzip it and then recursively hash the output. There's no option.

The motivation behind this is that sometimes, the content is always the same, but the archive may change. This is because zip files are inherently non-deterministic, and might be generated automatically. If they are regenerated, they'll have a different hash, although the content is the same. recursiveHash works around that.

Tools

• nix-hash
• nix hash (nix command)

When dealing with remote files, nix-prefetch-url offers a handy shortcut for downloading the file into the Nix store and printing out its hash. (nix-prefetch-url --unpack is its fetchzip equivalent.)

Libraries

• Original C++ implementation
• Rust implementation
• Go implementation
• PHP Implementation

Further reading

• Eelco Dolstra's PhD thesis, section 5.1.
• Github Issue about which encoding is used where, and what pitfalls can arise from it.