How to decrypt the data payload of a XBEE 802.15.4 packet with the key ?

On the devices Xbee S1, how to encrypt and decrypt the data payload of the packet when we know the key ?
Normally, the security level is known thanks to a flag in the packet but with the XBEE S1, the flag is None…
AES-CTR is used ?

You are not suppose to decrypt a packet without knowing what the Key is. That is the purpose of using encryption.

Thank you for your answer.

I do not think I made myself clear enough though. My objective is to decrypt a 802.15.4 packet sniffed and encrypted by a xbee s1 module. Of course, I know the key…
Normally a nonce is used and I don’t know what it is. Also, I 'm not sure what Aes (CTR, CCM…) is used.
Someone know all the parameters to encrypt and decrypt a xbee s1 encrypted packet ?
Thanks

You are going to need to look throiugh the IEEE 802.15.4 protocol to locate the required AES encryption altroythum to figure that out.

Normally, in the 802.15.4 packet, there is a flag that indicate what AES is used.
Here, with the Xbee S1, the flag is not used. Do you know what AES xbee s1 use ?