Add hash customization traits (#1334)

Some hash function constructions allow to have a customization string to have domain separation in hash functions. This is the case for [CSHAKE](https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-185.pdf#page=14) and [Blake2](https://www.blake2.net/blake2.pdf#page=6): 

With these new traits, the construction of hash functions with customization strings would be simpler. From currently for CSHAKE:
```rust
let mut hasher = CShake128::from_core(CShake128Core::new(b"test")); 
```

To:
```rust
let mut hasher = CShake128:new_personalization(b"test"));
```

digest/CHANGELOG.md

@@ -6,6 +6,9 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## UNRELEASED
+### Added
+- `CustomizedInit` trait ([#1334]).
+
 ### Changed
 - `crypto-common` dependency bumped to v0.2 ([#1173])
 - Edition changed to 2021 and MSRV bumped to 1.57 ([#1173])
@@ -14,6 +17,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - `Mac::new`, `Mac::new_from_slice`, and `Mac::generate_key` methods ([#1173])
 
 [#1173]: https://github.com/RustCrypto/traits/pull/1173
+[#1334]: https://github.com/RustCrypto/traits/pull/1334
 
 ## 0.10.7 (2023-05-19)
 ### Changed

digest/src/core_api/wrapper.rs

@@ -3,7 +3,8 @@ use super::{
     UpdateCore, XofReaderCoreWrapper,
 };
 use crate::{
-    ExtendableOutput, ExtendableOutputReset, FixedOutput, FixedOutputReset, HashMarker, Update,
+    CustomizedInit, ExtendableOutput, ExtendableOutputReset, FixedOutput, FixedOutputReset,
+    HashMarker, Update,
 };
 use block_buffer::BlockBuffer;
 use core::{
@@ -160,8 +161,28 @@ impl<T: BufferKindUser> Drop for CoreWrapper<T> {
 #[cfg(feature = "zeroize")]
 impl<T: BufferKindUser + zeroize::ZeroizeOnDrop> zeroize::ZeroizeOnDrop for CoreWrapper<T> {}
 
+impl<T> CustomizedInit for CoreWrapper<T>
+where
+    T: BufferKindUser + CustomizedInit,
+{
+    type CustomizedExtArg = T::CustomizedExtArg;
+
+    #[inline]
+    fn new_customized(customization: &[u8]) -> Self {
+        Self::from_core(T::new_customized(customization))
+    }
+
+    #[inline]
+    fn new_ext_customized(customization_ext: &Self::CustomizedExtArg) -> Self {
+        Self::from_core(T::new_ext_customized(customization_ext))
+    }
+}
+
 #[cfg(feature = "oid")]
-impl<T: BufferKindUser + AssociatedOid> AssociatedOid for CoreWrapper<T> {
+impl<T> AssociatedOid for CoreWrapper<T>
+where
+    T: BufferKindUser + AssociatedOid,
+{
     const OID: ObjectIdentifier = T::OID;
 }
 

digest/src/lib.rs

@@ -268,6 +268,21 @@ pub trait VariableOutputReset: VariableOutput + Reset {
     }
 }
 
+/// Trait for hash functions with customization string for domain separation.
+pub trait CustomizedInit: Sized {
+    // TODO: It would be nice to define a default value equal to `[u8]`, but unfortunately
+    // associated type defaults are currently unstable
+
+    /// Extended customization
+    type CustomizedExtArg;
+
+    /// Create new hasher instance with the given customization string.
+    fn new_customized(customization: &[u8]) -> Self;
+
+    /// Create new hasher instance with the given extended customization.
+    fn new_ext_customized(customization_ext: &Self::CustomizedExtArg) -> Self;
+}
+
 /// The error type used in variable hash traits.
 #[derive(Clone, Copy, Debug, Default)]
 pub struct InvalidOutputSize;