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| Donald | Apr 28 2008, 03:35 PM |
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Elite member
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I'm sorry, I didn't make myself clear. You see, the problem I'm worried about is keeping the receiver and transmitter both in sync. For example, suppose power went out at the embassy for 10 seconds. When they come back online, they will suddenly be out of sync with the transmitted signal coming from home, and since they are out of sync, they will be out of communication, unable to read the oncoming stream anymore. One method to avoid that is to make the key a hash of the secret key, and the date/time. This gives us a pseudo random stream cipher, but one that both parties can easily resynchronize at any time. We aren't actually giving any information to Eve about the time of our transmission, because we are transmitting all the time. Most of the time we simply transmit true random data, and send that. But when the ambassador wants to warn home that the enemy is planning a war! Then the secret key is hashed with the date/time and then xored with "message begins", followed by the message, followed by "message ends". All of this is transmitted, then followed by more random data. To Eve, the data stream is continuous. The random data looks like random data, AND the stream cipher encrypted message looks like random data. She doesn't have any way to detect that a message has begun or ended. Or whether there was a message at all. Of course, this all depends on the strength of the stream cipher. If the pseudo random function shows patterns, then all bets are off and Eve has a chance to detect the message. BUT, modern stream ciphers are VERY strong cryptographically and are (as far as we know) indistinguishable from random data. |
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| Needle In The Haystack Cipher · General | |
6:03 PM Nov 27
