path: root/p2p/crypto.go

package p2p

import (
    "crypto/ecdsa"
    "crypto/rand"
    "fmt"
    "io"

    "github.com/ethereum/go-ethereum/crypto"
    "github.com/obscuren/ecies"
    "github.com/obscuren/secp256k1-go"
)

var (
    sskLen int = 16  // ecies.MaxSharedKeyLength(pubKey) / 2
    sigLen int = 65  // elliptic S256
    pubLen int = 64  // 512 bit pubkey in uncompressed representation without format byte
    keyLen int = 32  // ECDSA
    msgLen int = 194 // sigLen + keyLen + pubLen + keyLen + 1 = 194
    resLen int = 97  // pubLen + keyLen + 1
)

// aesSecret, macSecret, egressMac, ingress
type secretRW struct {
    aesSecret, macSecret, egressMac, ingressMac []byte
}

type cryptoId struct {
    prvKey  *ecdsa.PrivateKey
    pubKey  *ecdsa.PublicKey
    pubKeyS []byte
}

func newCryptoId(id ClientIdentity) (self *cryptoId, err error) {
    // will be at server  init
    var prvKeyS []byte = id.PrivKey()
    if prvKeyS == nil {
        err = fmt.Errorf("no private key for client")
        return
    }
    // initialise ecies private key via importing keys (known via our own clientIdentity)
    // the key format is what elliptic package is using: elliptic.Marshal(Curve, X, Y)
    var prvKey = crypto.ToECDSA(prvKeyS)
    if prvKey == nil {
        err = fmt.Errorf("invalid private key for client")
        return
    }
    self = &cryptoId{
        prvKey: prvKey,
        // initialise public key from the imported private key
        pubKey: &prvKey.PublicKey,
        // to be created at server init shared between peers and sessions
        // for reuse, call wth ReadAt, no reset seek needed
    }
    self.pubKeyS = id.Pubkey()
    return
}

func (self *cryptoId) Run(conn io.ReadWriter, remotePubKeyS []byte, sessionToken []byte, initiator bool) (token []byte, rw *secretRW, err error) {
    var auth, initNonce, recNonce []byte
    var randomPrivKey *ecdsa.PrivateKey
    var remoteRandomPubKey *ecdsa.PublicKey
    if initiator {
        if auth, initNonce, randomPrivKey, _, err = self.startHandshake(remotePubKeyS, sessionToken); err != nil {
            return
        }
        conn.Write(auth)
        var response []byte
        conn.Read(response)
        // write out auth message
        // wait for response, then call complete
        if recNonce, remoteRandomPubKey, _, err = self.completeHandshake(response); err != nil {
            return
        }
    } else {
        conn.Read(auth)
        // we are listening connection. we are responders in the handshake.
        // Extract info from the authentication. The initiator starts by sending us a handshake that we need to respond to.
        // so we read auth message first, then respond
        var response []byte
        if response, recNonce, initNonce, randomPrivKey, remoteRandomPubKey, err = self.respondToHandshake(auth, remotePubKeyS, sessionToken); err != nil {
            return
        }
        conn.Write(response)
    }
    return self.newSession(initNonce, recNonce, auth, randomPrivKey, remoteRandomPubKey)
}

/* startHandshake is called by peer if it initiated the connection.
 By protocol spec, the party who initiates the connection (initiator) will send an 'auth' packet
New: authInitiator -> E(remote-pubk, S(ecdhe-random, ecdh-shared-secret^nonce) || H(ecdhe-random-pubk) || pubk || nonce || 0x0)
     authRecipient -> E(remote-pubk, ecdhe-random-pubk || nonce || 0x0)

Known: authInitiator = E(remote-pubk, S(ecdhe-random, token^nonce) || H(ecdhe-random-pubk) || pubk || nonce || 0x1)
       authRecipient = E(remote-pubk, ecdhe-random-pubk || nonce || 0x1) // token found
       authRecipient = E(remote-pubk, ecdhe-random-pubk || nonce || 0x0) // token not found

The caller provides the public key of the peer as conjuctured from lookup based on IP:port, given as user input or proven by signatures. The caller must have access to persistant information about the peers, and pass the previous session token as an argument to cryptoId.

The handshake is the process by which the peers establish their connection for a session.

*/

func ImportPublicKey(pubKey []byte) (pubKeyEC *ecdsa.PublicKey, err error) {
    var pubKey65 []byte
    switch len(pubKey) {
    case 64:
        pubKey65 = append([]byte{0x04}, pubKey...)
    case 65:
        pubKey65 = pubKey
    default:
        return nil, fmt.Errorf("invalid public key length %v (expect 64/65)", len(pubKey))
    }
    return crypto.ToECDSAPub(pubKey65), nil
}

func ExportPublicKey(pubKeyEC *ecdsa.PublicKey) (pubKey []byte, err error) {
    if pubKeyEC == nil {
        return nil, fmt.Errorf("no ECDSA public key given")
    }
    return crypto.FromECDSAPub(pubKeyEC)[1:], nil
}

func (self *cryptoId) startHandshake(remotePubKeyS, sessionToken []byte) (auth []byte, initNonce []byte, randomPrvKey *ecdsa.PrivateKey, remotePubKey *ecdsa.PublicKey, err error) {
    // session init, common to both parties
    if remotePubKey, err = ImportPublicKey(remotePubKeyS); err != nil {
        return
    }

    var tokenFlag byte
    if sessionToken == nil {
        // no session token found means we need to generate shared secret.
        // ecies shared secret is used as initial session token for new peers
        // generate shared key from prv and remote pubkey
        if sessionToken, err = ecies.ImportECDSA(self.prvKey).GenerateShared(ecies.ImportECDSAPublic(remotePubKey), sskLen, sskLen); err != nil {
            return
        }
        // tokenFlag = 0x00 // redundant
    } else {
        // for known peers, we use stored token from the previous session
        tokenFlag = 0x01
    }

    //E(remote-pubk, S(ecdhe-random, ecdh-shared-secret^nonce) || H(ecdhe-random-pubk) || pubk || nonce || 0x0)
    // E(remote-pubk, S(ecdhe-random, token^nonce) || H(ecdhe-random-pubk) || pubk || nonce || 0x1)
    // allocate msgLen long message,
    var msg []byte = make([]byte, msgLen)
    initNonce = msg[msgLen-keyLen-1 : msgLen-1]
    if _, err = rand.Read(initNonce); err != nil {
        return
    }
    // create known message
    // ecdh-shared-secret^nonce for new peers
    // token^nonce for old peers
    var sharedSecret = Xor(sessionToken, initNonce)

    // generate random keypair to use for signing
    if randomPrvKey, err = crypto.GenerateKey(); err != nil {
        return
    }
    // sign shared secret (message known to both parties): shared-secret
    var signature []byte
    // signature = sign(ecdhe-random, shared-secret)
    // uses secp256k1.Sign
    if signature, err = crypto.Sign(sharedSecret, randomPrvKey); err != nil {
        return
    }

    // message
    // signed-shared-secret || H(ecdhe-random-pubk) || pubk || nonce || 0x0
    copy(msg, signature) // copy signed-shared-secret
    // H(ecdhe-random-pubk)
    var randomPubKey64 []byte
    if randomPubKey64, err = ExportPublicKey(&randomPrvKey.PublicKey); err != nil {
        return
    }
    copy(msg[sigLen:sigLen+keyLen], crypto.Sha3(randomPubKey64))
    // pubkey copied to the correct segment.
    copy(msg[sigLen+keyLen:sigLen+keyLen+pubLen], self.pubKeyS)
    // nonce is already in the slice
    // stick tokenFlag byte to the end
    msg[msgLen-1] = tokenFlag

    // encrypt using remote-pubk
    // auth = eciesEncrypt(remote-pubk, msg)

    if auth, err = crypto.Encrypt(remotePubKey, msg); err != nil {
        return
    }

    return
}

// verifyAuth is called by peer if it accepted (but not initiated) the connection
func (self *cryptoId) respondToHandshake(auth, remotePubKeyS, sessionToken []byte) (authResp []byte, respNonce []byte, initNonce []byte, randomPrivKey *ecdsa.PrivateKey, remoteRandomPubKey *ecdsa.PublicKey, err error) {
    var msg []byte
    var remotePubKey *ecdsa.PublicKey
    if remotePubKey, err = ImportPublicKey(remotePubKeyS); err != nil {
        return
    }

    // they prove that msg is meant for me,
    // I prove I possess private key if i can read it
    if msg, err = crypto.Decrypt(self.prvKey, auth); err != nil {
        return
    }

    var tokenFlag byte
    if sessionToken == nil {
        // no session token found means we need to generate shared secret.
        // ecies shared secret is used as initial session token for new peers
        // generate shared key from prv and remote pubkey
        if sessionToken, err = ecies.ImportECDSA(self.prvKey).GenerateShared(ecies.ImportECDSAPublic(remotePubKey), sskLen, sskLen); err != nil {
            return
        }
        // tokenFlag = 0x00 // redundant
    } else {
        // for known peers, we use stored token from the previous session
        tokenFlag = 0x01
    }

    // the initiator nonce is read off the end of the message
    initNonce = msg[msgLen-keyLen-1 : msgLen-1]
    // I prove that i own prv key (to derive shared secret, and read nonce off encrypted msg) and that I own shared secret
    // they prove they own the private key belonging to ecdhe-random-pubk
    // we can now reconstruct the signed message and recover the peers pubkey
    var signedMsg = Xor(sessionToken, initNonce)
    var remoteRandomPubKeyS []byte
    if remoteRandomPubKeyS, err = secp256k1.RecoverPubkey(signedMsg, msg[:sigLen]); err != nil {
        return
    }
    // convert to ECDSA standard
    if remoteRandomPubKey, err = ImportPublicKey(remoteRandomPubKeyS); err != nil {
        return
    }

    // now we find ourselves a long task too, fill it random
    var resp = make([]byte, resLen)
    // generate keyLen long nonce
    respNonce = resp[pubLen : pubLen+keyLen]
    if _, err = rand.Read(respNonce); err != nil {
        return
    }
    // generate random keypair for session
    if randomPrivKey, err = crypto.GenerateKey(); err != nil {
        return
    }
    // responder auth message
    // E(remote-pubk, ecdhe-random-pubk || nonce || 0x0)
    var randomPubKeyS []byte
    if randomPubKeyS, err = ExportPublicKey(&randomPrivKey.PublicKey); err != nil {
        return
    }
    copy(resp[:pubLen], randomPubKeyS)
    // nonce is already in the slice
    resp[resLen-1] = tokenFlag

    // encrypt using remote-pubk
    // auth = eciesEncrypt(remote-pubk, msg)
    // why not encrypt with ecdhe-random-remote
    if authResp, err = crypto.Encrypt(remotePubKey, resp); err != nil {
        return
    }
    return
}

func (self *cryptoId) completeHandshake(auth []byte) (respNonce []byte, remoteRandomPubKey *ecdsa.PublicKey, tokenFlag bool, err error) {
    var msg []byte
    // they prove that msg is meant for me,
    // I prove I possess private key if i can read it
    if msg, err = crypto.Decrypt(self.prvKey, auth); err != nil {
        return
    }

    respNonce = msg[pubLen : pubLen+keyLen]
    var remoteRandomPubKeyS = msg[:pubLen]
    if remoteRandomPubKey, err = ImportPublicKey(remoteRandomPubKeyS); err != nil {
        return
    }
    if msg[resLen-1] == 0x01 {
        tokenFlag = true
    }
    return
}

func (self *cryptoId) newSession(initNonce, respNonce, auth []byte, privKey *ecdsa.PrivateKey, remoteRandomPubKey *ecdsa.PublicKey) (sessionToken []byte, rw *secretRW, err error) {
    // 3) Now we can trust ecdhe-random-pubk to derive new keys
    //ecdhe-shared-secret = ecdh.agree(ecdhe-random, remote-ecdhe-random-pubk)
    var dhSharedSecret []byte
    pubKey := ecies.ImportECDSAPublic(remoteRandomPubKey)
    if dhSharedSecret, err = ecies.ImportECDSA(privKey).GenerateShared(pubKey, sskLen, sskLen); err != nil {
        return
    }
    // shared-secret = crypto.Sha3(ecdhe-shared-secret || crypto.Sha3(nonce || initiator-nonce))
    var sharedSecret = crypto.Sha3(append(dhSharedSecret, crypto.Sha3(append(respNonce, initNonce...))...))
    // token = crypto.Sha3(shared-secret)
    sessionToken = crypto.Sha3(sharedSecret)
    // aes-secret = crypto.Sha3(ecdhe-shared-secret || shared-secret)
    var aesSecret = crypto.Sha3(append(dhSharedSecret, sharedSecret...))
    // # destroy shared-secret
    // mac-secret = crypto.Sha3(ecdhe-shared-secret || aes-secret)
    var macSecret = crypto.Sha3(append(dhSharedSecret, aesSecret...))
    // # destroy ecdhe-shared-secret
    // egress-mac = crypto.Sha3(mac-secret^nonce || auth)
    var egressMac = crypto.Sha3(append(Xor(macSecret, respNonce), auth...))
    // # destroy nonce
    // ingress-mac = crypto.Sha3(mac-secret^initiator-nonce || auth),
    var ingressMac = crypto.Sha3(append(Xor(macSecret, initNonce), auth...))
    // # destroy remote-nonce
    rw = &secretRW{
        aesSecret:  aesSecret,
        macSecret:  macSecret,
        egressMac:  egressMac,
        ingressMac: ingressMac,
    }
    return
}

// should use cipher.xorBytes from crypto/cipher/xor.go for fast xor
func Xor(one, other []byte) (xor []byte) {
    xor = make([]byte, len(one))
    for i := 0; i < len(one); i++ {
        xor[i] = one[i] ^ other[i]
    }
    return
}