you find that the documents do not exist until the encrypted data is combined with the decryption key; that view provides even an even stronger case for fifth amendment protection, as in this view the defendant is being required to _create_ the evidence against him.
It is a potentially interesting angle, but I doubt it would fly. It would mean that any and all laws that make some information illegal would cease to apply the moment it is encrypted, because it is "no longer there" (and later it magically reappears when the key is applied). This could be stretched even further by saying that e.g. compression is also similar, in that the output is not directly usable; and the compression code is the "key".
This would then apply across the board: copyright, national secrets etc. So for example if a guy steals some top secret data, encrypts it, and hands it over to Chinese in two separate transactions - one for the data, and the other for the key - it would seem that nothing illegal has occurred, because the data in its encrypted form is not top secret - only the decryption output would be, and it doesn't exist until encrypted data and key are combined in a very specific way.
Somehow, I doubt that this will work out that way.