# Foldhash
  
This repository contains foldhash, a fast, non-cryptographic, minimally
DoS-resistant hashing algorithm implemented in Rust designed for computational
uses such as hash maps, bloom filters, count sketching, etc.

When should you **not** use foldhash:

- You are afraid of people studying your long-running program's behavior to
  reverse engineer its internal random state and using this knowledge to create
  many colliding inputs for computational complexity attacks. For more details
  see the section "HashDoS resistance".

- You expect foldhash to have a consistent output across versions or
  platforms, such as for persistent file formats or communication protocols.
  
- You are relying on foldhash's properties for any kind of security.
  Foldhash is **not appropriate for any cryptographic purpose**.

Foldhash has two variants, one optimized for speed which is ideal for data
structures such as hash maps and bloom filters, and one optimized for
statistical quality which is ideal for algorithms such as
[HyperLogLog](https://en.wikipedia.org/wiki/HyperLogLog) and
[MinHash](https://en.wikipedia.org/wiki/MinHash).

Foldhash can be used in a `#![no_std]` environment by disabling its default
`"std"` feature.
  

## Performance

We evaluated foldhash against three commonly used hashes in Rust:
[aHash](https://github.com/tkaitchuck/aHash) v0.8.11,
[fxhash](https://github.com/cbreeden/fxhash) v0.2.1, and
[SipHash-1-3](https://en.wikipedia.org/wiki/SipHash), the default hash algorithm
in Rust at the time of writing. We evaluated both variants foldhash provides,
`foldhash-f` and `foldhash-q`, which correspond to `foldhash::fast` and
`foldhash::quality` in the crate respectively.

First we note that hashers with random state inflate the size of your `HashMap`,
which may or may not be important for your performance:
```rust
std::mem::size_of::<foldhash::HashMap<u32, u32>>() = 40  // (both variants)
std::mem::size_of::<ahash::HashMap<u32, u32>>() = 64
std::mem::size_of::<fxhash::FxHashMap<u32, u32>>() = 32
std::mem::size_of::<std::collections::HashMap<u32, u32>>() = 48
```

We tested runtime performance on two machines, one with a 2023 Apple M2 CPU, one
with a 2023 Intel Xeon Platinum 8481C server CPU, both with stable Rust 1.80.1.
Since one of our competitors (aHash) is reliant on AES-based instructions for
optimal performance we have included both a benchmark with and without
`-C target-cpu=native` for the Intel machine.

We tested across a wide variety of data types we consider representative of
types / distributions one might hash in the real world, in the context of a hash
table key:

- `u32` - random 32-bit unsigned integers,
- `u32pair` - pairs of random 32-bit unsigned integers,
- `u64` - random 64-bit unsigned integers,
- `u64pair` - pairs of random 64-bit unsigned integers,
- `u64lobits` - 64-bit unsigned integers where only the bottom 16 bits vary,
- `u64hibits` - 64-bit unsigned integers where only the top 16 bits vary,
- `ipv4` - [`std::net::Ipv4Addr`](https://doc.rust-lang.org/std/net/struct.Ipv4Addr.html), which is equivalent to `[u8; 4]`, 
- `ipv6` - [`std::net::Ipv6Addr`](https://doc.rust-lang.org/std/net/struct.Ipv6Addr.html), which is equivalent to `[u8; 16]`, 
- `rgba` - random `(u8, u8, u8, u8)` tuples,
- `strenglishword` - strings containing words sampled uniformly from the top 10,000 most common English words,
- `struuid` - random UUIDs, hashed in string representation,
- `strurl` - strings containing URLs sampled uniformly from a corpus of 10,000 URLs,
- `strdate` - random `YYYY-MM-DD` date strings,
- `accesslog` - `(u128, u32, chrono::NaiveDate, bool)`, meant to simulate a typical
   larger compound type, in this case `(resource_id, user_id, date, success)`
   for an access log.
- `kilobyte` - random bytestrings one kilobyte in length,
- `tenkilobyte` - random bytestrings ten kilobytes in length.

We tested the performance of hashing the above data types in the following four contexts:

- `hashonly` - only the time it takes to hash the value,
- `lookupmiss` - the time it takes to do a lookup in a 1,000 element hash map
of random elements, only sampling keys of which we know that are not in the hash map,
- `lookuphit` - similar to `lookupmiss`, except the keys are sampled from keys
known to be in the hash map,
- `setbuild` - the time it takes to construct a `HashSet` of 1,000 elements
from 1,000 randomly sampled elements each repeated 10 times (so 10,000 inserts,
with ~90% duplicates).

All times are reported as expected time per operation, so one hash, one lookup,
or one insert respectively. The full results [can be found
here](https://gist.github.com/orlp/1271ad5b8b775c651cc55773888858eb). To
summarize, we will only show the results for `u64` and `strenglishword` here, as
well as the observed geometric mean and average rank over the full benchmark.

```
Xeon 8481c

┌────────────────┬────────────┬────────────┬────────────┬─────────┬─────────┬─────────┐
│              avg_rank       ┆       1.58 ┆       2.66 ┆    2.09 ┆    3.70 ┆    4.97 │
│        geometric_mean       ┆       6.21 ┆       7.01 ┆    7.56 ┆    8.74 ┆   28.70 │
╞════════════════╪════════════╪════════════╪════════════╪═════════╪═════════╪═════════╡
│          distr ┆      bench ┆ foldhash-f ┆ foldhash-q ┆  fxhash ┆   ahash ┆ siphash │
╞════════════════╪════════════╪════════════╪════════════╪═════════╪═════════╪═════════╡
│            u64 ┆   hashonly ┆       0.79 ┆       1.03 ┆    0.67 ┆    1.23 ┆    9.09 │
│            u64 ┆ lookupmiss ┆       2.01 ┆       2.44 ┆    1.73 ┆    2.73 ┆   12.03 │
│            u64 ┆  lookuphit ┆       3.04 ┆       3.59 ┆    2.64 ┆    3.84 ┆   12.65 │
│            u64 ┆   setbuild ┆       6.13 ┆       6.52 ┆    4.88 ┆    6.66 ┆   17.80 │
|            ... ┆        ... ┆        ... ┆        ... ┆     ... ┆     ... ┆     ... |
│ strenglishword ┆   hashonly ┆       2.63 ┆       2.98 ┆    3.24 ┆    3.57 ┆   11.87 │
│ strenglishword ┆ lookupmiss ┆       4.63 ┆       5.03 ┆    4.51 ┆    5.86 ┆   15.19 │
│ strenglishword ┆  lookuphit ┆       8.62 ┆       9.25 ┆    8.28 ┆   10.06 ┆   21.35 │
│ strenglishword ┆   setbuild ┆      14.77 ┆      15.57 ┆   18.86 ┆   15.72 ┆   35.36 │
└────────────────┴────────────┴────────────┴────────────┴─────────┴─────────┴─────────┘

Xeon 8481c with RUSTFLAGS="-C target-cpu=native"

┌────────────────┬────────────┬────────────┬────────────┬─────────┬─────────┬─────────┐
│              avg_rank       ┆       1.89 ┆       3.12 ┆    2.25 ┆    2.77 ┆    4.97 │ 
│        geometric_mean       ┆       6.00 ┆       6.82 ┆    7.39 ┆    6.94 ┆   29.49 │ 
╞════════════════╪════════════╪════════════╪════════════╪═════════╪═════════╪═════════╡
│          distr ┆      bench ┆ foldhash-f ┆ foldhash-q ┆  fxhash ┆   ahash ┆ siphash │
╞════════════════╪════════════╪════════════╪════════════╪═════════╪═════════╪═════════╡
│            u64 ┆   hashonly ┆       0.79 ┆       1.01 ┆    0.67 ┆    1.34 ┆    9.24 │
│            u64 ┆ lookupmiss ┆       1.68 ┆       2.12 ┆    1.62 ┆    1.96 ┆   12.04 │
│            u64 ┆  lookuphit ┆       2.68 ┆       3.19 ┆    2.28 ┆    3.16 ┆   13.09 │
│            u64 ┆   setbuild ┆       6.16 ┆       6.42 ┆    4.75 ┆    7.03 ┆   18.88 │
|            ... ┆        ... ┆        ... ┆        ... ┆     ... ┆     ... ┆     ... |
│ strenglishword ┆   hashonly ┆       2.60 ┆       2.97 ┆    3.25 ┆    3.04 ┆   11.58 │
│ strenglishword ┆ lookupmiss ┆       4.41 ┆       4.96 ┆    4.82 ┆    4.79 ┆   32.31 │
│ strenglishword ┆  lookuphit ┆       8.68 ┆       9.35 ┆    8.46 ┆    8.63 ┆   21.48 │
│ strenglishword ┆   setbuild ┆      15.01 ┆      16.34 ┆   19.34 ┆   15.37 ┆   35.22 │
└────────────────┴────────────┴────────────┴────────────┴─────────┴─────────┴─────────┘

Apple M2

┌────────────────┬────────────┬────────────┬────────────┬─────────┬─────────┬─────────┐
│              avg_rank       ┆       1.62 ┆       2.81 ┆    2.02 ┆    3.58 ┆    4.97 │
│        geometric_mean       ┆       4.41 ┆       4.86 ┆    5.39 ┆    5.71 ┆   21.94 │
╞════════════════╪════════════╪════════════╪════════════╪═════════╪═════════╪═════════╡
│          distr ┆      bench ┆ foldhash-f ┆ foldhash-q ┆  fxhash ┆   ahash ┆ siphash │
╞════════════════╪════════════╪════════════╪════════════╪═════════╪═════════╪═════════╡
│            u64 ┆   hashonly ┆       0.60 ┆       0.70 ┆    0.41 ┆    0.78 ┆    6.61 │
│            u64 ┆ lookupmiss ┆       1.50 ┆       1.61 ┆    1.23 ┆    1.65 ┆    8.28 │
│            u64 ┆  lookuphit ┆       1.78 ┆       2.10 ┆    1.57 ┆    2.25 ┆    8.53 │
│            u64 ┆   setbuild ┆       4.74 ┆       5.19 ┆    3.61 ┆    5.38 ┆   15.36 │
|            ... ┆        ... ┆        ... ┆        ... ┆     ... ┆     ... ┆     ... |
│ strenglishword ┆   hashonly ┆       1.84 ┆       2.13 ┆    1.85 ┆    2.13 ┆   11.61 │
│ strenglishword ┆ lookupmiss ┆       2.71 ┆       2.96 ┆    2.47 ┆    2.99 ┆    9.27 │
│ strenglishword ┆  lookuphit ┆       7.54 ┆       8.77 ┆    7.83 ┆    8.77 ┆   18.65 │
│ strenglishword ┆   setbuild ┆      16.61 ┆      17.09 ┆   14.83 ┆   16.52 ┆   26.42 │
└────────────────┴────────────┴────────────┴────────────┴─────────┴─────────┴─────────┘
```

We note from the above benchmark that for hash table performance the extra
quality that `foldhash-q` provides is almost never actually worth the small but
also non-negligible computational overhead it has over `foldhash-f`. This is our
justification for providing `foldhash::fast` as a default choice for hash
tables, even though it has measurable biases (see also the "Quality" section).

fxhash generally does fairly well for small inputs on the benchmarks, however it
has structural weaknesses as a hash which makes it ill-advised to use as a
general-purpose hash function in our opinion. For example the `lookuphit`
benchmark on Apple M2 for `u64hibits` takes 1.77 nanoseconds per lookup for
foldhash, but 67.72 nanoseconds for fxhash (due to everything colliding - the
effects would be even worse with a larger hash map). In our opinion foldhash-f
strikes the right balance between quality and performance for hash tables,
whereas fxhash flies a bit too close to the sun.

aHash is faster than foldhash for medium-long strings when compiled with
AES instruction support, but is slower in almost every other scenario or when
AES instructions are unavailable.

## Quality

Foldhash-f is a fairly strong hash in terms of collisions *on its full 64-bit
output*. However, statistical tests such as
[SMHasher3](https://gitlab.com/fwojcik/smhasher3) can distinguish it from an ideal
hash function in tests that focus on the relationship between individual
input/output bits. One such property is avalanching: changing a single bit in
the input does not flip every other bit with 50% probability when using
foldhash-f like it should if it behaved like a proper random oracle.

As the benchmarks above show, spending more effort in foldhash-f to improve the
hash quality does not lead to better hash table performance. However, there are
also use cases for hash functions where it is important that (each bit of) the
hash is unbiased and a random function of all bits of the input, such as in
algorithms as HyperLogLog or MinHash.

For this purpose we also provide foldhash-q, which is simply a post-processed
version of foldhash-f to properly avalanche all the bits. Foldhash-q passes the
[SMHasher3](https://gitlab.com/fwojcik/smhasher3) test suite [without any
failures](https://github.com/orlp/foldhash_smhasher3). You can also plot the
worst-case probability (where 50% is ideal) that any particular output bit flips
if you flip an input bit, which nicely visualizes how fxhash and foldhash-f
fail this avalanche property but foldhash-q and SipHash-1-3 pass:

| FxHash | Foldhash-f | Foldhash-q | SipHash-1-3 |
|--------|------------|------------|-------------|
|  |  |  | 

## Background

The name foldhash is derived from the *folded multiply*. This technique
compresses two 64-bit words into a single 64-bit word while simultaneously
thoroughly mixing the bits. It does this using a 64 x 64 bit -> 128 bit
multiplication followed by folding the two halves of the 128-bit product
together using a XOR operation:

```rust
let full = (x as u128) * (y as u128);
let lo = full as u64;
let hi = (full >> 64) as u64;
let folded = lo ^ hi;
```

We're not aware of a formal analysis of this operation, but empirically it works
very well. An informal intuition for why it should work is that multiplication
can be seen as the sum of many shifted copies of one of the arguments, only
including those shifted copies where the other argument has set bits, e.g. for
multiplying 4-bit words `abcd` and `efgh`:

```
abcd * efgh =

  abcd    * e
   abcd   * f
    abcd  * g
     abcd * h
--------------- +
```

Note that the middle bits of the product are a function of many of the input
bits, whereas the top-most and bottom-most bits are impacted by fewer of the
input bits. By folding the top half back onto the bottom half these effects
compensate each other, making all the output bits affected by much of the input.

We did not invent the folded multiply, it was previously used in essentially the
same way in [aHash](https://github.com/tkaitchuck/aHash),
[wyhash](https://github.com/wangyi-fudan/wyhash), and
[xxhash3](https://github.com/Cyan4973/xxHash). The operation was also used
in [mum-hash](https://github.com/vnmakarov/mum-hash), and probably others.
We do not know who originally invented it, the earliest reference
we could find was Steven Fuerst [blogging about it](https://web.archive.org/web/20121213174842/http://locklessinc.com/articles/crypto_hash/)
in 2012.


## HashDoS resistance

The folded multiply has a fairly glaring flaw: if one of the halves is zero, the
output is zero. This makes it trivial to create a large number of hash
collisions (even by accident, as zeroes are a common input to hashes). To combat
this, every folded multiply in foldhash has the following form:

```rust
folded_multiply(input1 ^ secret1, input2 ^ secret2)
```

Here `secret1` or `secret2` are either secret random numbers generated by
foldhash beforehand, or partial hash results influenced by such a secret prior.
This (plus other careful design throughout the hash function) ensures that it is
not possible to create a list of inputs that collide for every instance of
foldhash, and also prevents certain access patterns on hash tables going
quadratric by ensuring that each hash table uses a different seed and thus a
different access pattern. It is these two properties that we refer to when we
claim foldhash is "minimally DoS-resistant": it does the bare minimum to defeat
very simple attacks.

However, to be crystal clear, **foldhash does not claim to provide HashDoS
resistance against interactive attackers**. For a student of cryptography it
should be trivial to derive the secret values from direct observation of hash
outputs, and feasible to derive the secret values from indirect observation of
hashes, such as through timing attacks or hash table iteration. Once an attacker
knows the secret values, they can once again create infinite hash collisions
with ease.


## Acknowledgements

We thank Liam Gray for their suggestions on improving string hashing
performance.
