function fromDER(x) {
  if (x[0] === 0x00) x = x.slice(1);
  const buffer = new Uint8Array(32);
  const bstart = Math.max(0, 32 - x.length);
  buffer.set(x, bstart);
  return buffer;
}
Converts a DER-encoded signature to a buffer.
If the first byte of the input buffer is 0x00, it is skipped.
The resulting buffer is 32 bytes long, filled with zeros if necessary.
@param x - The DER-encoded signature.
@returns The converted buffer.