ts_tunnel: add replay protection for incoming packets

[danderson]

Rejects packets for an established session that have excessively old counter values. To allow for some packet reordering on the wire, we maintain a sliding window of acceptable counter values, and track already received counter values in that window with a bit ring.

Fixes #33

Change-Id: Ib71e5ea78078bb2e84314bc0f2fee94e6a6a6964

---

Adding the replay window made the receive session state a fair bit larger, which tripped a clippy check for unbalanced enum variants. I dealt with this by boxing the receive sessions, but it feels a little ad-hoc to me. Suggestions on better layouts welcome.

[npry]

lgtm with the panic doc, the other comments are nonblocking

[npry]

Nit: I would typically use Formatter::debug_struct here, since it handles and forwards additional formatting options automatically — you can get the indented pretty-printed repr with the {:#?} specifier:

Suggested change

	fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
	writeln!(f, "ReplayWindow{{\n last: {}\n bits:", self.last)?;
	for block in self.blocks {
	write!(f, " ")?;
	for octet in 0usize..8 {
	let v = ((block >> (octet * 8)) & 0xff) as u8;
	write!(f, "{:08b} ", v.reverse_bits())?;
	}
	writeln!(f)?;
	}
	write!(f, "}}")
	}
	fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
	struct BlockFormatter<'b>(&'b [u8]);
	impl Debug for BlockFormatter {
	fn fmt(&self, f: &mut std::fmt::Formatter<'_> -> std::fmt::Result {
	for b in self.0 {
	f.write("{:08b} ", b.reverse_bits())?;
	}
	Ok(())
	}
	}
	f.debug_struct("ReplayWindow")
	.field("last", &self.last)
	.field("bits", &BitsFormatter(&self.blocks))
	.finish()
	}

[npry]

Also document the previously-set case

[npry]

Is the Box because the session size is large due to the contained bitset? afaik this lives on the heap anyway — is there a copy/clone somewhere that would otherwise be expensive?