Half a secret Linguistics 294L Modern uses of cryptography Linguistics 294L Modern uses of cryptography Information has value Some secrets are obviously worth keeping PIN numbers SSN login passwords product blueprints What secrets? Would I like to see my competitors sales figures? Would I like to see my competitors customer list? Did it cost me money or effort to make? If so, consider protecting it? Information has value Information asymmetries can be useful. Potential stealer cant safely eat any of the melons. One of these melons is poisoned Information has value Information asymmetries can be negated. Potential stealer cant safely eat any of the melons. But neither can owner. They now need to pool information. Now two of the melons are poisoned Other similar things Three signatures on a company check Vaults with multiple keys Nuclear submarine launch protocols Diffie- Hellman Public Key Cryptography Whitfield Diffie Public Key Cryptography Whitfield Diffie Diffie- Hellman Public Key Cryptography Martin Hellman Public Key Cryptography Martin Hellman Key distribution Up to now, we have been assuming the need for an agreed shared key. The crucial idea Diffie and Hellman had was to imagine cutting this key in two. One half for encryption The other half for decryption Uses of half secrets I can publish my encryption key, anyone can send me confidential messages. If I keep my decryption key secret, no- one else can read the messages Uses of half secrets I can publish my decryption key, anyone can decrypt my messages If I keep my encryption key secret,anything encrypted with that key must have come from me.