In Theory: perfect

In Practice: too many problems
 
 
 
 

Problem: How do two people who have not communicated before start sending encrypted messages?

Answer: Public Key Cryptography

Whitfield Diffie Martin E. Hellman

"New Directions in Cryptography"

IEEE Transactions on Information Theory, November 1976.

WHITFIELD DIFFIE

http://www.wired.com/wired/archive//3.33/diffie.html?person=whitfield_diffie&topic_set=wiredpeople
 

These have the three properties:

    Each undoes the other

    Each is efficient to apply

    Knowledge of E gives no useful knowledge of D.
 
 

Suppose Alice wants to send a message to Bob

Call the message M.

Alice looks ups Bob's public key EB and computes the encrypted message

EB(M) which she sends to Bob.

Bob then applies the private decryption key DB to

obtain

Bob is the only person who knows DBso he is the only person who can decrypt this message.
 
 

How does Bob know the message really came from Alice? How could he prove in court that Alice sent it?

In addition to the message M, Alice signs it with her signature S to which she first applies her secret decryption key,

She sends Bob DA(S)

He looks up the public EA and applies it to obtain

So Public-Key Crypography solves many problems of Privacy and Authentication and provides an easy way for two people to begin commnicating.

The only question is: How do we find a pair of functions E and D which fit the criteria?

Part 4