1. Public Key Encryption 3 Digital Signatures or The Box within a Box

2. Ken Coder sends Dee a secret message only she can unlock.

3. It could have come from Pre Tender! Yves Dropper's evil cousin

4. Ken's idea: Lock with my private key

5. Unlock with my public key: Only I could have sent it!

6. But Pre Tender can still read it.

7. Dee's Idea: Lock your box in a second one using my public key.

8. Only Ken can send it, and only Dee can read it!

9. Digital Signatures In any public key cryptosystem, one can implement digital signatures. To send a signed message M from A to B using keys A E, A D, B E, B D : A sends cyphertext C = B E (A D (M)) B recovers text T = A E (B D (C)) T = A E (B D (B E (A D (M)))) = A E (A D (M)) = M B's private key A's Private Key

10. It's the law Documents with Digital Signatures became legally binding in the United States in the year 2000.