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97 lines
4.1 KiB
Markdown
# go-digest
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[](https://godoc.org/github.com/opencontainers/go-digest) [](https://goreportcard.com/report/github.com/opencontainers/go-digest) [](https://travis-ci.org/opencontainers/go-digest)
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Common digest package used across the container ecosystem.
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Please see the [godoc](https://godoc.org/github.com/opencontainers/go-digest) for more information.
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# What is a digest?
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A digest is just a [hash](https://en.wikipedia.org/wiki/Hash_function).
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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:
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```go
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id := digest.FromBytes([]byte("my content"))
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```
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In the example above, the id can be used to uniquely identify the byte slice "my content".
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This allows two disparate applications to agree on a verifiable identifier without having to trust one another.
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An identifying digest can be verified, as follows:
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```go
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if id != digest.FromBytes([]byte("my content")) {
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return errors.New("the content has changed!")
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}
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```
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A `Verifier` type can be used to handle cases where an `io.Reader` makes more sense:
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```go
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rd := getContent()
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verifier := id.Verifier()
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io.Copy(verifier, rd)
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if !verifier.Verified() {
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return errors.New("the content has changed!")
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}
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```
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Using [Merkle DAGs](https://en.wikipedia.org/wiki/Merkle_tree), this can power a rich, safe, content distribution system.
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# Usage
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While the [godoc](https://godoc.org/github.com/opencontainers/go-digest) is considered the best resource, a few important items need to be called out when using this package.
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1. Make sure to import the hash implementations into your application or the package will panic.
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You should have something like the following in the main (or other entrypoint) of your application:
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```go
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import (
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_ "crypto/sha256"
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_ "crypto/sha512"
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)
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```
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This may seem inconvenient but it allows you replace the hash
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implementations with others, such as https://github.com/stevvooe/resumable.
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2. Even though `digest.Digest` may be assemblable as a string, _always_ verify your input with `digest.Parse` or use `Digest.Validate` when accepting untrusted input.
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While there are measures to avoid common problems, this will ensure you have valid digests in the rest of your application.
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3. While alternative encodings of hash values (digests) are possible (for example, base64), this package deals exclusively with hex-encoded digests.
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# Stability
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The Go API, at this stage, is considered stable, unless otherwise noted.
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As always, before using a package export, read the [godoc](https://godoc.org/github.com/opencontainers/go-digest).
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# Contributing
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This package is considered fairly complete.
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It has been in production in thousands (millions?) of deployments and is fairly battle-hardened.
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New additions will be met with skepticism.
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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.
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## Code of Conduct
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Participation in the OpenContainers community is governed by [OpenContainer's Code of Conduct][code-of-conduct].
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## Security
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If you find an issue, please follow the [security][security] protocol to report it.
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# Copyright and license
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Copyright © 2019, 2020 OCI Contributors
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Copyright © 2016 Docker, Inc.
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All rights reserved, except as follows.
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Code is released under the [Apache 2.0 license](LICENSE).
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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).
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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/.
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[security]: https://github.com/opencontainers/org/blob/master/security
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[code-of-conduct]: https://github.com/opencontainers/org/blob/master/CODE_OF_CONDUCT.md
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