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Simplify the way to compute the remainder modulo 3 in PDF20Hash function

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I noticed the 256 % 3 (which is equal to 1) so I slighty simplify the code.
The sum of the 16 Uint8 doesn't exceed 2^12, hence we can just take the
sum modulo 3.
This commit is contained in:
Calixte Denizet 2022-10-07 14:38:39 +02:00
parent 8629a55215
commit c0e165bf97
1 changed files with 5 additions and 9 deletions
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14
src/core/crypto.js
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@ -1304,15 +1304,11 @@ const PDF20 = (function PDF20Closure() {
e = cipher.encrypt(k1, k.subarray(16, 32)); e = cipher.encrypt(k1, k.subarray(16, 32));
// Now we have to take the first 16 bytes of an unsigned big endian // Now we have to take the first 16 bytes of an unsigned big endian
// integer and compute the remainder modulo 3. That is a fairly large // integer and compute the remainder modulo 3. That is a fairly large
// number and JavaScript isn't going to handle that well, so we're using // number and JavaScript isn't going to handle that well.
// a trick that allows us to perform modulo math byte by byte. // The number is e0 + 256 * e1 + 256^2 * e2... and 256 % 3 === 1, hence
let remainder = 0; // the powers of 256 are === 1 modulo 3 and finally the number modulo 3
for (let z = 0; z < 16; z++) { // is equal to the remainder modulo 3 of the sum of the e_n.
remainder *= 256 % 3; const remainder = e.slice(0, 16).reduce((a, b) => a + b, 0) % 3;
remainder %= 3;
remainder += (e[z] >>> 0) % 3;
remainder %= 3;
}
if (remainder === 0) { if (remainder === 0) {
k = calculateSHA256(e, 0, e.length); k = calculateSHA256(e, 0, e.length);
} else if (remainder === 1) { } else if (remainder === 1) {