Traditionally, all secure communications relied on the encoder and the decoder having the same key.
This caused enormous difficulties - and expense - because the key material had to be distributed to senders and recipients. The UK military turned
to GCHQ to solve this problem and in 1973 they developed what is now known as public-key encryption.
This involves the recipient making a key available to a wide number of people (a public key) which any sender can use to encode a message. Despite
the fact that the key is public, an eavesdropper cannot decode the message because the recipient has some secret information to decrypt the message
(known as the private key).
This same solution was developed by a group of American academics in 1977 and has been used to good effect on the Internet to effect secure transactions.
The Americans were credited with the invention but GCHQ actually got there first.
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