title: How to prove inconstructable cryptographic scheme?
body: <p>I realize this question might not be that programming related, and that it by many will sound like a silly question due to the intuitive logical fault of this idéa. </p>
  
  <p>My question is: is it provable impossible to construct a cryptographic scheme (implementable with a turing-complete programming language) where the encrypted data can be decrypted, without exposing a decryption key to the decrypting party?</p>
  
  <p>Of course, I can see the intuitive logical fault to such a scheme, but as so often with formal logic and math, a formal proof have to be constructed before assuming such a statement. Is such a proof present, or can it easely be constructed?</p>
  
  <p>Thank you for advice on this one!</p>
  
  <p>Edit: Thank you all for valuable input to this discussion! </p>