Struct curve25519_dalek::scalar::Scalar

pub struct Scalar(pub [u8; 32]);

The Scalar struct represents an element in ℤ/lℤ, where

l = 2²⁵² + 27742317777372353535851937790883648493

is the order of the basepoint. The Scalar is stored as bytes.

Methods

impl Scalar

fn random<T: Rng>(csprng: &mut T) -> Self

Return a Scalar chosen uniformly at random using a CSPRNG. Panics if the operating system's CSPRNG is unavailable.

Inputs

• cspring: any cryptographically secure PRNG which implements the rand::Rng interface.

Returns

A random scalar within ℤ/lℤ.

fn zero() -> Self

Construct the additive identity

fn one() -> Self

Construct the multiplicative identity

fn non_adjacent_form(&self) -> [i8; 256]

Compute a width-5 "Non-Adjacent Form" of this scalar.

A width-w NAF of a positive integer k is an expression k = sum(k[i]*2^i for i in range(l)), where each nonzero coefficient k[i] is odd and bounded by |k[i]| < 2^(w-1), k[l-1] is nonzero, and at most one of any w consecutive coefficients is nonzero. (Hankerson, Menezes, Vanstone; def 3.32).

Intuitively, this is like a binary expansion, except that we allow some coefficients to grow up to 2^(w-1) so that the nonzero coefficients are as sparse as possible.

fn to_radix_16(&self) -> [i8; 64]

Write this scalar in radix 16, with coefficients in [-8,8), i.e., compute a_i such that

a = a_0 + a_1*16¹ + ... + a_63*16^63,

with -8 ≤ a_i < 8 for 0 ≤ i < 63 and -8 ≤ a_63 ≤ 8.

Precondition: self[31] <= 127. This is the case whenever self is reduced.

fn multiply_add(a: &Scalar, b: &Scalar, c: &Scalar) -> Scalar

Compute ab+c (mod l). XXX should this exist, or should we just have Mul, Add etc impls that unpack and then call UnpackedScalar::multiply_add ?

fn reduce(input: &[u8; 64]) -> Scalar

Reduce a 512-bit little endian number mod l

Trait Implementations

impl Copy for Scalar

impl Clone for Scalar

fn clone(&self) -> Scalar

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Index<usize> for Scalar

type Output = u8

The returned type after indexing

fn index<'a>(&'a self, _index: usize) -> &'a u8

The method for the indexing (container[index]) operation

impl IndexMut<usize> for Scalar

fn index_mut<'a>(&'a mut self, _index: usize) -> &'a mut u8

The method for the mutable indexing (container[index]) operation

impl CTAssignable for Scalar

fn conditional_assign(&mut self, other: &Scalar, choice: u8)

Conditionally assign another Scalar to this one.

let a = Scalar([0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
                0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0]);
let b = Scalar([1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,
                1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1]);
let mut t = a;
t.conditional_assign(&b, 0u8);
assert!(t[0] == a[0]);
t.conditional_assign(&b, 1u8);
assert!(t[0] == b[0]);

Preconditions

• choice in {0,1}