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## Simple Summary
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This EIP will enable the BLAKE2b hash function and other higher-round 64-bit BLAKE2 variants to run cheaply on the EVM, allowing easier interoperability between Ethereum and Zcash as well as other Equihash-based PoW coins.
This EIP introduces a new precompiled contract which implements the compression function `F` used in the BLAKE2 cryptographic hashing algorithm, for the purpose of allowing interoperability between the EVM and Zcash, as well as introducing more flexible cryptographic hash primitives to the EVM.
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Besides being a useful cryptographic hash function and SHA3 finalist, BLAKE2 allows for efficient verification of the Equihash PoW used in Zcash, making a BTC Relay - style SPV client possible on Ethereum. A single verification of an Equihash PoW verification requires 512 iterations of the hash function, making verification of Zcash block headers prohibitively expensive if a Solidity implementation of BLAKE2 is used.
Interoperability with Zcash could enable contracts like trustless atomic swaps between the chains, which could provide a much needed aspect of privacy to the very public Ethereum blockchain.
## Specification
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We propose adding a precompiled contract at address `0x09` wrapping the [BLAKE2 `F` compression function](https://tools.ietf.org/html/rfc7693#section-3.2).
The precompile requires 6 inputs tightly encoded, taking exactly 213 bytes, as explained below. The encoded inputs are corresponding to the ones specified in the [BLAKE2 RFC Section 3.2](https://tools.ietf.org/html/rfc7693#section-3.2):
All other values yield an invalid encoding of`f`error.
The precompile should compute the `F` function as [specified in the RFC](https://tools.ietf.org/html/rfc7693#section-3.2) and return the updated state vector `h` with unchanged encoding (little-endian).
### Example Usage in Solidity
The precompile can be wrapped easily in Solidity to provide a more development-friendly interface to `F`.
```solidity
function F(uint32 rounds, bytes32[2] memory h, bytes32[4] memory m, bytes8[2] memory t, bool f) public view returns (bytes32[2] memory) {
Each operation will cost `GFROUND * rounds` gas, where `GFROUND = 1`. Detailed benchmarks are presented in the benchmarks appendix section.
## Rationale
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BLAKE2 is an excellent candidate for precompilation. BLAKE2 is heavily optimized for modern 64-bit CPUs, specifically utilizing 24 and 63-bit rotations to allow parallelism through SIMD instructions and little-endian arithmetic. These characteristics provide exceptional speed on native CPUs: 3.08 cycles per byte, or 1 gibibyte per second on an Intel i5.
In contrast, the big-endian 32 byte semantics of the EVM are not conducive to efficient implementation of BLAKE2, and thus the gas cost associated with computing the hash on the EVM is disproportionate to the true cost of computing the function natively.
An obvious implementation would be a direct BLAKE2b hash function precompile. At first glance, a BLAKE2b precompile satisfies most hashing and interoperability requirements on the EVM. Once we started digging in, however, it became clear that any BLAKE2b implementation would need specific features and internal modifications based on different projects' requirements and libraries.
A [thread with the Zcash team](https://github.com/ethereum/EIPs/issues/152#issuecomment-499240310) makes the issue clear.
> The minimal thing that is necessary for a working ZEC-ETH relay is an implementation of BLAKE2b Compression F in a precompile.
> A BLAKE2b Compression Function F precompile would also suffice for the Filecoin and Handshake interop goals.
> A full BLAKE2b precompile would suffice for a ZEC-ETH relay, provided that the implementation provided the parts of the BLAKE2 API that we need (personalization, maybe something else—I'm not sure).
> I'm not 100% certain if a full BLAKE2b precompile would also suffice for the Filecoin and Handshake goals. It almost certainly could, provided that it supports all the API that they need.
> BLAKE2s — whether the Compression Function F or the full hash — is only a nice-to-have for the purposes of a ZEC-ETH relay.
From this and other conversations with teams in the space, we believe we should focus first on the `F` precompile as a strictly necessary piece for interoperability projects. A BLAKE2b precompile is a nice-to-have, and we support any efforts to add one-- but it's unclear whether complete requirements and a flexible API can be found in time for Istanbul.
Implementation of only the core F compression function also allows substantial flexibility and extensibility while keeping changes at the protocol level to a minimum. This will allow functions like tree hashing, incremental hashing, and keyed, salted, and personalized hashing as well as variable length digests, none of which are currently available on the EVM.
## Backwards Compatibility
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There is very little risk of breaking backwards-compatibility with this EIP, the sole issue being if someone were to build a contract relying on the address at `0x09` being empty. The likelihood of this is low, and should specific instances arise, the address could be chosen to be any arbitrary value with negligible risk of collision.
## Test Cases
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An initial implementation of the `F` function in Go, adapted from the standard library, can be found in our [Golang BLAKE2 library fork](https://github.com/keep-network/blake2-f). There's also an implementation of the precompile in our fork of [go-ethereum](https://github.com/keep-network/go-ethereum/pull/4).
Assuming ecRecover precompile is perfectly priced, we executed a set of benchmarks comparing Blake2b F compression function precompile with ecRecover precompile. For benchmarks, we used 3.1 GHz Intel Core i7 64-bit machine.
```
$ sysctl -n machdep.cpu.brand_string
Intel(R) Core(TM) i7-7920HQ CPU @ 3.10GHz
```
### 12 rounds
An average gas price of F precompile call with 12 rounds compared to ecRecover should have been `6.74153` and it gives `0.5618` gas per round.
```
Name Gascost Time (ns) MGas/S Gasprice for 10MGas/S Gasprice for ECDSA eq
An average gas price of F precompile call with 1 round compared to ecRecover should have been `2.431701`. However, in this scenario the call cost would totally overshadow the dynamic cost anyway.
```
Name Gascost Time (ns) MGas/S Gasprice for 10MGas/S Gasprice for ECDSA eq
