>That's precisely what I would do.
>
>I bet in Sedna this already exists.

Yes, I completed that last night. Basically, I can now do something like this from VFP:

loEncrypt=CREATEOBJECT('LevelExtreme.Encrypt')
IF NOT loEncrypt.Decrypt(lcToken,PADR(AI,8," "))
   MESSAGEBOX("Erreur: "+loEncrypt.cError)
   RETURN
ENDIF
IF lcPrimaryKey==loEncrypt.cDecrypt
   llSuccess=.T.
   ELSE
   llSuccess=.F.
ENDIF

The encryption mechanism is encapsulated in the .NET class. So, the .NET Web Service uses that class to encrypt and VFP does the same, by the use of COM, to decrypt.

It took me a while to figure out the proper syntax in .NET to achieve the COM to be visible, as registered and as the methods and properties accessibility. But, once I figured it out, it worked perfectly.