The prime
number codes solve a problem that dogged all previous
codes.
Two spies have to meet in advance to decide the
code that they will use to encode messages. Conventional
codes were like a door where the same key is used
to lock and unlock the door. To hide a message,
you use the key to lock the door. To decode the
message the same key is used to unlock the door.
But if you visit an internet site and they give
you a copy of the key then everyone has copies
of the same key and can see each others credit
cards.
What we want is a new lock on this door, where
a different key is used to unlock the door.
The clock calculator is such a lock. The number
E scrambles the credit card. It is a different
number D which unscrambles the credit card number.
The only way to find the decoding key is to crack
the number N into pxq and solve the equation E
x D = 1 (modulo (p-1)x(q-1)) on the secret clock
calculator.
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