lora_decrypt.js

var aesjs = require('aes-js');

exports.lora_decrypt = function(payload_hex, sequence_counter, appskey, addr){
      
	
	var key = new Buffer(appskey, 'hex');
	var aesEcb = new aesjs.ModeOfOperation.ecb(key);
    var dev_addr = new Buffer(addr, 'hex');
    var buffer = new Buffer(payload_hex, 'hex');
    var size = buffer.length;

    
    var bufferIndex = 0;
    
    var ctr = 1;
    
    var encbuffer = new Uint8Array(buffer.length);
    
    var Block_A = new Uint8Array([0x01, 0x00 ,0x00 ,0x00 ,0x00, 0 ,dev_addr[3] ,dev_addr[2] ,dev_addr[1]
    				 ,dev_addr[0] ,sequence_counter & 0xff , (sequence_counter >> 8) & 0xff , (sequence_counter >> 16) & 0xff
    				 , (sequence_counter >> 24) & 0xff, 0x00, 0x00]);
    var sBlock = new Uint8Array(16);
	/*
	this is an example how Block a is formed according to Lorawan specs
	Block_A[0]  = 0x01;
	Block_A[1]  = 0x00;
	Block_A[2]  = 0x00;
	Block_A[3]  = 0x00;
	Block_A[4]  = 0x00;
	Block_A[5]  = 0;        // 0 for uplink frames 1 for downlink frames;
	Block_A[6]  = dev_addr[3]; // LSB devAddr 4 bytes
	Block_A[7]  = dev_addr[2];  // ..
	Block_A[8]  = dev_addr[1];  // ..
	Block_A[9]  = dev_addr[0];  // MSB
	Block_A[10] = sequence_counter & 0xff;  // LSB framecounter
	Block_A[11] = (sequence_counter >> 8) & 0xff;  // MSB framecounter
	Block_A[12] = (sequence_counter >> 16) & 0xff;     // Frame counter upper Bytes
	Block_A[13] = (sequence_counter >> 24) & 0xff;
	Block_A[14] = 0x00;
	*/
				
    while (size >= 16){
        Block_A[15] = ctr & 0xFF;
        ctr += 1;
        sBlock = aesEcb.encrypt(Block_A);
       
        for (i=0;i<16;i++){
            encbuffer[bufferIndex + i] = buffer[bufferIndex + i] ^ sBlock[i];
		}
        size -= 16
        bufferIndex += 16
    }
    if (size > 0){
        Block_A[15] = ctr & 0xFF;
        ctr += 1;
        sBlock = aesEcb.encrypt(Block_A);
     
        for (i=0;i<16;i++){
            encbuffer[bufferIndex + i] = buffer[bufferIndex + i] ^ sBlock[i];   
		}
    }
 
	return encbuffer;
	

}