digest

digest is the foundational trait crate of the RustCrypto ecosystem, defining the interfaces that cryptographic hash functions and message authentication codes implement. Rather than providing a specific algorithm, it specifies the common Digest, Update, FixedOutput, ExtendableOutput, and MAC traits so that every hasher, from SHA-256 to BLAKE2 to SHA-3, presents the same update/finalize API.

This shared abstraction lets application and library code be written generically over any hash function, swap algorithms without rewrites, and compose hashing into higher-level protocols. It is a no_std crate depending only on crypto-common, with optional features for block-level APIs, MAC support, zeroization, and OID metadata.

What You Get

• The high-level Digest trait plus Update, FixedOutput, and Reset building blocks
• Extendable-output-function (XOF) traits for algorithms like SHAKE
• Message authentication code (MAC) traits including constant-time output comparison
• A block-level API for implementing new hash functions with less boilerplate
• Optional zeroize, const-oid, and blobby-based test-vector support

Common Use Cases

• Writing functions generic over any hash algorithm via the Digest trait bound
• Implementing a new cryptographic hash function against a standard interface
• Building MAC-based authentication with a uniform verification API
• Swapping hash algorithms in a protocol without changing call sites

Under The Hood

Architecture - The crate layers its abstractions: digest.rs defines the high-level object-safe Digest trait built from the lower-level Update/FixedOutput/Reset traits in lib.rs, mac.rs adds authentication-code traits with constant-time output types, xof_fixed.rs covers extendable-output functions, and block_api.rs plus buffer_macros.rs provide a reusable block-processing scaffold (backed by block-buffer) for implementers. A dev.rs/dev/ module supplies test-vector harnesses.

Tech Stack - Rust edition 2024 (MSRV 1.85), one crate in the RustCrypto/traits Cargo workspace. Its only required dependency is crypto-common; optional dependencies include block-buffer, const-oid, zeroize, ctutils, and blobby, each behind a feature flag, and it is no_std with an opt-in alloc feature.

Code Quality - As a keystone crate depended on by nearly the entire Rust hashing ecosystem, it is held to a high bar: it ships dev macros and blobby-driven known-answer test infrastructure (tests/, src/dev/), uses sha2 as a dev-dependency to validate the traits end to end, and follows RustCrypto’s strict MSRV and API-stability conventions.

API Design - The design is deliberately minimal and composable: most users only touch Digest::new/update/finalize (usually re-exported by a concrete hash crate like sha2), while implementers pick the exact sub-traits their algorithm supports. Generic bounds like impl Digest make algorithm-agnostic code natural; the learning curve is mainly in the generic-array/typenum output types.