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Suppose the following.

Someone wishes to encrypt a message so it is not intercepted. With traditional ciphers, if the key is guessed correctly, the message is revealed. This cipher is similar– except that if an incorrect, but intentionally weaker key is guessed, a valid decoy message is revealed to deceive the adversary.

So

ciphertext + realPassword = "We attack at 14:30 sharp."

chiphertext + weakPassword = "We attack at dawn."

Does such a cipher exist, and if so what is its name.

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    $\begingroup$ You planning to attack anybody soon? $\endgroup$ – Guy Apr 1 '14 at 5:17
  • $\begingroup$ I was actually wondering about how it could be used to protect mission critical data, by convincing the attacker that an actually important file is innocuous. $\endgroup$ – awiebe Apr 1 '14 at 5:19
  • $\begingroup$ You can certainly make up trivial examples such as "WWEEAATTTTAACCKKAATT1D4A:W3N0XSXHXAXRXPX". $\endgroup$ – David Apr 1 '14 at 5:19
  • $\begingroup$ Ok so, that interleaves the messages, and pads with x(as a programer I would use 0). In the simple case, perform a checksum of the weak and strong password, then mod n. Where n is the number of interleaved passwords. Not a bad, although to make the cryptography stronger, the ciphertext will grow n*(maxLength(Messages)). $\endgroup$ – awiebe Apr 1 '14 at 5:31
  • $\begingroup$ This is real, and available. $\endgroup$ – vadim123 Apr 1 '14 at 6:30
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Honey Encryption does even better than what you are asking for. With honey encryption, every key from the key space results in a valid looking ciphertext. This primitive is only just now starting to be understood with applications also being developed. One target the inventors are looking at is password databases.

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