dissertation-2-code/proxy/packet.go

package proxy

import (
	"encoding/binary"
	"time"
)

type Packet struct {
	Data      []byte
	timestamp time.Time
}

// create a packet from the raw data of an IP packet
func NewPacket(data []byte) Packet {
	return Packet{
		Data:      data,
		timestamp: time.Now(),
	}
}

// rebuild a packet from the wrapped format
func UnmarshalPacket(raw []byte, verifier MacVerifier) (Packet, error) {
	// the MAC is the last N bytes
	data := raw[:len(raw)-verifier.CodeLength()]
	sum := raw[len(raw)-verifier.CodeLength():]

	if err := verifier.Verify(data, sum); err != nil {
		return Packet{}, err
	}

	p := Packet{
		Data: data[:len(data)-8],
	}

	unixTime := int64(binary.LittleEndian.Uint64(data[len(data)-8:]))
	p.timestamp = time.Unix(unixTime, 0)

	return p, nil
}

// get the raw data of the IP packet
func (p Packet) Raw() []byte {
	return p.Data
}

// produce the wrapped format of a packet
func (p Packet) Marshal(generator MacGenerator) []byte {
	// length of data + length of timestamp (8 byte) + length of checksum
	slice := make([]byte, len(p.Data)+8+generator.CodeLength())

	copy(slice, p.Data)

	unixTime := uint64(p.timestamp.Unix())
	binary.LittleEndian.PutUint64(slice[len(p.Data):], unixTime)

	mac := generator.Generate(slice)
	copy(slice[len(p.Data)+8:], mac)

	return slice
}