Pretty Good Privacy

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Motivations

Symmetric cryptography—Alice wants to send Bob a message. They both agree on a single password to use as an encryption/decryption key. Alice can use that password to encrypt data to send to Bob, and Bob can use that password to decrypt messages received from Alice.

Symmetric cryptography has a number of disadvantages:

  • At the very beginning, when Alice and Bob initially exchanged the key, the key exchange could have been transparently intercepted by a third party (assume Alice and Bob were not using a secure channel because that is precisely what we are trying to build!).
  • For a group of n people to communicate privately, the number of unique keys is polynomial:
    # people # keys
    2 1
    3 3
    4 6
    5 10
    6 15
    n n(n-1)/2