while (entropyBin.length < expectedBits) {
entropyBin = "0" + entropyBin;
}
+ // Calculate the number of bits per event
+ var bitsPerEvent = Math.log2(base.asInt);
// Cards binary must be handled differently, since they're not replaced
if (base.asInt == 52) {
- entropyBin = getCardBinary(base.parts);
+ var cardEntropy = processCardEntropy(base.parts);
+ entropyBin = cardEntropy.binaryStr;
+ bitsPerEvent = cardEntropy.bitsPerEvent;
}
// Supply a 'filtered' entropy string for display purposes
var entropyClean = base.parts.join("");
binaryStr: entropyBin,
cleanStr: entropyClean,
cleanHtml: entropyHtml,
+ bitsPerEvent: bitsPerEvent,
base: base,
}
return e;
// total possible entropy is measured using n!, not base^n.
// eg the second last card can be only one of two, not one of fifty two
// so the added entropy for that card is only one bit at most
- function getCardBinary(cards) {
+ function processCardEntropy(cards) {
// Track how many instances of each card have been used, and thus
// how many decks are in use.
var cardCounts = {};
}
// Work out the total number of bits for this many decks
// See http://crypto.stackexchange.com/q/41886
- var gainedBits = Math.log2(factorial(52 * numberOfDecks));
+ var gainedBits = 0;
+ // Equivalent of Math.log2(factorial(52*numberOfDecks))
+ // which becomes infinity for numberOfDecks > 4
+ for (var i=1; i<=52*numberOfDecks; i++) {
+ gainedBits = gainedBits + Math.log2(i);
+ }
var lostBits = 52 * Math.log2(factorial(numberOfDecks));
var maxBits = gainedBits - lostBits;
// Convert the drawn cards to a binary representation.
// Create a normalized string of the selected cards
var normalizedCards = cards.join("").toUpperCase();
// Convert to binary using the SHA256 hash of the normalized cards.
- // If the number of bits is more than 256, multiple rounds of hashing
+ // If the number of bits is more than 256, multiple hashes
// are used until the required number of bits is reached.
var entropyBin = "";
var iterations = 0;
while (entropyBin.length < numberOfBits) {
- var hashedCards = sjcl.hash.sha256.hash(normalizedCards);
- for (var j=0; j<iterations; j++) {
- hashedCards = sjcl.hash.sha256.hash(hashedCards);
- }
+ var hashedCards = sjcl.hash.sha256.hash(normalizedCards + ":" + iterations);
var hashHex = sjcl.codec.hex.fromBits(hashedCards);
for (var i=0; i<hashHex.length; i++) {
var decimal = parseInt(hashHex[i], 16);
}
// Truncate to the appropriate number of bits.
entropyBin = entropyBin.substring(0, numberOfBits);
- return entropyBin;
+ // Get the number of bits per event
+ bitsPerEvent = maxBits / totalCards;
+ return {
+ binaryStr: entropyBin,
+ bitsPerEvent: bitsPerEvent,
+ }
}
// Polyfill for Math.log2