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noir-bignum

[!WARNING]
If you're updating from release 0.3.7 to 0.4.0, please read these migration notes, because there are significant breaking changes.

An optimized big number library for Noir.

noir-bignum evaluates modular arithmetic for large integers of any length.

BigNum instances are parametrised by a struct that satisfies BigNumParamsTrait.

Multiplication operations for a 2048-bit prime field cost approx. 930 gates.

bignum can evaluate large integer arithmetic by defining a modulus() that is a power of 2.

Benchmarks

TODO

Dependencies

Refer to Noir's docs and Barretenberg's docs for installation steps.

Installation

In your Nargo.toml file, add the version of this library you would like to install under dependency:

[dependencies]
bignum = { tag = "v0.2.2", git = "https://github.com/noir-lang/noir-bignum" }

Import

Add imports at the top of your Noir code, for example:

use dep::bignum::fields::U256::U256Params;
use dep::bignum::BigNum;

bignum

BigNum members are represented as arrays of 120-bit limbs. The number of 120-bit limbs required to represent a given BigNum object must be defined at compile-time.

If your field moduli is also known at compile-time, use the BigNumTrait definition in lib.nr

BigNum struct

Big numbers are instantiated with the BigNum struct:

struct BigNum<let N: u32, let MOD_BITS: u32, Params> {
    limbs: [Field; N]
}

Usage

Example

A simple 1 + 2 = 3 check in 256-bit unsigned integers:

use dep::bignum::fields::U256::U256Params;
use dep::bignum::BigNum;

type U256 = BigNum<3, 257, U256Params>;

fn main() {
    let one: U256 = BigNum::from_array([1, 0, 0]);
    let two: U256 = BigNum::from_array([2, 0, 0]);
    let three: U256 = BigNum::from_array([3, 0, 0]);
    assert((one + two) == three);
}

Methods

TODO: Document all available methods

Moduli presets

Big Unsigned Integers

BigNum supports operations over unsigned integers, with predefined types for 256, 384, 512, 768, 1024, 2048, 4096 and 8192 bit integers.

All arithmetic operations are supported including integer div and mod functions (udiv, umod). Bit shifts and comparison operators are not yet implemented.

e.g.

use dep::bignum::fields::U256::U256Params;
use dep::bignum::BigNum;

type U256 = BigNum<3, 257, U256Params>;

fn foo(x: U256, y: U256) -> U256 {
    x.udiv(y)
}
Fields

BigNum::fields contains BigNumParams for common fields.

Feature requests and/or pull requests welcome for missing fields you need.

TODO: Document existing field presets (e.g. bls, ed25519, secp256k1)

RuntimeBigNum

If your field moduli is not known at compile-time (e.g. RSA verification), use the RuntimeBigNum struct defined in runtime_bignum.nr: runtime_bignum::RuntimeBigNum.

use dep::bignum::fields::bn254Fq::BN254_Fq_Params;

// Notice how we don't provide the params here, because we're pretending they're
// not known at compile-time, for illustration purposes.
type My_RBN = RuntimeBigNum<3, 254>;

fn main() {
    let params = BN254_Fq_Params::get_params(); // or some other params known at runtime.

    // Notice how we feed the params in, because we're pretending they're not
    // known at compile-time.
    let one: My_RBN = RuntimeBigNum::from_array(params, [1, 0, 0]);
    let two: My_RBN = RuntimeBigNum::from_array(params, [2, 0, 0]);
    let three: My_RBN = RuntimeBigNum::from_array(params, [3, 0, 0]);

    assert((one + two) == three);
}

Types

User-facing structs:

BigNum: big numbers whose parameters are all known at compile-time.

RuntimeBigNum: big numbers whose parameters are only known at runtime. (Note: the number of bits of the modulus of the bignum must be known at compile-time).

If creating custom bignum params:

BigNumParams is needed, to declare your params. These parameters (modulus, redc_param) can be provided at runtime via witnesses (e.g. RSA verification). The redc_param is only used in unconstrained functions and does not need to be derived from modulus in-circuit.

BigNumParamsGetter is a convenient wrapper around params, which is needed if declaring a new type of BigNum.

Methods

Arithmetics

Basic expressions can be evaluated using the BigNum and RuntimeBigNum operators +,-,*,/. However, when evaluating relations (up to degree 2) that are more complex than single operations, the static methods BigNum::evaluate_quadratic_expression or RuntimeBigNum::evaluate_quadratic_expression are much more efficient (due to needing only a single modular reduction).

Unconstrained arithmetics

Unconstrained functions __mul, __add, __sub, __div, __pow etc. can be used to compute witnesses that can then be fed into BigNumInstance::evaluate_quadratic_expression.

Note: __div, __pow and div are expensive due to requiring modular exponentiations during witness computation. It is worth modifying witness generation algorithms to minimize the number of modular exponentiations required. (for example, using batch inverses).

e.g. if we wanted to compute (a + b) * c + (d - e) * f = g by evaluating the above example, g can be derived via:

let a: BigNumInstance<3, 254, BN254_Fq_Params> = BigNum::new();
let t0 = c.__mul(a.__add(b));
let t1 = f.__mul(d.__sub(e));
let g = bn.__add(t0, t1);

then the values can be arranged and fed-into evaluate_quadratic_expression.

See bignum_test.nr and runtime_bignum_test.nr for more examples.

evaluate_quadratic_expression

The method evaluate_quadratic_expression has the following interface:

    fn evaluate_quadratic_expression<let LHS_N: u64, let RHS_N: u64, let NUM_PRODUCTS: u64, let ADD_N: u64>(
        self,
        lhs_terms: [[BN; LHS_N]; NUM_PRODUCTS],
        lhs_flags: [[bool; LHS_N]; NUM_PRODUCTS],
        rhs_terms: [[BN; RHS_N]; NUM_PRODUCTS],
        rhs_flags: [[bool; RHS_N]; NUM_PRODUCTS],
        linear_terms: [BN; ADD_N],
        linear_flags: [bool; ADD_N]
    );

NUM_PRODUCTS represents the number of multiplications being summed (e.g. for a*b + c*d == 0, NUM_PRODUCTS = 2).

LHS_N, RHS_N represents the number of BigNum objects being summed in the left and right operands of each product. For example, for (a + b) * c + (d + e) * f == 0, LHS_N = 2, RHS_N = 1.

ADD_N represents the number of BigNum objects being added into the product (e.g. for a * b + c + d == 0, ADD_N = 2).

The flag parameters lhs_flags, rhs_flags, add_flags define whether an operand in the expression will be negated. For example, for (a + b) * c + (d - e) * f - g == 0, we would have:

let lhs_terms = [[a, b], [d, e]];
let lhs_flags = [[false, false], [false, true]];
let rhs_terms = [[c], [f]];
let rhs_flags = [[false], [false]];
let add_terms = [g];
let add_flags = [true];
BigNum::evaluate_quadratic_expresson(lhs_terms, lhs_flags, rhs_terms, rhs_flags, linear_terms, linear_flags);
TODO: Document other available methods

Deriving BigNumParams parameters: modulus, redc_param

For common fields, BigNumParams parameters can be pulled from the presets in bignum/fields/.

For other moduli (e.g. those used in RSA verification), both modulus and redc_param must be computed and formatted according to the following speficiations:

modulus represents the BigNum modulus, encoded as an array of Field elements that each encode 120 bits of the modulus. The first array element represents the least significant 120 bits.

redc_param is equal to (1 << (2 * Params::modulus_bits())) / modulus . This must be computed outside of the circuit and provided either as a private witness or hardcoded constant. (computing it via an unconstrained function would be very expensive until noir witness computation times improve)

double_modulus is derived via the method compute_double_modulus in runtime_bignum.nr. If you want to provide this value as a compile-time constant (see fields/bn254Fq.nr for an example), follow the algorithm compute_double_modulus as this parameter is not structly 2 * modulus. Each limb except the most significant limb borrows 2^120 from the next most significant limb. This ensure that when performing limb subtractions double_modulus.limbs[i] - x.limbs[i], we know that the result will not underflow.

BigNumParams parameters can be derived from a known modulus using the rust crate noir-bignum-paramgen (https://crates.io/crates/noir-bignum-paramgen)

Additional usage examples

use dep::bignum::fields::bn254Fq::BN254_Fq_Params;

use dep::bignum::BigNum;

type Fq = BigNum<3, 254, BN254_Fq_Params>;

fn example_mul(Fq a, Fq b) -> Fq {
    a * b
}

fn example_ecc_double(Fq x, Fq y) -> (Fq, Fq) {
    // Step 1: construct witnesses
    // lambda = 3*x*x / 2y
    let mut lambda_numerator = x.__mul(x);
    lambda_numerator = lambda_numerator.__add(lambda_numerator.__add(lambda_numerator));
    let lambda_denominator = y.__add(y);
    let lambda = lambda_numerator / lambda_denominator;
    // x3 = lambda * lambda - x - x
    let x3 = lambda.__mul(lambda).__sub(x.__add(x));
    // y3 = lambda * (x - x3) - y
    let y3 = lambda.__mul(x.__sub(x3)).__sub(y);

    // Step 2: constrain witnesses to be correct using minimal number of modular reductions (3)
    // 2y * lambda - 3*x*x = 0
    BigNum::evaluate_quadratic_expression(
        [[lambda]],
        [[false]],
        [[y,y]],
        [[false, false]],
        [x,x,x],
        [true, true, true]
    );
    // lambda * lambda - x - x - x3 = 0
    BigNum::evaluate_quadratic_expression(
        [[lambda]],
        [[false]],
        [[lambda]],
        [[false]],
        [x3,x,x],
        [true, true, true]
    );
    // lambda * (x - x3) - y = 0
     BigNum::evaluate_quadratic_expression(
        [[lambda]],
        [[false]],
        [[x, x3]],
        [[false, true]],
        [y],
        [true]
    );
    (x3, y3)
}