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package radio
import (
"bytes"
"crypto/aes"
"crypto/cipher"
"encoding/binary"
"fmt"
)
// CreateNonce creates a 128-bit nonce.
// It takes a uint32 packetId, converts it to a uint64, and a uint32 fromNode.
// The nonce is concatenated as [64-bit packetId][32-bit fromNode][32-bit block counter].
func CreateNonce(packetId uint32, fromNode uint32) ([]byte, error) {
// Expand packetId to 64 bits
packetId64 := uint64(packetId)
// Initialize block counter (32-bit, starts at zero)
blockCounter := uint32(0)
// Create a buffer for the nonce
buf := new(bytes.Buffer)
// Write packetId, fromNode, and block counter to the buffer
err := binary.Write(buf, binary.LittleEndian, packetId64)
if err != nil {
return nil, err
}
err = binary.Write(buf, binary.LittleEndian, fromNode)
if err != nil {
return nil, err
}
err = binary.Write(buf, binary.LittleEndian, blockCounter)
if err != nil {
return nil, err
}
return buf.Bytes(), nil
}
// XOR encrypts or decrypts text with the specified key. It requires the packetID and sending node ID for the AES IV
func XOR(text []byte, key []byte, packetID, fromNode uint32) ([]byte, error) {
if len(key) != 16 && len(key) != 24 && len(key) != 32 {
return nil, fmt.Errorf("key length must be 16, 24, or 32 bytes")
}
block, err := aes.NewCipher(key)
if err != nil {
return nil, err
}
// The IV needs to be unique, but not secure. It's common to include it at
// the beginning of the text. In CTR mode, the IV size is equal to the block size.
//if len(text) < aes.BlockSize {
// return nil, fmt.Errorf("text too short")
//}
iv, err := CreateNonce(packetID, fromNode)
if err != nil {
return nil, err
}
//text = text[aes.BlockSize:]
// CTR mode is the same for both encryption and decryption, so we use
// the NewCTR function rather than NewCBCDecrypter.
stream := cipher.NewCTR(block, iv)
// XORKeyStream can work in-place if the two arguments are the same.
plaintext := make([]byte, len(text))
stream.XORKeyStream(plaintext, text)
return plaintext, nil
}
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