Merge pull request RustCrypto#131 from RustCrypto/digest/more-readme-…

…content

digest: add more content to README.md

digest/README.md

@@ -6,10 +6,10 @@
 ![Rust Version][rustc-image]
 [![Build Status][build-image]][build-link]
 
-Traits which describe functionality of [cryptographic hash functions], a.k.a.
+Traits which describe functionality of [cryptographic hash functions][0], a.k.a.
 digest algorithms.
 
-See [RustCrypto/hashes] for implementations which use this trait.
+See [RustCrypto/hashes][1] for implementations which use this trait.
 
 [Documentation][docs-link]
 
@@ -25,6 +25,105 @@ done with a minor version bump.
 - All on-by-default features of this library are covered by SemVer
 - MSRV is considered exempt from SemVer as noted above
 
+## Usage
+
+Let us demonstrate how to use crates in this repository using BLAKE2b as an
+example.
+
+First add `blake2` crate to your `Cargo.toml`:
+
+```toml
+[dependencies]
+blake2 = "0.8"
+```
+
+`blake2` and other crates re-export `digest` crate and `Digest` trait for
+convenience, so you don't have to add `digest` crate as an explicit dependency.
+
+Now you can write the following code:
+
+```rust
+use blake2::{Blake2b, Digest};
+
+let mut hasher = Blake2b::new();
+let data = b"Hello world!";
+hasher.input(data);
+// `input` can be called repeatedly and is generic over `AsRef<[u8]>`
+hasher.input("String data");
+// Note that calling `result()` consumes hasher
+let hash = hasher.result();
+println!("Result: {:x}", hash);
+```
+
+In this example `hash` has type [`GenericArray<u8, U64>`][2], which is a generic
+alternative to `[u8; 64]`.
+
+Alternatively you can use chained approach, which is equivalent to the previous
+example:
+
+```rust
+let hash = Blake2b::new()
+    .chain(b"Hello world!")
+    .chain("String data")
+    .result();
+
+println!("Result: {:x}", hash);
+```
+
+If the whole message is available you also can use convinience `digest` method:
+
+```rust
+let hash = Blake2b::digest(b"my message");
+println!("Result: {:x}", hash);
+```
+
+### Hashing `Read`-able objects
+
+If you want to hash data from [`Read`][3] trait (e.g. from file) you can rely on
+implementation of [`Write`][4] trait (requires enabled-by-default `std` feature):
+
+```rust
+use blake2::{Blake2b, Digest};
+use std::{fs, io};
+
+let mut file = fs::File::open(&path)?;
+let mut hasher = Blake2b::new();
+let n = io::copy(&mut file, &mut hasher)?;
+let hash = hasher.result();
+
+println!("Path: {}", path);
+println!("Bytes processed: {}", n);
+println!("Hash value: {:x}", hash);
+```
+
+### Generic code
+
+You can write generic code over `Digest` (or other traits from `digest` crate)
+trait which will work over different hash functions:
+
+```rust
+use digest::Digest;
+
+// Toy example, do not use it in practice!
+// Instead use crates from: https://github.com/RustCrypto/password-hashing
+fn hash_password<D: Digest>(password: &str, salt: &str, output: &mut [u8]) {
+    let mut hasher = D::new();
+    hasher.input(password.as_bytes());
+    hasher.input(b"$");
+    hasher.input(salt.as_bytes());
+    output.copy_from_slice(hasher.result().as_slice())
+}
+
+use blake2::Blake2b;
+use sha2::Sha256;
+
+hash_password::<Blake2b>("my_password", "abcd", &mut buf);
+hash_password::<Sha256>("my_password", "abcd", &mut buf);
+```
+
+If you want to use hash functions with trait objects, use `digest::DynDigest`
+trait.
+
 ## License
 
 Licensed under either of:
@@ -53,5 +152,10 @@ dual licensed as above, without any additional terms or conditions.
 
 [//]: # (general links)
 
-[cryptographic hash functions]: https://en.wikipedia.org/wiki/Cryptographic_hash_function
-[RustCrypto/hashes]: https://github.com/RustCrypto/hashes
+[0]: https://en.wikipedia.org/wiki/Cryptographic_hash_function
+[1]: https://github.com/RustCrypto/hashes
+[2]: https://docs.rs/generic-array
+[3]: https://doc.rust-lang.org/std/io/trait.Read.html
+[4]: https://doc.rust-lang.org/std/io/trait.Write.html
+[5]: https://en.wikipedia.org/wiki/Hash-based_message_authentication_code
+[6]: https://github.com/RustCrypto/MACs