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Unformatted text preview: r example, to get K’. She stores both C and the XOR on her hard disk. Now, when the secret police interrogate her, she can explain that she is an amateur cryptographer and that K’ is a merely one-time pad for C. The secret police might suspect something, but unless they know K they cannot prove that Alice’s explanation isn’t valid. Another method is to encrypt P with a symmetric algorithm and K, and D with K’. Intertwine bits (or bytes) of the ciphertext to make the final ciphertexts. If the secret police demand the key, Alice gives them K’ and says that the alternating bits (or bytes) are random noise designed to frustrate cryptanalysis. The trouble is the explanation is so implausible that the secret police will probably not believe her (especially considering it is suggested in this book). A better way is for Alice to create a dummy message, D, such that the concatenation of P and D, compressed, is about the same size as D. Call this concatenation P’. Alice then encrypts P’ with whatever algorithm she and Bob share to get C. Then she sends C to Bob. Bob decrypts C to get P’, and then P and D. Then they both compute C • D = K’. This K’ becomes the dummy one-time pad they use in case the secret police break their doors down. Alice has to transmit D so that hers and Bob’s alibis match. Another method is for Alice to take an innocuous message and run it through some error-correcting code. Then she can introduce errors that correspond to the secret encrypted message. On the receiving end, Bob can extract the errors to reconstruct the secret message and decrypt it. He can also use the error-correcting code to recover the innocuous message. Alice and Bob might be hard pressed to explain to the secret police why they consistently get a 30 percent bit-error rate on an otherwise noise-free computer network, but in some circumstances this scheme can work. Finally, Alice and Bob can use the subliminal channels in their digital signature algorithms (see Sections 4.2 and 23.3). This is undetect...
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- Fall '10