Finish first pass of SECIO spec

secio/README.md

@@ -53,3 +53,108 @@ required to implement SECIO.
 ## Protocol
 
 ### Proposal Generation
+
+SECIO channel negotiation begins with a proposal phase. Both sides generate a
+proposal as follows:
+
+*Note: the public key should be serialized per the `Bytes` method from
+[go-libp2p-crypto](https://godoc.org/github.com/libp2p/go-libp2p-crypto#Key).*
+
+```
+Propose{
+  Rand: <16 secure random bytes>,
+  Pubkey: <public key bytes>,
+  Exchanges: <comma separated string of supported key exchanges>,
+  Ciphers: <comma separated string of supported ciphers>,
+  Hashes: <comma separated string of supported hashes>,
+}
+```
+
+Both peers serialize this message and send it over the wire. Upon deserializing
+their peer's message, they verify that the pubkey matches that described by the
+peer's peer ID (NOTE: this is sometimes only possible for the peer *initiating*
+the connection.) If the key doesn't match the peer ID, the peer can close the
+connection. If the peer doesn't know the remote peer's ID, they can compute and
+store the remote peer ID for later use.
+
+### Determining Roles and Algorithms
+
+Next, the peers use a deterministic formula to compute their roles in the coming
+exchanges. Each peer computes:
+
+```
+oh1 := sha256(concat(remotePeerPubKeyBytes, myNonce))
+oh2 := sha256(concat(myPubKeyBytes, remotePeerNonce))
+```
+
+With these hashes, determine which peer's preferences to favor. This peer will
+be referred to as the "preferred peer". If `oh1 == oh2`, then the peer is
+communicating with itself and should return an error. If `oh1 < oh2`, use the
+remote peer's preferences. If `oh1 > oh2`, prefer the local peer's preferences.
+
+Given our preference, we now sort through each of the `Exchanges`, `Ciphers`,
+and `Hashes` provided by both peers, selecting the first item from our preferred
+peer's set that is also shared by the other peer.
+
+### Key Exchange
+
+Now the peers prepare a key exchange. Both peers generate an ephemeral key based
+on the agreed upon exchange (currently support is only available for elliptic
+curve algorithms). Ephemeral keys are generated via
+[go-libp2p-crypto](https://godoc.org/github.com/libp2p/go-libp2p-crypto#GenerateEKeyPair).
+
+With keys generated, both peers create a `Exchange` message. First, they start by
+generating a "corpus" that they will sign.
+
+```
+corpus := concat(myProposalBytes, remotePeerProposalBytes, ephemeralPubKey)
+```
+
+Then, generate the `Exchange`:
+
+```
+Exchange{
+  Epubkey: <ephemeral pubkey>,
+  Signature: <sign corpus with local private key>,
+}
+```
+
+The peers serialize these and write them over the wire. Upon receiving the
+remote peer's `Exchange`, validate the signature by computing the `corpus` you
+expect them to have generated with their public key. Peers should close the
+connection if the signature does not validate.
+
+### Key Stretching
+
+Peers now generate their shared secret based on the function generated by the
+ephemeral key generation, passing it the remote peer's ephemeral key. With the
+shared secret generated, both peers stretch the key using the algorithm
+described by
+[go-libp2p-crypto](https://godoc.org/github.com/libp2p/go-libp2p-crypto#KeyStretcher).
+
+```
+k1, k2 := KeyStretcher(sharedSecret)
+```
+
+With `k1` and `k2` computed, swap the two values if the remote peer is the
+preferred peer. After swapping if necessary, `k1` becomes the local peer's key
+and `k2` the remote peer's key.
+
+Each peer now generates a MAC key and cipher for the remote and local keys
+generated in the previous step using the `MacKey` and `CipherKey` from the
+generated
+[`StretchedKeys`](https://godoc.org/github.com/libp2p/go-libp2p-crypto#StretchedKeys)
+objects.
+
+### Initiate Secure Channel
+
+With the cipher and HMAC signer created, the secure channel is ready to be
+opened. Each packet is of the form:
+
+```
+[uint32 length of packet | encrypted body | hmac signature of encrypted body]
+```
+
+The first packet transmitted by each peer must be the remote peer's nonce. Peers
+validate that the remote peer sent them their nonce, closing if unsuccessful.
+Otherwise, a secure channel has been successfully opened.