Only increment nonce for valid messages

[dapplion]

When receiving a message we increment the nonce before validating message integrity

js-libp2p-noise/src/handshakes/abstract-handshake.ts

Lines 24 to 29 in 78921b9

	public decryptWithAd (cs: CipherState, ad: Uint8Array, ciphertext: Uint8Array): {plaintext: bytes, valid: boolean} {
	const { plaintext, valid } = this.decrypt(cs.k, cs.n, ad, ciphertext)
	cs.n.increment()
	return { plaintext, valid }
	}

And attacker could fill nonce spots and break comms if it can MITM the connection

[mpetrunic]

The whole point of noise is to prevent MITM. And even if somebody succeeds it can only reset con with that one peer he managed to corrupt so it isn't really important I guess?

[wemeetagain]

Tracing thru the code it seems that at most a single message can decrypt incorrectly before the stream is reset.
two cases:

1. during handshake, the handshake throws, https://github.com/ChainSafe/js-libp2p-noise/blob/master/src/handshake-xx.ts L74, L88, L126
2. after handshake, during streaming, iterable throws, https://github.com/ChainSafe/js-libp2p-noise/blob/master/src/crypto/streaming.ts L34