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Remove unnecessary index shuffling #10

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Remove unnecessary index shuffling #10

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31 changes: 2 additions & 29 deletions src/index.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -174,31 +174,6 @@ function newCoefficients(intercept: number, degree: number): Readonly<Uint8Array
return coefficients;
}

// Creates a set of values from [1, 256).
// Returns a psuedo-random shuffling of the set.
function newCoordinates(): Readonly<Uint8Array> {
const coordinates = new Uint8Array(255);
for (let i = 0; i < 255; i++) {
coordinates[i] = i + 1;
}

// Pseudo-randomize the array of coordinates.
//
// This impl maps almost perfectly because both of the lists (coordinates and randomIndices)
// have a length of 255 and byte values are between 0 and 255 inclusive. The only value that
// does not map neatly here is if the random byte is 255, since that value used as an index
// would be out of bounds. Thus, for bytes whose value is 255, wrap around to 0.
const randomIndices = getRandomBytes(255);
for (let i = 0; i < 255; i++) {
const j = randomIndices[i]! % 255; // Make sure to handle the case where the byte is 255.
const temp = coordinates[i]!;
coordinates[i] = coordinates[j]!;
coordinates[j] = temp;
}

return coordinates;
}

// Helpers for declarative argument validation.
const AssertArgument = {
instanceOf(object: any, constructor: Function, message: string) {
Expand Down Expand Up @@ -256,11 +231,10 @@ export async function split(

const result: Uint8Array[] = [];
const secretLength = secret.byteLength;
const xCoordinates = newCoordinates();

for (let i = 0; i < shares; i++) {
const share = new Uint8Array(secretLength + 1);
share[secretLength] = xCoordinates[i]!;
share[secretLength] = i + 1;
result.push(share);
}

Expand All @@ -271,8 +245,7 @@ export async function split(
const coefficients = newCoefficients(byte, degree);

for (let j = 0; j < shares; ++j) {
const x = xCoordinates[j]!;
const y = evaluate(coefficients, x, degree);
const y = evaluate(coefficients, j + 1, degree);
result[j]![i] = y;
}
}
Expand Down