FRMPayload decryption

TheStudent227 · December 18, 2021, 9:42pm

Hello I have some unclear points in decryption procedure of FRMPayload. The picture below shows some unclear moments.

• What is the value of i ? What is initial value of it ?

• How the S looks for FRMPayload in the example in picture ?

• What means : to the first len(pld) octets ?

Maybe someone please, can give an example how S, i, k and performed steps will look like, for FRMPayload, shown in picture ?

brocaar · January 3, 2022, 9:07pm

You can find the ChirpStack implementation of the encryption / decryption of the FRMPayload here:

github.com

brocaar/lorawan/blob/master/phypayload.go#L854

    
      
          
          
	hb := hash.Sum([]byte{})
          	if len(hb) < 4 {
          		return mic, errors.New("lorawan: the hash returned less than 4 bytes")
          	}
          
          
	copy(mic[:], hb[0:4])
          	return mic, nil
          }
          
          
// EncryptFRMPayload encrypts the FRMPayload (slice of bytes).
          // Note that EncryptFRMPayload is used for both encryption and decryption.
          func EncryptFRMPayload(key AES128Key, uplink bool, devAddr DevAddr, fCnt uint32, data []byte) ([]byte, error) {
          	pLen := len(data)
          	if pLen%16 != 0 {
          		// append with empty bytes so that len(data) is a multiple of 16
          		data = append(data, make([]byte, 16-(pLen%16))...)
          	}
          
          
	block, err := aes.NewCipher(key[:])
          	if err != nil {

cstratton · January 6, 2022, 4:49pm

Basically what happens is that a vector of cyphertext S is produced some whole number of 16 byte blocks (S1, S2…) at least as long as the message. The plaintext A that’s encrypted to do this has each block (A1, A2…) stuffed with a variety of things shown in the picture, one of which is i the counter of blocks, starting from 1 and incrementing in each following block.

The message is then XOR’d with the vector of cyphertext, trimmed before or after to the actual length of the message.

One of the things to note is that it’s always the AES encrypt operation, never decrypt, as this keeps the device firmware smaller.