func()

in otr/otr.go [272:444]

• 160 lines of code
• 44 McCabe index (conditional complexity)

func (c *Conversation) Receive(in []byte) (out []byte, encrypted bool, change SecurityChange, toSend [][]byte, err error) {
	if bytes.HasPrefix(in, fragmentPrefix) {
		in, err = c.processFragment(in)
		if in == nil || err != nil {
			return
		}
	}

	if bytes.HasPrefix(in, msgPrefix) && in[len(in)-1] == '.' {
		in = in[len(msgPrefix) : len(in)-1]
	} else if version := isQuery(in); version > 0 {
		c.authState = authStateAwaitingDHKey
		c.reset()
		toSend = c.encode(c.generateDHCommit())
		return
	} else {
		// plaintext message
		out = in
		return
	}

	msg := make([]byte, base64.StdEncoding.DecodedLen(len(in)))
	msgLen, err := base64.StdEncoding.Decode(msg, in)
	if err != nil {
		err = errors.New("otr: invalid base64 encoding in message")
		return
	}
	msg = msg[:msgLen]

	// The first two bytes are the protocol version (2)
	if len(msg) < 3 || msg[0] != 0 || msg[1] != 2 {
		err = errors.New("otr: invalid OTR message")
		return
	}

	msgType := int(msg[2])
	msg = msg[3:]

	switch msgType {
	case msgTypeDHCommit:
		switch c.authState {
		case authStateNone:
			c.authState = authStateAwaitingRevealSig
			if err = c.processDHCommit(msg); err != nil {
				return
			}
			c.reset()
			toSend = c.encode(c.generateDHKey())
			return
		case authStateAwaitingDHKey:
			// This is a 'SYN-crossing'. The greater digest wins.
			var cmp int
			if cmp, err = c.compareToDHCommit(msg); err != nil {
				return
			}
			if cmp > 0 {
				// We win. Retransmit DH commit.
				toSend = c.encode(c.serializeDHCommit())
				return
			} else {
				// They win. We forget about our DH commit.
				c.authState = authStateAwaitingRevealSig
				if err = c.processDHCommit(msg); err != nil {
					return
				}
				c.reset()
				toSend = c.encode(c.generateDHKey())
				return
			}
		case authStateAwaitingRevealSig:
			if err = c.processDHCommit(msg); err != nil {
				return
			}
			toSend = c.encode(c.serializeDHKey())
		case authStateAwaitingSig:
			if err = c.processDHCommit(msg); err != nil {
				return
			}
			c.reset()
			toSend = c.encode(c.generateDHKey())
			c.authState = authStateAwaitingRevealSig
		default:
			panic("bad state")
		}
	case msgTypeDHKey:
		switch c.authState {
		case authStateAwaitingDHKey:
			var isSame bool
			if isSame, err = c.processDHKey(msg); err != nil {
				return
			}
			if isSame {
				err = errors.New("otr: unexpected duplicate DH key")
				return
			}
			toSend = c.encode(c.generateRevealSig())
			c.authState = authStateAwaitingSig
		case authStateAwaitingSig:
			var isSame bool
			if isSame, err = c.processDHKey(msg); err != nil {
				return
			}
			if isSame {
				toSend = c.encode(c.serializeDHKey())
			}
		}
	case msgTypeRevealSig:
		if c.authState != authStateAwaitingRevealSig {
			return
		}
		if err = c.processRevealSig(msg); err != nil {
			return
		}
		toSend = c.encode(c.generateSig())
		c.authState = authStateNone
		c.state = stateEncrypted
		change = NewKeys
	case msgTypeSig:
		if c.authState != authStateAwaitingSig {
			return
		}
		if err = c.processSig(msg); err != nil {
			return
		}
		c.authState = authStateNone
		c.state = stateEncrypted
		change = NewKeys
	case msgTypeData:
		if c.state != stateEncrypted {
			err = errors.New("otr: encrypted message received without encrypted session established")
			return
		}
		var tlvs []tlv
		out, tlvs, err = c.processData(msg)
		encrypted = true

	EachTLV:
		for _, inTLV := range tlvs {
			switch inTLV.typ {
			case tlvTypeDisconnected:
				change = ConversationEnded
				c.state = stateFinished
				break EachTLV
			case tlvTypeSMP1, tlvTypeSMP2, tlvTypeSMP3, tlvTypeSMP4, tlvTypeSMPAbort, tlvTypeSMP1WithQuestion:
				var reply tlv
				var complete bool
				reply, complete, err = c.processSMP(inTLV)
				if err == smpSecretMissingError {
					err = nil
					change = SMPSecretNeeded
					c.smp.saved = &inTLV
					return
				}
				if err == smpFailureError {
					err = nil
					change = SMPFailed
				} else if complete {
					change = SMPComplete
				}
				if reply.typ != 0 {
					toSend = c.encode(c.generateData(nil, &reply))
				}
				break EachTLV
			default:
				// skip unknown TLVs
			}
		}
	default:
		err = errors.New("otr: unknown message type " + strconv.Itoa(msgType))
	}

	return
}