computeNonce function

Uint8List computeNonce(Uint8List iv, int packetNumber)

Calculates the QUIC AEAD nonce. Nonce is computed as IV XOR (big-endian 64-bit Packet Number). The IV is 12 bytes. The 64-bit Packet Number is left-padded with zeros and XORed with the rightmost 8 bytes of the IV.

Implementation

Uint8List computeNonce(Uint8List iv, int packetNumber) {
  if (iv.length != 12) {
    throw ArgumentError("IV must be 12 bytes for QUIC AEAD.");
  }

  // Create a mutable copy of the IV
  final nonce = Uint8List.fromList(iv);

  // Create an 8-byte buffer for the packet number (64-bit, big-endian)
  final pnBuffer = Uint8List(8);

  // Use ByteData to safely write the 64-bit integer in Big Endian format.
  final byteData = ByteData(8);
  byteData.setUint64(0, packetNumber, Endian.big);

  // Copy to pnBuffer
  pnBuffer.setAll(0, byteData.buffer.asUint8List());

  // XOR the rightmost 8 bytes of the IV (indices 4-11) with the 8 bytes of pnBuffer.
  // The first 4 bytes of the 12-byte IV remain untouched.
  for (var i = 0; i < 8; i++) {
    // pnBuffer[i] is byte 0 to 7 of the 64-bit packet number
    // nonce[i + 4] is byte 4 to 11 of the 12-byte IV/Nonce
    nonce[i + 4] ^= pnBuffer[i];
  }

  return nonce;
}