Hi Noam,
The crypto:encrypt function returns the result as a string, so you'll need to wrap it with an additional element:
return <record>{ <CK>{ crypto:encrypt(...) }</CK>, ... }</record>
However, the returned string will contain non-ASCII characters, so it may not be what you expect, so you could possibly encode it as Base64:
convert:string-to-base64( ... )
Hope this helps, Christian