Public Key Encryption 3

Digital Signatures

or

The Box within a Box

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

It could have come from Pre Tender!

Yves Dropper'sevil cousin

Ken's idea: Lock with my private key

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

But Pre Tender can still read it.

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

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

Digital Signatures

In any public key cryptosystem, one can implement digital signatures.

To send a signed message M from A to B using keys AE, AD, BE, BD:

A sends cyphertext C = BE(AD(M))

B recovers text T = AE(BD(C))

T = AE(BD(BE(AD(M))))

= AE(AD(M))

= MB's

private key

A'sPrivate Key

It's the law

Documents with Digital Signatures became legally binding in the United States in the year 2000.