# go-digest

[![GoDoc](https://godoc.org/github.com/docker/go-digest?status.svg)](https://godoc.org/github.com/docker/go-digest) [![Go Report Card](https://goreportcard.com/badge/github.com/docker/go-digest)](https://goreportcard.com/report/github.com/docker/go-digest) [![Build Status](https://travis-ci.org/docker/go-digest.svg?branch=master)](https://travis-ci.org/docker/go-digest)

Common digest package used across the container ecosystem.

Please see the [godoc](https://godoc.org/github.com/docker/go-digest) for more information.

# What is a digest?

A digest is just a hash.

The most common use case for a digest is to create a content
identifier for use in [Content Addressable Storage](https://en.wikipedia.org/wiki/Content-addressable_storage)
systems:

```go
id := digest.FromBytes([]byte("my content"))
```

In the example above, the id can be used to uniquely identify 
the byte slice "my content". This allows two disparate applications
to agree on a verifiable identifier without having to trust one
another.

An identifying digest can be verified, as follows:

```go
if id != digest.FromBytes([]byte("my content")) {
  return errors.New("the content has changed!")
}
```

A `Verifier` type can be used to handle cases where an `io.Reader`
makes more sense:

```go
rd := getContent()
verifier := id.Verifier()
io.Copy(verifier, rd)

if !verifier.Verified() {
  return errors.New("the content has changed!")
}
```

Using [Merkle DAGs](https://en.wikipedia.org/wiki/Merkle_tree), this
can power a rich, safe, content distribution system.

# Usage

While the [godoc](https://godoc.org/github.com/docker/go-digest) is 
considered the best resource, a few important items need to be called 
out when using this package.

1. Make sure to import the hash implementations into your application
    or the package will panic. You should have something like the 
    following in the main (or other entrypoint) of your application:
   
    ```go
    import (
        _ "crypto/sha256"
   	    _ "crypto/sha512"
    )
    ```
    This may seem inconvenient but it allows you replace the hash 
    implementations with others, such as https://github.com/stevvooe/resumable.
 
2. Even though `digest.Digest` may be assemable as a string, _always_ 
    verify your input with `digest.Parse` or use `Digest.Validate`
    when accepting untrusted input. While there are measures to 
    avoid common problems, this will ensure you have valid digests
    in the rest of your application.

# Stability

The Go API, at this stage, is considered stable, unless otherwise noted.

As always, before using a package export, read the [godoc](https://godoc.org/github.com/docker/go-digest).

# Contributing

This package is considered fairly complete. It has been in production
in thousands (millions?) of deployments and is fairly battle-hardened.
New additions will be met with skepticism. If you think there is a 
missing feature, please file a bug clearly describing the problem and 
the alternatives you tried before submitting a PR.

# Reporting security issues

The maintainers take security seriously. If you discover a security 
issue, please bring it to their attention right away!

Please DO NOT file a public issue, instead send your report privately
to security@docker.com.

Security reports are greatly appreciated and we will publicly thank you 
for it. We also like to send gifts—if you're into Docker schwag, make 
sure to let us know. We currently do not offer a paid security bounty 
program, but are not ruling it out in the future.

# Copyright and license

Copyright © 2016 Docker, Inc. All rights reserved, except as follows. Code is released under the [Apache 2.0 license](LICENSE.code). This `README.md` file and the [`CONTRIBUTING.md`](CONTRIBUTING.md) file are licensed under the Creative Commons Attribution 4.0 International License under the terms and conditions set forth in the file [`LICENSE.docs`](LICENSE.docs). You may obtain a duplicate copy of the same license, titled CC BY-SA 4.0, at http://creativecommons.org/licenses/by-sa/4.0/.
