# bls12_381_plus [![Crates.io](https://img.shields.io/crates/v/bls12_381_plus.svg)](https://crates.io/crates/bls12_381_plus) # This crate provides an implementation of the BLS12-381 pairing-friendly elliptic curve construction with [hash to curve](https://www.ietf.org/archive/id/draft-irtf-cfrg-hash-to-curve-10.html) and multiexponentiation methods like Straus and Pippenger. * **This implementation has not been reviewed or audited. Use at your own risk.** * This implementation targets Rust `1.56` or later. * This implementation does not require the Rust standard library. * All operations are constant time unless explicitly noted. ## Features * `bits` (on by default): Enables APIs for obtaining bit iterators for scalars. * `groups` (on by default): Enables APIs for performing group arithmetic with G1, G2, and GT. * `pairings` (on by default): Enables some APIs for performing pairings. * `hashing` (on by default): Enables hash to curve methods as defined by [IETF](https://datatracker.ietf.org/doc/draft-irtf-cfrg-hash-to-curve/?include_text=1). * `alloc` (on by default): Enables APIs that require an allocator; these include pairing optimizations. * `nightly`: Enables `subtle/nightly` which tries to prevent compiler optimizations that could jeopardize constant time operations. Requires the nightly Rust compiler. * `experimental`: Enables experimental features. These features have no backwards-compatibility guarantees and may change at any time; users that depend on specific behaviour should pin an exact version of this crate. The current list of experimental features: * Hashing to curves ([Internet Draft v12](https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-hash-to-curve-12)) ## [Documentation](https://docs.rs/bls12_381) ## Testing Test for non-wasm32 targets: ```sh cargo test ``` Test wasm32 targets: ```sh cargo test --target wasm32-unknown-unknown ``` ## Curve Description BLS12-381 is a pairing-friendly elliptic curve construction from the [BLS family](https://eprint.iacr.org/2002/088), with embedding degree 12. It is built over a 381-bit prime field `GF(p)` with... * z = `-0xd201000000010000` * p = (z - 1)2(z4 - z2 + 1) / 3 + z * = `0x1a0111ea397fe69a4b1ba7b6434bacd764774b84f38512bf6730d2a0f6b0f6241eabfffeb153ffffb9feffffffffaaab` * q = z4 - z2 + 1 * = `0x73eda753299d7d483339d80809a1d80553bda402fffe5bfeffffffff00000001` ... yielding two **source groups** G1 and G2, each of 255-bit prime order `q`, such that an efficiently computable non-degenerate bilinear pairing function `e` exists into a third **target group** GT. Specifically, G1 is the `q`-order subgroup of E(Fp) : y2 = x3 + 4 and G2 is the `q`-order subgroup of E'(Fp2) : y2 = x3 + 4(u + 1) where the extension field Fp2 is defined as Fp(u) / (u2 + 1). BLS12-381 is chosen so that `z` has small Hamming weight (to improve pairing performance) and also so that `GF(q)` has a large 232 primitive root of unity for performing radix-2 fast Fourier transforms for efficient multi-point evaluation and interpolation. It is also chosen so that it exists in a particularly efficient and rigid subfamily of BLS12 curves. ### Curve Security Pairing-friendly elliptic curve constructions are (necessarily) less secure than conventional elliptic curves due to their small "embedding degree". Given a small enough embedding degree, the pairing function itself would allow for a break in DLP hardness if it projected into a weak target group, as weaknesses in this target group are immediately translated into weaknesses in the source group. In order to achieve reasonable security without an unreasonably expensive pairing function, a careful choice of embedding degree, base field characteristic and prime subgroup order must be made. BLS12-381 uses an embedding degree of 12 to ensure fast pairing performance but a choice of a 381-bit base field characteristic to yield a 255-bit subgroup order (for protection against [Pollard's rho algorithm](https://en.wikipedia.org/wiki/Pollard%27s_rho_algorithm)) while reaching close to a 128-bit security level. There are [known optimizations](https://ellipticnews.wordpress.com/2016/05/02/kim-barbulescu-variant-of-the-number-field-sieve-to-compute-discrete-logarithms-in-finite-fields/) of the [Number Field Sieve algorithm](https://en.wikipedia.org/wiki/General_number_field_sieve) which could be used to weaken DLP security in the target group by taking advantage of its structure, as it is a multiplicative subgroup of a low-degree extension field. However, these attacks require an (as of yet unknown) efficient algorithm for scanning a large space of polynomials. Even if the attack were practical it would only reduce security to roughly 117 to 120 bits. (This contrasts with 254-bit BN curves which usually have less than 100 bits of security in the same situation.) ### Alternative Curves Applications may wish to exchange pairing performance and/or G2 performance by using BLS24 or KSS16 curves which conservatively target 128-bit security. In applications that need cycles of elliptic curves for e.g. arbitrary proof composition, MNT6/MNT4 curve cycles are known that target the 128-bit security level. In applications that only need fixed-depth proof composition, curves of this form have been constructed as part of Zexe. ## Acknowledgements Please see `Cargo.toml` for a list of primary authors of this codebase. ## License Licensed under either of * Apache License, Version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0) * MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT) 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.