Nix Hash

Hashes in Nix

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 extend, 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 fetchurl:

src = fetchurl {
  url = "https://example.org/downloads/source-code.zip";
  sha256 = "1g6ycnji10q5dd0avm6bz4lqpif82ppxjjq4x7vd8xihpgg3dm91";
};

You can specify the hash in any base that's supported. Thus, sha256 = "21d536debb3076d4f6e9044bd9ef15c8c58b29f9cbd4ad406b058310a565debc"; is equally allowed.

An alternative – and supposedly preferred – way of specifying hashes are so-called "SRI hashes". They're pretty simple, as the hash contains the algorithm used and always is in base64:

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

If you find a hash that uses colon as a separator (<type>:<hash>), don't use that. This relies on undocumented behavior and is not officially supported.

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

The tool of choice for hashing is nix-hash, although it will be deprecated one day and replaced by subcommands of the nix command. Below is a comparison between the current nix-hash and the experimental replacements which already can be used today, but are subject to change.

nix-hash	nix command	explanation
nix-hash --flat --type $HASHTYPE	nix hash file --base16 --type $HASHTYPE, see Nix_command/hash-file	Hash a file by using a “flat” hash which directly hashes a file and behaves like the {md5,sha1,sha256,sha512}sum utilities.
nix-hash --flat --base32 --type $HASHTYPE	nix hash file --base32 --type $HASHTYPE	Like above, but with the more used base32 output.
nix-hash --type $HASHTYPE	nix hash path --base16 --type $HASHTYPE, see Nix_command/hash-path	Compute the hash of a given path's dump in the NAR format.
nix-hash --base32 --type $HASHTYPE	nix hash path --base32 --type $HASHTYPE	Like above, but with the more common base32 representation.
nix-hash --to-base32 --type $HASHTYPE	nix to-base32 --type $HASHTYPE, see Nix_command/to-base32	Convert a hash of $HASHTYPE to its (nix-specific) base32 representation.
nix-hash --to-base16 --type $HASHTYPE	nix to-base16 --type $HASHTYPE, see Nix_command/to-base16	Convert a hash of $HASHTYPE to its base16 representation.
(not supported)	nix hash to-sri --type $HASHTYPE, see Nix_command/to-sri	Convert a hash of $HASHTYPE to its SRI representation.
(not supported)	nix hash to-base64 --type $HASHTYPE, see Nix_command/to-base64	Convert a hash of $HASHTYPE to its base64 representation which is the SRI representation without the hash type indication.

$HASHTYPE is either md5 (deprecated in nixpkgs), sha1, sha256 (current nixpkgs standard) or sha512 (candidate for the next standard hash). The main differences between nix-hash and the nix subcommands is the lack of support for SRI and base64 in the former and the lack of stability in the latter. The defaults settings for the two tools are as follows:

setting	nix-hash default	nix command default
output format	base16	SRI with base64 hash representation
hash algorithm	md5	sha256

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.