Encrypt Decrypt Problem

I am suddenly getting an error in a script that has been working for years where I encrypt an html file so that is can't be accessed via a browser and then decrypt it for later reference

The encrypting scheme is very basic:

<cfset archivefile="<HTML><BODY>" & paynowbutton & invoiceheader & invoicebody & "</body></html>">

<cfset tempinvoice=encrypt(archivefile,session.key)><!---session.key is a 4 digit PIN number--->

<cffile    action = "write"  file = "#application.LocalRoot#\invoices\#brokerid#_invoice.html"  output = "#tempinvoice#">

THEN TO UNENCRYPT

<cffile action="read"    file = "#application.LocalRoot#\invoices\#form.brokerid2#_invoice.html"  variable="invoice">

<cfset invoice=decrypt(invoice,session.key)>

THEN THE ERROR WHICH HAS BEEN INCREASING IN FREQUENCY

An error occurred while trying to encrypt or decrypt your input string: The input and output encodings are not same

ANY IDEAS??