ch7: pretty good privacy

digsig  
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.

Diffie Helmann digital signature

  • one-way function: given f( ), finding f-1( ) hard

  • e.g. Alice's RSA public encryption function f(m) = me (mod n)

  • f-1( ) is Alice's RSA secret decryption function

  • recall f-1(x) = xd (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