applied cryptography - protocols, algorithms, and source code in c

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Unformatted text preview: that it is hard to recognize Alice’s voice, but the prosecution argues that since only Alice’s escrowed key will decipher the call it must have been made from her telephone. Alice argues that the call was forged like so [984, 1339]: Given the ciphertext and the plaintext, it is possible to XOR them to get the keystream. This keystream can then be XORed with an entirely different plaintext to form a forged ciphertext, which can then be converted to forged plaintext when fed into the Clipper decryptor. True or not, this argument could easily put enough doubt in a jury’s mind to disregard the telephone call as evidence. Another attack, called the Squeeze attack, allows Alice to frame Bob. Here’s how [575]: Alice calls Bob using Clipper. She saves a copy of his LEAF as well as the session key. Then, she calls Carol (who she knows is being wiretapped). During the key setup, Alice forces the session key to be identical to the one she used with Bob; this requires hacking the phone, but it is not hard. Then, instead of sending her LEAF she sends Bob’s. It’s a valid LEAF, so Carol’s phone will not notice. Now she can say whatever she wants to Carol; when the police decrypt the LEAF, they will find that it is Bob’s. Even if Bob wasn’t framed by Alice, the mere fact that he can claim this in court undermines the purpose of the scheme. The law enforcement authorities of the United States should not be in the business of collecting information in criminal investigations that is useless in court. Even if key escrow were a good idea, Clipper is a bad way of implementing it. 24.17 Capstone Capstone (also known as the MYK-80) is the other NSA-developed VLSI cryptographic chip that implements the U.S. government’s Escrowed Encryption Standard [1153]. Capstone includes the following functions [1155, 462]: Table 24.2 EES Participating Agencies Justice—System Sponsor and Family Key Agent NIST—Program Manager and Escrow Agent FBI—Decrypt User and Family Key Agent Treasury—Escrow Agent NSA—Progra...
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