lora/lorawan: completed functions needed for Join OTAA process

Signed-off-by: deadprogram <ron@hybridgroup.com>

lora/lorawan/session.go

@@ -4,7 +4,6 @@ import (
 	"crypto/aes"
 	"encoding/binary"
 	"encoding/hex"
-	"errors"
 	"math"
 )
 
@@ -23,7 +22,7 @@ type Session struct {
 // SetDevAddr configures the Session DevAddr
 func (s *Session) SetDevAddr(devAddr []uint8) error {
 	if len(devAddr) != 4 {
-		return errors.New("invalid length")
+		return ErrInvalidDevAddrLength
 	}
 
 	copy(s.DevAddr[:], devAddr)
@@ -35,6 +34,14 @@ func (s *Session) GetDevAddr() string {
 	return hex.EncodeToString(s.DevAddr[:])
 }
 
+func (s *Session) GetNwkSKey() string {
+	return hex.EncodeToString(s.NwkSKey[:])
+}
+
+func (s *Session) GetAppSKey() string {
+	return hex.EncodeToString(s.AppSKey[:])
+}
+
 // GenMessage Forge an uplink message
 func (s *Session) GenMessage(dir uint8, payload []uint8) ([]uint8, error) {
 	var buf []uint8
@@ -75,7 +82,7 @@ func (s *Session) genFRMPayload(dir uint8, fCnt uint32, payload []byte, isFOpts
 		k++
 	}
 	if k > math.MaxUint8 {
-		return nil, errors.New("Payload too big !")
+		return nil, ErrFrmPayloadTooLarge
 	}
 	encrypted := make([]byte, 0, k*16)
 	cipher, err := aes.NewCipher(s.AppSKey[:])