ValueError: Invalid Key while trying to upload code to ESP8266 device

I'm trying to connect my NodeMCU (ESP8266) device running Micropython to AWS IoT but wwhile running the code I'm getting the following error:

start connecting
Connected:  True
Key and Certificate files Loaded
Connecting to AWS IoT...
Traceback (most recent call last):
    File "<stdin>", line 49, in <module>
    File "<stdin>", line 45, in mqtt_connect
    File "umqtt/simple.py", line 74, in connect
    File "ssl.py", line 1, in wrap_socket
    File "ssl.py", line 1, in wrap_socket
ValueError: invalid key

I read somewhere that key should be in 'der' format, so I converted the downloaded key from AWS to der format using OPENSSL but still I'm getting the same error. Please help.

I'm running the following code:

import time
import network
import urequests
SSID = "SHEKHAR"
PASSWORD = "shekhar123"

print("start connecting")
    
wlan = network.WLAN(network.STA_IF)
wlan.active(True)
wlan.connect(SSID, PASSWORD)

while wlan.isconnected() == False:
    pass

print("Connected: ", wlan.isconnected())


import sys
import machine
from umqtt.simple import MQTTClient

clientId = 'nodemcu'
AWS_ENDPOINT = 'a2xf5sr3dys5uo-ats.iot.us-east-1.amazonaws.com'
PORT = 8883

certfile = '/certificate.crt.der'
with open(certfile, 'rb') as f:
    cert = f.read()
f.close()

keyfile = '/private.key.der'
with open(keyfile, 'rb') as f:
    key = f.read()
f.close()

print("Key and Certificate files Loaded")

SSL_PARAMS = {'key': key, 'cert': cert, 'server_side': False}

# client = MQTTClient(clientId, AWS_ENDPOINT, port=PORT, keepalive=10000, ssl=True, ssl_params=SSL_PARAMS)
def mqtt_connect(client=clientId, endpoint=AWS_ENDPOINT, sslp=SSL_PARAMS):
    mqtt = MQTTClient(client_id=client, server=endpoint, port=8883, keepalive=1200, ssl=True, ssl_params=sslp)
    print("Connecting to AWS IoT...")
    mqtt.connect()
    print("Done")
    return mqtt

mqtt_connect()