fdh 0.4.0 - Docs.rs

Full Domain Hash

A Full Domain Hash (FDH) is a useful cryptographic construction that extends the size of a hash digest to an arbitrary length. For example, SHA256 can be expanded to 1024 bits instead of the usual 256 bits.

We construct an FDH by computing a number of cycles where cycles=(target length)/(digest length) + 1. We then compute FDH(M) = HASH(M||0) || HASH(M||1) || ... || HASH(M||cycles−1), where HASH is any hash function, M is the message, || denotes concatenation, and numerical values are single-byte u8.

FDHs are usually used with an RSA signature scheme where the target length is the size of the key. See https://en.wikipedia.org/wiki/Full_Domain_Hash

This crate makes extensive use of the digest crate's cryptograhic hash traits, so most useful methods are implemented as part of digest traits. These traits are re-exported for convenience. See https://github.com/RustCrypto/hashes for a list of compatible hashes.

Example

  use sha2::Sha256;
  use fdh::{FullDomainHash, VariableOutput, Input};

  // Expand SHA256 from 256 bits to 1024 bits.
  let output_bits = 1024;
  let output_bytes = 1024 / 8;
  let mut hasher = FullDomainHash::<Sha256>::new(output_bytes)?;
  hasher.input(b"ATTACK AT DAWN");
  let result = hasher.vec_result();

`no_std`

This crate also supports no_std, so it can be used in embedded or other settings with no allocation.

#![no_std]
use sha2::Sha256;
use fdh::{FullDomainHash, Input, ExtendableOutput, XofReader};
   
// Expand SHA256 from 256 bits to 512 bits (and beyond!), reading it in 16 byte chunks.
let mut hasher = FullDomainHash::<Sha256>::default();
hasher.input(b"ATTACK AT DAWN");
let mut reader = hasher.xof_result();
let mut read_buf = <[u8; 16]>::default();

// Read the first 16 bytes into read_buf
reader.read(&mut read_buf);

// Read the second 16 bytes into read_buf
reader.read(&mut read_buf);

// If we want, we can just keep going, reading as many bits as we want indefinitely.
reader.read(&mut read_buf);
reader.read(&mut read_buf);