Alice and Bob have agreed a symmetric key to be used for encrypting messages to be sent over an insecure (wrt a passive adversary) communication channel. Both of them have hardware devices for carrying out the encryption, but no (hardware or software) decrypting tools.

Nevertheless, they have to setup a simple scheme for securing the privacy, based on hardware encryptors, allowing the sender to encrypt a message and the recipient to decrypt a message.

How to Design the scheme?

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    How can you only be able to encrypt? Who would even design such hard- or software? – AdHominem Feb 8 '16 at 18:43
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    This sounds like a homework problem, and we are missing the context. – Mike Ounsworth Feb 8 '16 at 19:08
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    I think that answer is like using a "forward" encryption in both sides. So maybe answer is just using a stream cipher with CFB or OFB mode of operation. The parties will use then the same hardware resources. – Leonardo Feb 8 '16 at 19:57
  • If there are no decrypting tools, how are the messages to be decrypted? – schroeder Feb 8 '16 at 21:41
  • because they will do the same operation just with slightly different input exploiting XOR properties! – Leonardo Feb 9 '16 at 13:33

I thought something about the XOR operation. If I'm not wrong XOR is symmetrical so if you apply a XOR k XOR k you obtain a again. So for encryption you can use just a XOR, and for the decryption you can use another XOR.

  • Yes this is correct. They will use the same available hardware both for encryption and decryption so this is the answer. – Leonardo Feb 9 '16 at 13:32

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