# 1976-1991

## The book of science

Tom Sharp

 Whitfield Diffie, Martin Hellman, Ralph Merkle, Phil Zimmermann cryptography

## Public-key cryptography

• If you and I have the same key
• and I send you an encrypted message,
• then you may decrypt my message.
• This would be symmetric-key cryptography.
• Not very good for private ad-hoc communication.
• Not, at least, until after Diffie and Hellman
• proposed how to do public-key encryption, and Merkle
• showed that a shared key could be sent
• across an insecure channel.
• What do you do? Each party, Alice and Bob,
• chooses a secret value and shares a common value.
• Now each derives a shared sum of the three values.
• First each shares a mix of their own value with the shared value
• and then each mixes their own with the mix of the other
• so that now each has a mix of the three values.
• The values are large prime numbers, difficult to factor.
• This is known as Diffie-Hellman key exchange.
• Public-key is asymmetric-key cryptography.
• Both Alice and generate two keys,
• one to be shared and used for encryption,
• and the other to be kept private and used for decryption.
• Typically, this technique is used for sharing a symmetric key
• that can be used to encrypt and decrypt messages in both directions.

## Pretty Good Privacy

• PGP uses hashing, data compression,
• symmetric-key cryptography,
• and public-key cryptography.
• It encrypts a message
• with a symmetric key, and
• uses the receiver’s public key
• to encrypt the symmetric key
• that it sends along
• with the encrypted message.
• The receiver uses his or her private key
• to decrypt the symmetric key,
• which decrypts the message.

## Identity

• Your identity should be inviolable,
• and it’s not.
• You should own your own name,
• your birth place and birth day,
• your photographs and social connections,
• and you don’t.
• You have your own physical being;
• this is normally inviolable.
• After that, it should be OK