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Use modulo arithmetic instead

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also I think this was just wrong in that it was subtracting 1
unnercessarily because we already used < rather than <= below.
This commit is contained in:
David Baker
2025-01-21 09:50:55 +00:00
parent 1f1d6f0bc8
commit f13967aaec
1 changed files with 3 additions and 2 deletions
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5
src/randomstring.ts
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@@ -74,7 +74,8 @@ export function secureRandomStringFrom(len: number, chars: string): string {
// this as we can't possibly map them onto the character set while keeping each character equally // this as we can't possibly map them onto the character set while keeping each character equally
// likely to be chosen (minus 1 to convert to indices in a string). (Essentially, we're using a d8 // likely to be chosen (minus 1 to convert to indices in a string). (Essentially, we're using a d8
// to choose between 7 possibilities and re-rolling on an 8, keeping all 7 outcomes equally likely.) // to choose between 7 possibilities and re-rolling on an 8, keeping all 7 outcomes equally likely.)
const maxRandValue = Math.floor(255 / chars.length) * chars.length - 1; // Our random values must be strictly less than this
const randomValueCutoff = 256 - (256 % chars.length);
// Grab 30% more entropy than we need. This should be enough that we can discard the values that are // Grab 30% more entropy than we need. This should be enough that we can discard the values that are
// too high without having to go back and grab more unless we're super unlucky. // too high without having to go back and grab more unless we're super unlucky.
@@ -92,7 +93,7 @@ export function secureRandomStringFrom(len: number, chars: string): string {
const randomByte = entropyBuffer[entropyBufferPos++]; const randomByte = entropyBuffer[entropyBufferPos++];
if (randomByte < maxRandValue) { if (randomByte < randomValueCutoff) {
result.push(chars[randomByte % chars.length]); result.push(chars[randomByte % chars.length]);
} }
} }