Encryption techniques can use either public keys or secret keys. Secret-key encryption techniques use a secret key which is only known by the two communicating parities, as illustrated in Figure 18.1. This key can be fixed or can be passed from the two parties over a secure communications link (for example over the postal network or a leased line). The two most popular private-key techniques are DES (Data Encryption Standard) and IDEA (International Data Encryption Algorithm) and a popular public-key technique is RSA (named after its inventors, Rivest, Shamir and Adleman). Public-key encryption uses two keys, one private and the other public.
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