PKCS7 Padding Validation
Cryptopals Set 2 Challenge 15 // no repo for this challenge // no demo for this challenge
Step 1: We’ve Already Done This (but let’s talk abt it)
The challenge is to write a function that takes a string and determines if it is PKCS#7 (doc) padded. If it is, then we remove the padding; if it is not, we throw an error.
function chopPKCS7(padArr) {
// accepts padded hex array, returns non-padded array
if (!Array.isArray(padArr) || !padArr.length) {
return null;
}
let last = Number(parseInt(padArr[padArr.length - 1], 16).toString(10));
if (last > 0 && last < 256) {
let possibleSlice = padArr.slice(padArr.length - last);
let lastCh = padArr[padArr.length - 1];
if (last >= padArr.length) {
throw new Error(
"Padding >= array length! Are you sure you wanna do that?"
);
} else if (!possibleSlice.every((c) => c == lastCh)) {
throw new Error("String is not PKCS7-padded.");
}
return padArr.slice(0, padArr.length - last);
}
return padArr;
}
This function takes an array and checks the last character. It converts that character into a number and, if that number is between 0 and 256, it checks with every (mdn docs) to see if that number of characters (possibleSlice) from the end of the string are all equal to the same value: namely, the length of the padding characters. If they are, then the numbers are removed and the chopped array is returned. If they aren’t, then an error is thrown. Done!