Pyspark: Prevent Column value from changing once calculated

I have a spark dataframe which I am using to store keys generated for encryption. I use a UDF to generate a key for each row but whenever the dataframe is queried the keys change. I have tried wrapping the function in lit() but it still changes. How can I make this column Immutable?

Code:

def generate_key():
    encryptionKey = Fernet.generate_key().decode('utf-8')
    return encryptionKey

get_key = udf(generate_key, StringType())

def key_table(df):
    df        = df.select("id").withColumn('encKey', lit(get_key()))
    return df

Output: https://i.stack.imgur.com/8zXcv.png