Distributed computing

USING PUBLIC-KEY ENCRYPTION

• To send a message m to u that only u can read:
• Send E_u(m)
• For user t to sign and send a message m to u, t can”
• Send D_t(E_u(m)).
• The receiver u can prove that t sent this message, because only t knows D_t.
• Further, only u (and t) can read the message.

SSL (Secure Socket Layer)

• Effect: client knows it is talking to a certain server; client remains anonymous to server; communication is secure.
• Useful for purchases, even anonymous ones.

SSL Method -- simplified

• Client uses the well known public encryption method of the server to communicate desire to communicate. Nobody can eavesdrop. Client also generates a session private key.
• Server responds with the private key and then communication proceeds.

One Way Functions

• Given a function like triple, if I tell you that 15 has been produced by triple(x), you can infer that x is 5.
• By contrast, given certain hash functions, if I tell you h(x) it is very hard to infer x. Such (hard to find the inverse) functions are called one-way.
• Ex: Given x, compute x2 then take the middle 20 digits. Given those digits, hard to find x.
• There is a standard one-way hash function called SHA-1.

One Way Functions for Spies

• You have a bunch of spies. They are good people, very trustworthy.
• They go into enemy territory. When they return they must say something to the guards so they don’t get shot. Passwords can’t get reused for security.
• Also guards might go into a bar and be tempted to reveal secrets.
• Could we use one way functions in some way?

Should I get a file from this site?

• Suppose you want to download some file (say a special kind of player) from a web site.
• You want to be sure that web site is to be trusted.
• Otherwise, you might be getting a “Trojan horse” (something that looks good but can do you harm).
• You believe that trusted entities will keep their private keys secret.