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Don't consider multiples of 3 and 5

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This commit is contained in:
KAYLukas 2018-03-06 23:11:49 +01:00
parent 3c420312a0
commit 389d6410c9
1 changed files with 18 additions and 11 deletions
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29
src/crypto/public_key/prime.js
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@ -39,20 +39,27 @@ export default {
*/
async function randomProbablePrime(bits, e, k) {
const min = new BN(1).shln(bits - 1);
const thirty = new BN(30);
/*
* We can avoid any multiples of 3 and 5 by looking at n mod 30
* n mod 30 = 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
* the next possible prime is mod 30:
* 1 7 7 7 7 7 7 11 11 11 11 13 13 17 17 17 17 19 19 23 23 23 23 29 29 29 29 29 29 1
*/
const adds = [1, 6, 5, 4, 3, 2, 1, 4, 3, 2, 1, 2, 1, 4, 3, 2, 1, 2, 1, 4, 3, 2, 1, 6, 5, 4, 3, 2, 1, 2];
let n = await random.getRandomBN(min, min.shln(1));
if (n.isEven()) {
n.iaddn(1); // force odd
}
let i = n.mod(thirty).toNumber();
// eslint-disable-next-line no-await-in-loop
while (!await isProbablePrime(n, e, k)) {
n.iaddn(2);
// If reached the maximum, go back to the minimum.
if (n.bitLength() > bits) {
n = n.mod(min.shln(1)).iadd(min);
}
}
do {
n.iaddn(adds[i]);
i = (i + adds[i]) % adds.length;
// If reached the maximum, go back to the minimum.
if (n.bitLength() > bits) {
n = n.mod(min.shln(1)).iadd(min);
}
// eslint-disable-next-line no-await-in-loop
} while (!await isProbablePrime(n, e, k));
return n;
}