pretty good privacy

information age: who encrypts ?

government military business crime

citizens ?

Phil Zimmermann

1970s Florida Atlantic University

1977 RSA: government military business (costly)

until then, large scale gov't privacy invasion prohibitive

1980s developed pgp

1991 asks friend to post pgp on internet

1993 US govt visits Phil

pgp features

msg: symmetric secret-key cryptosystem

secret key: asymmetric public-key cryptosystem

signature: asymmetric public-key cryptosystem

automated pkc key generation, session key

spoofing

e.g. Eve uses Alice's RSA public key, sends Alice this message: Hi Alice, this is Bob. I would like to purchase an expensive gift for Eve. Please send the gift directly to Eve, and charge my account. Thank you. Bob.

one-way function: given f( ), finding f

^{-1}( ) harde.g. Alice's RSA public encryption function f(m) = m

^{e}(mod n)f

^{-1}( ) is Alice's RSA secret decryption functionrecall f

^{-1}(x) = x^{d}(mod n), where d*e = 1 (mod phi(n)), where phi(n) = (p-1)(q-1)f( ) is a one-way function, but so is g = f

^{-1}( )Alice sends

**signed**message m to Bob:uses public hash function h( )

computes j = h(m)

computes c = g(j)

sends m,j,c to Bob

Bob verifies Alice's signature:

confirms j = h(m)

confirms j = g

^{-1}(c) = f(c)

security ?

determining g( ) from g

( ) infeasible for Eve only Alice could have created signature