Encryption has a remarkably long and varied history. Spies have been using it to convey secret messages ever since there were secret messages to convey. One powerful method of encryption uses random digits. Two documents are prepared with the same random sequence of numbers. The spy is sent out with one and the spy master retains the other. The digits are used as follows. Suppose that the word to be encrypted is SPY and the random digits are 352. Then S becomes V (three letters after S), P becomes U (five letters after P), and Y becomes A (two letters after Y, restarting at A after Z). The spy would encrypt a message and then destroy the document used to encrypt it. This is an early example of
◦ a hashing algorithm.
◦ asymmetric key encryption.
◦ symmetric key encryption.
◦ public key encryption.
Molecular mechanisms that convey positional information.
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