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crypto: fix randomInt bias #36894

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crypto: fix randomInt bias #36894

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crypto: fix randomInt bias 
Co-authored-by: Andrey Pechkurov <apechkurov@gmail.com>
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@tniessen @puzpuzpuz
tniessen and puzpuzpuz committed Jan 12, 2021
commit b0d845f8aca42d8720e90c9fc29294c89790ac53
19 changes: 9 additions & 10 deletions lib/internal/crypto/random.js
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Original file line number Diff line number Diff line change
Expand Up @@ -218,30 +218,29 @@ function randomInt(min, max, callback) {
`<= ${RAND_MAX}`, range);
}

const excess = RAND_MAX % range;
const randLimit = RAND_MAX - excess;
// For (x % range) to produce an unbiased value greater than or equal to 0 and
// less than range, x must be drawn randomly from the set of integers greater
// than or equal to 0 and less than randLimit.
const randLimit = RAND_MAX - (RAND_MAX % range);

if (isSync) {
// Sync API
while (true) {
const x = randomBytes(6).readUIntBE(0, 6);
// If x > (maxVal - (maxVal % range)), we will get "modulo bias"
if (x > randLimit) {
// Try again
if (x >= randLimit) {
// Try again.
continue;
}
const n = (x % range) + min;
return n;
return (x % range) + min;
}
} else {
// Async API
const pickAttempt = () => {
randomBytes(6, (err, bytes) => {
if (err) return callback(err);
const x = bytes.readUIntBE(0, 6);
// If x > (maxVal - (maxVal % range)), we will get "modulo bias"
if (x > randLimit) {
// Try again
if (x >= randLimit) {
// Try again.
return pickAttempt();
}
const n = (x % range) + min;
Expand Down