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Просмотр файла: montgomery.js
"use strict";
Object.defineProperty(exports, "__esModule", { value: true });
exports.montgomery = void 0;
/*! noble-curves - MIT License (c) 2022 Paul Miller (paulmillr.com) */
const modular_js_1 = require("./modular.js");
const utils_js_1 = require("./utils.js");
const _0n = BigInt(0);
const _1n = BigInt(1);
function validateOpts(curve) {
(0, utils_js_1.validateObject)(curve, {
a: 'bigint',
}, {
montgomeryBits: 'isSafeInteger',
nByteLength: 'isSafeInteger',
adjustScalarBytes: 'function',
domain: 'function',
powPminus2: 'function',
Gu: 'bigint',
});
// Set defaults
return Object.freeze({ ...curve });
}
// NOTE: not really montgomery curve, just bunch of very specific methods for X25519/X448 (RFC 7748, https://www.rfc-editor.org/rfc/rfc7748)
// Uses only one coordinate instead of two
function montgomery(curveDef) {
const CURVE = validateOpts(curveDef);
const { P } = CURVE;
const modP = (n) => (0, modular_js_1.mod)(n, P);
const montgomeryBits = CURVE.montgomeryBits;
const montgomeryBytes = Math.ceil(montgomeryBits / 8);
const fieldLen = CURVE.nByteLength;
const adjustScalarBytes = CURVE.adjustScalarBytes || ((bytes) => bytes);
const powPminus2 = CURVE.powPminus2 || ((x) => (0, modular_js_1.pow)(x, P - BigInt(2), P));
// cswap from RFC7748. But it is not from RFC7748!
/*
cswap(swap, x_2, x_3):
dummy = mask(swap) AND (x_2 XOR x_3)
x_2 = x_2 XOR dummy
x_3 = x_3 XOR dummy
Return (x_2, x_3)
Where mask(swap) is the all-1 or all-0 word of the same length as x_2
and x_3, computed, e.g., as mask(swap) = 0 - swap.
*/
function cswap(swap, x_2, x_3) {
const dummy = modP(swap * (x_2 - x_3));
x_2 = modP(x_2 - dummy);
x_3 = modP(x_3 + dummy);
return [x_2, x_3];
}
// Accepts 0 as well
function assertFieldElement(n) {
if (typeof n === 'bigint' && _0n <= n && n < P)
return n;
throw new Error('Expected valid scalar 0 < scalar < CURVE.P');
}
// x25519 from 4
// The constant a24 is (486662 - 2) / 4 = 121665 for curve25519/X25519
const a24 = (CURVE.a - BigInt(2)) / BigInt(4);
/**
*
* @param pointU u coordinate (x) on Montgomery Curve 25519
* @param scalar by which the point would be multiplied
* @returns new Point on Montgomery curve
*/
function montgomeryLadder(pointU, scalar) {
const u = assertFieldElement(pointU);
// Section 5: Implementations MUST accept non-canonical values and process them as
// if they had been reduced modulo the field prime.
const k = assertFieldElement(scalar);
const x_1 = u;
let x_2 = _1n;
let z_2 = _0n;
let x_3 = u;
let z_3 = _1n;
let swap = _0n;
let sw;
for (let t = BigInt(montgomeryBits - 1); t >= _0n; t--) {
const k_t = (k >> t) & _1n;
swap ^= k_t;
sw = cswap(swap, x_2, x_3);
x_2 = sw[0];
x_3 = sw[1];
sw = cswap(swap, z_2, z_3);
z_2 = sw[0];
z_3 = sw[1];
swap = k_t;
const A = x_2 + z_2;
const AA = modP(A * A);
const B = x_2 - z_2;
const BB = modP(B * B);
const E = AA - BB;
const C = x_3 + z_3;
const D = x_3 - z_3;
const DA = modP(D * A);
const CB = modP(C * B);
const dacb = DA + CB;
const da_cb = DA - CB;
x_3 = modP(dacb * dacb);
z_3 = modP(x_1 * modP(da_cb * da_cb));
x_2 = modP(AA * BB);
z_2 = modP(E * (AA + modP(a24 * E)));
}
// (x_2, x_3) = cswap(swap, x_2, x_3)
sw = cswap(swap, x_2, x_3);
x_2 = sw[0];
x_3 = sw[1];
// (z_2, z_3) = cswap(swap, z_2, z_3)
sw = cswap(swap, z_2, z_3);
z_2 = sw[0];
z_3 = sw[1];
// z_2^(p - 2)
const z2 = powPminus2(z_2);
// Return x_2 * (z_2^(p - 2))
return modP(x_2 * z2);
}
function encodeUCoordinate(u) {
return (0, utils_js_1.numberToBytesLE)(modP(u), montgomeryBytes);
}
function decodeUCoordinate(uEnc) {
// Section 5: When receiving such an array, implementations of X25519
// MUST mask the most significant bit in the final byte.
const u = (0, utils_js_1.ensureBytes)('u coordinate', uEnc, montgomeryBytes);
if (fieldLen === 32)
u[31] &= 127; // 0b0111_1111
return (0, utils_js_1.bytesToNumberLE)(u);
}
function decodeScalar(n) {
const bytes = (0, utils_js_1.ensureBytes)('scalar', n);
const len = bytes.length;
if (len !== montgomeryBytes && len !== fieldLen)
throw new Error(`Expected ${montgomeryBytes} or ${fieldLen} bytes, got ${len}`);
return (0, utils_js_1.bytesToNumberLE)(adjustScalarBytes(bytes));
}
function scalarMult(scalar, u) {
const pointU = decodeUCoordinate(u);
const _scalar = decodeScalar(scalar);
const pu = montgomeryLadder(pointU, _scalar);
// The result was not contributory
// https://cr.yp.to/ecdh.html#validate
if (pu === _0n)
throw new Error('Invalid private or public key received');
return encodeUCoordinate(pu);
}
// Computes public key from private. By doing scalar multiplication of base point.
const GuBytes = encodeUCoordinate(CURVE.Gu);
function scalarMultBase(scalar) {
return scalarMult(scalar, GuBytes);
}
return {
scalarMult,
scalarMultBase,
getSharedSecret: (privateKey, publicKey) => scalarMult(privateKey, publicKey),
getPublicKey: (privateKey) => scalarMultBase(privateKey),
utils: { randomPrivateKey: () => CURVE.randomBytes(CURVE.nByteLength) },
GuBytes: GuBytes,
};
}
exports.montgomery = montgomery;
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