// Copyright 2019, 2020 OCI Contributors // Copyright 2017 Docker, Inc. // // Licensed under the Apache License, Version 2.0 (the "License"); // you may not use this file except in compliance with the License. // You may obtain a copy of the License at // // https://www.apache.org/licenses/LICENSE-2.0 // // Unless required by applicable law or agreed to in writing, software // distributed under the License is distributed on an "AS IS" BASIS, // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. // See the License for the specific language governing permissions and // limitations under the License. // Package digest provides a generalized type to opaquely represent message // digests and their operations within the registry. The Digest type is // designed to serve as a flexible identifier in a content-addressable system. // More importantly, it provides tools and wrappers to work with // hash.Hash-based digests with little effort. // // Basics // // The format of a digest is simply a string with two parts, dubbed the // "algorithm" and the "digest", separated by a colon: // // <algorithm>:<digest> // // An example of a sha256 digest representation follows: // // sha256:7173b809ca12ec5dee4506cd86be934c4596dd234ee82c0662eac04a8c2c71dc // // The "algorithm" portion defines both the hashing algorithm used to calculate // the digest and the encoding of the resulting digest, which defaults to "hex" // if not otherwise specified. Currently, all supported algorithms have their // digests encoded in hex strings. // // In the example above, the string "sha256" is the algorithm and the hex bytes // are the "digest". // // Because the Digest type is simply a string, once a valid Digest is // obtained, comparisons are cheap, quick and simple to express with the // standard equality operator. // // Verification // // The main benefit of using the Digest type is simple verification against a // given digest. The Verifier interface, modeled after the stdlib hash.Hash // interface, provides a common write sink for digest verification. After // writing is complete, calling the Verifier.Verified method will indicate // whether or not the stream of bytes matches the target digest. // // Missing Features // // In addition to the above, we intend to add the following features to this // package: // // 1. A Digester type that supports write sink digest calculation. // // 2. Suspend and resume of ongoing digest calculations to support efficient digest verification in the registry. // package digest