#hash #hashmap #aes #no-std #aes-ni

no-std ahash

A non-cryptographic hash function using AES-NI for high performance

54 releases

new 0.4.5 Sep 13, 2020
0.4.4 Jul 23, 2020
0.3.8 Jun 9, 2020
0.3.2 Jan 24, 2020
0.1.18 Mar 31, 2019

#19 in Cryptography

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aHash

AHash is a high speed keyed hashing algorithm intended for use in in-memory hashmaps. It provides a high quality 64bit hash. AHash is designed for performance and is not cryptographically secure.

Goals

AHash is the fastest DOS resistant hash for use in HashMaps available in the Rust language. Failing in any of these criteria will be treated as a bug.

Design

AHash is a keyed hash, so two instances initialized with different keys will produce completely different hashes, and the resulting hashes cannot be predicted without knowing the keys. This prevents DOS attacks where an attacker sends a large number of items whose hashes collide that get used as keys in a hashmap.

AHash takes advantage of specialized hardware instructions whenever possible including the hardware AES instruction on X86 processors when it is available. If it is not available it falls back on a somewhat slower (but still DOS resistant) algorithm based on multiplication.

As such aHash does not have a fixed standard for its output. This is not a problem for Hashmaps, and allows aHash to achieve high performance and improve over time.

Non-Goals

Because different computers or computers on versions of the code will observe different outputs Hash is not recommended for use other than in-memory maps. Specifically aHash does not intend to be:

  • Used as a MACs or other application requiring a cryptographically secure hash
  • Used for distributed applications or ones requiring persisting hashed values

Hash quality

Both aHash's aes variant and the fallback pass the full SMHasher test suite (the output of the tests is checked into the smhasher subdirectory.)

At over 50GB/s aHash is the fastest algorithm to pass the full test suite by more than a factor of 2. Even the fallback algorithm is in the top 5 in terms of throughput.

Speed

When it is available aHash uses AES rounds using the AES-NI instruction. AES-NI is very fast (on an intel i7-6700 it is as fast as a 64 bit multiplication.) and handles 16 bytes of input at a time, while being a very strong permutation.

This is obviously much faster than most standard approaches to hashing, and does a better job of scrambling data than most non-secure hashes.

On an intel i7-6700 compiled on nightly Rust with flags -C opt-level=3 -C target-cpu=native -C codegen-units=1:

Input SipHash 1-3 time FnvHash time FxHash time aHash time aHash Fallback*
u8 9.3271 ns 0.808 ns 0.594 ns 0.7704 ns 0.7664 ns
u16 9.5139 ns 0.803 ns 0.594 ns 0.7653 ns 0.7704 ns
u32 9.1196 ns 1.4424 ns 0.594 ns 0.7637 ns 0.7712 ns
u64 10.854 ns 3.0484 ns 0.628 ns 0.7788 ns 0.7888 ns
u128 12.465 ns 7.0728 ns 0.799 ns 0.6174 ns 0.6250 ns
1 byte string 11.745 ns 2.4743 ns 2.4000 ns 1.4921 ns 1.5861 ns
3 byte string 12.066 ns 3.5221 ns 2.9253 ns 1.4745 ns 1.8518 ns
4 byte string 11.634 ns 4.0770 ns 1.8818 ns 1.5206 ns 1.8924 ns
7 byte string 14.762 ns 5.9780 ns 3.2282 ns 1.5207 ns 1.8933 ns
8 byte string 13.442 ns 4.0535 ns 2.9422 ns 1.6262 ns 1.8929 ns
15 byte string 16.880 ns 8.3434 ns 4.6070 ns 1.6265 ns 1.7965 ns
16 byte string 15.155 ns 7.5796 ns 3.2619 ns 1.6262 ns 1.8011 ns
24 byte string 16.521 ns 12.492 ns 3.5424 ns 1.6266 ns 2.8311 ns
68 byte string 24.598 ns 50.715 ns 5.8312 ns 4.8282 ns 5.4824 ns
132 byte string 39.224 ns 119.96 ns 11.777 ns 6.5087 ns 9.1459 ns
1024 byte string 254.00 ns 1087.3 ns 156.41 ns 25.402 ns 54.566 ns
  • Fallback refers to the algorithm aHash would use if AES instructions are unavailable. For reference a hash that does nothing (not even reads the input data takes) 0.520 ns. So that represents the fastest possible time.

As you can see above aHash like FxHash provides a large speedup over SipHash-1-3 which is already nearly twice as fast as SipHash-2-4.

Rust's HashMap by default uses SipHash-1-3 because faster hash functions such as FxHash are predictable and vulnerable to denial of service attacks. While aHash has both very strong scrambling and very high performance.

AHash performs well when dealing with large inputs because aHash reads 8 or 16 bytes at a time. (depending on availability of AES-NI)

Because of this, and its optimized logic, aHash is able to outperform FxHash with strings. It also provides especially good performance dealing with unaligned input. (Notice the big performance gaps between 3 vs 4, 7 vs 8 and 15 vs 16 in FxHash above)

For more a more representative performance comparison which includes the overhead of using a HashMap, see HashBrown's benchmarks as HashBrown now uses aHash as its hasher by default.

Security

AHash is designed to prevent an adversary that does not know the key from being able to create hash collisions or partial collisions.

This achieved by ensuring that:

  • aHash is designed to resist differential crypto analysis. Meaning it should not be possible to devise a scheme to "cancel" out a modification of the internal state from a block of input via some corresponding change in a subsequent block of input.
    • This is achieved by not performing any "premixing" - This reversible mixing gave previous hashes such as murmurhash confidence in their quality, but could be undone by a deliberate attack.
    • Before it is used each chunk of input is "masked" such as by xoring it with an unpredictable value.
  • aHash obeys the 'strict avalanche criterion': Each bit of input has the potential to flip every bit of the output.
  • Similarly, each bit in the key can affect every bit in the output.
  • Input bits never affect just one, or a very few, bits in intermediate state. This is specifically designed to prevent the sort of differential attacks launched by the sipHash authors which cancel previous inputs.
  • The finish call at the end of the hash is designed to not expose individual bits of the internal state.
    • For example in the main algorithm 256bits of state and 256bits of keys are reduced to 64 total bits using 3 rounds of AES encryption. Reversing this is more than non-trivial. Most of the information is by definition gone, and any given bit of the internal state is fully diffused across the output.
  • In both aHash and its fallback the internal state is divided into two halves which are updated by two unrelated techniques using the same input. - This means that if there is a way to attack one of them it likely won't be able to attack both of them at the same time.
  • It is deliberately difficult to 'chain' collisions.
    • To attack Previous attacks on hash functions have relied on the ability

More details are available on the wiki.

aHash is not cryptographically secure

AHash should not be used for situations where cryptographic security is needed. It is not intended for this and will likely fail to hold up for several reasons.

  1. aHash relies on random keys which are assumed to not be observable by an attacker. For a cryptographic hash all inputs can be seen and controlled by the attacker.
  2. aHash has not yet gone through peer review.
  3. Because aHash uses reduced rounds of AES as opposed to the standard of 10. Things like the SQUARE attack apply to part of the internal state. (These are mitigated by other means to prevent producing collections, but would be a problem in other contexts).
  4. Like any cypher based hash, it will show certain statistical deviations from truly random output when comparing a (VERY) large number of hashes. (By definition cyphers have fewer collisions than truly random data.)

There are several efforts to build a secure hash function that uses AES-NI for acceleration, but aHash is not one of them.

Accelerated CPUs

Hardware AES instructions are built into Intel processors built after 2010 and AMD processors after 2012. It is also available on many other CPUs should in eventually be able to get aHash to work. However, only X86 and X86-64 are the only supported architectures at the moment, as currently they are the only architectures for which Rust provides an intrinsic.

aHash also uses sse2 and sse3 instructions. X86 processors that have aesni also have these instruction sets.

Why not use a cryptographic hash in a hashmap.

Cryptographic hashes are designed to make is nearly impossible to find two items that collide when the attacker has full control over the input. This has several implications:

  • They are very difficult to construct, and have to go to a lot of effort to ensure that collisions are not possible.
  • They have no notion of a 'key'. Rather, they are fully deterministic and provide exactly one hash for a given input.

For a HashMap the requirements are different.

  • Speed is very important, especially for short inputs. Often the key for a HashMap is a single u32 or similar, and to be effective the bucket that it should be hashed to needs to be computed in just a few CPU cycles.
  • A hashmap does not need to provide a hard and fast guarantee that no two inputs will ever collide. Hence, hashCodes are not 256bits but are just 64 or 32 bits in length. Often the first thing done with the hashcode is to truncate it further to compute which among a few buckets should be used for a key.
    • Here collisions are expected, and a cheap to deal with provided there is no systematic way to generated huge numbers of values that all go to the same bucket.
    • This also means that unlike a cryptographic hash partial collisions matter. It doesn't do a hashmap any good to produce a unique 256bit hash if the lower 12 bits are all the same. This means that even a provably irreversible hash would not offer protection from a DOS attack in a hashmap because an attacker can easily just brute force the bottom N bits.

From a cryptography point of view, a hashmap needs something closer to a block cypher. Where the input can be quickly mixed in a way that cannot be reversed without knowing a key.

Why use aHash over X

SipHash

For a hashmap: Because aHash nearly 10x faster.

SipHash is however useful in other contexts, such as for a HMAC, where aHash would be completely inappropriate.

SipHash-2-4 is designed to provide DOS attack resistance, and has no presently known attacks against this claim that doesn't involve learning bits of the key.

SipHash is also available in the "1-3" variant which is about twice as fast as the standard version. The SipHash authors don't recommend using this variation when DOS attacks are a concern, but there are still no known practical DOS attacks against the algorithm. Rust has opted for the "1-3" version as the default in std::collections::HashMap, because the speed trade off of "2-4" was not worth it.

As you can see in the table above, aHash is much faster than even SipHash-1-3, but it also provides DOS resistance, and any attack against the accelerated form would likely involve a weakness in AES.

FxHash

In terms of performance, aHash is faster than the FXhash for strings and byte arrays but not primitives. So it might seem like using Fxhash for hashmaps when the key is a primitive is a good idea. This is not the case.

When FX hash is operating on a 4 or 8 byte input such as a u32 or a u64, it reduces to multiplying the input by a fixed constant. This is a bad hashing algorithm because it means that lower bits can never be influenced by any higher bit. In the context of a hashmap where the low order bits are used to determine which bucket to put an item in, this isn't any better than the identity function. Any keys that happen to end in the same bit pattern will all collide. Some examples of where this is likely to occur are:

  • Strings encoded in base64
  • Null terminated strings (when working with C code)
  • Integers that have the lower bits as zeros. (IE any multiple of small power of 2, which isn't a rare pattern in computer programs.)
    • For example when taking lengths of data or locations in data it is common for values to have a multiple of 1024, if these were used as keys in a map they will collide and end up in the same bucket.

Like any non-keyed hash FxHash can be attacked. But FxHash is so prone to this that you may find yourself doing it accidentally.

For example, it is possible to accidentally introduce quadratic behavior by reading from one map in iteration order and writing to another.

Fxhash flaws make sense when you understand it for what it is. It is a quick and dirty hash, nothing more. it was not published and promoted by its creator, it was found!

Because it is error-prone, FxHash should never be used as a default. In specialized instances where the keys are understood it makes sense, but given that aHash is faster on almost any object, it's probably not worth it.

FnvHash

FnvHash is also a poor default. It only handles one byte at a time, so its performance really suffers with large inputs. It is also non-keyed so it is still subject to DOS attacks and accidentally quadratic behavior.

MurmurHash, CityHash, MetroHash, FarmHash, HighwayHash, XXHash, SeaHash

Murmur, City, Metro, Farm and Highway are all related, and appear to directly replace one another. Sea and XX are independent and compete.

They are all fine hashing algorithms, they do a good job of scrambling data, but they are all targeted at a different usecase. They are intended to work in distributed systems where the hash is expected to be the same over time and from one computer to the next, efficiently hashing large volumes of data.

This is quite different from the needs of a Hasher used in a hashmap. In a map the typical value is under 10 bytes. None of these algorithms scale down to handle that small of data at a competitive time. What's more the restriction that they provide consistent output prevents them from taking advantage of different hardware capabilities on different CPUs. It makes sense for a hashmap to work differently on a phone than on a server, or in wasm.

If you need to persist or transmit a hash of a file, then using one of these is probably a good idea. HighwayHash seems to be the preferred solution du jour. But inside a simple Hashmap, stick with aHash.

AquaHash

AquaHash is structured similarly to aHash. (Though the two were designed completely independently). AquaHash does not scale down nearly as well and does poorly with for example a single i32 as input. Its only implementation at this point is in C++.

t1ha

T1ha is fairly fast at large sizes, and the output is of fairly high quality, but it is not clear what usecase it aims for. It has many different versions and is very complex, and uses hardware tricks, so one might infer it is meant for hashmaps like aHash. But any hash using it take at least 20ns, and it doesn't outperform even SipHash until the input sizes are larger than 128 bytes and is not designed to be DOS resistant. So uses are likely niche.

License

Licensed under either of:

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.

Dependencies