/**
* Convert a list of words to a byte string
* that has the same binary representation with a checksum
* @param array $words
* @return string
*/
public function toEntropy(array $words)
{
// collect up words, compact into bit array
$tmp = new BitArray([]);
for ($i = 0; $i < count($words); $i++) {
$wordIndex = array_search($words[$i], $this->words);
for ($j = 0; $j < 11; $j++) {
$tmp[$i * 11 + $j] = ($wordIndex & 1 << 10 - $j) != 0;
}
}
$concatLenBits = count($tmp);
$checksumLengthBits = $concatLenBits / 33;
$entropyLengthBits = $concatLenBits - $checksumLengthBits;
$entropy = $tmp->slice(0, $entropyLengthBits)->toBytes();
$hashBits = new BitArray(hash("sha256", $entropy, true));
$checksum = $tmp->slice($entropyLengthBits, $checksumLengthBits);
if ($checksum != $hashBits->slice(0, $checksumLengthBits)) {
throw new \RuntimeException("Invalid checksum");
}
return $entropy;
}
