1. CSIS 5857: Encoding and Encryption Lecture 27: Pretty Good Privacy for Email and the “Web of Trust”

2. Pretty Good Privacy • First comprehensive cryptosystem designed for use by private individuals • Singlehandedly developed by Phillip Zimmermann • Provides email confidentiality and authentication • Solves many very difficult problems: • How do individuals who have never met agree on which (of many) encryption algorithms to use? • How can key management and security be implemented on simple PCs? • How trust about the correctness of public keys be established without certification authorities?

3. PGP Message Contents Alice sends email to Bob containing: • Symmetric session key encrypted with Bob’s public key • Message (compressed for efficiency) encrypted with the session key • Message hashed to form a digest, signed with Alice’s private key, and encrypted with the session key • IDs to indicate which encryption algorithms used

4. PGP Message Contents When Bob receives the message: • Determines which encryption algorithms Alice used • Uses his private key to decrypt the session key • Uses the session key to decrypt the message and digest • Uses Alice’s public key to decrypt the digest • Hashes the message to make sure it matches the digest

5. PGP Algorithms

6. PGP Key Management • Each user has: • Public key ring of other people’s public keys • Private key ring of own private keys • May use different keys to communicate with different people

7. Private Key Ring • Indexed by user and key ID • Encrypted on user’s machine

8. Public Key Ring • User ID = email of person whose key this is • Trusts/Legitimacy determined by “web of trust”

9. Key Use by Sender • Alice indicates private and public keys to use • PGP prompts for passphrase to access private key ring • Hash of passphrase used as decryption key • Alice hits keys to generate “random” session key

10. Key Use by Recipient • Bob retrieves ID of private and public keys used from message • PGP looks up keys in Bob’s key ring • Prompts for passphrase to decrypt the private key

11. Trust Levels in PGP • No central CA assumed in PGP • User indicates trust in a certificate based on: • Provider: entity who provided and/or signed the certificate • Other criteria, such as how certificate provided (in person, over phone, over email, etc.) • PGP Trust Levels: • Full • Partial • None

12. Introducers and Trust Levels • Users can indicate trust level in certificate providers • That trust level automatically assigned to any certificates they provide Asok’s certificate Ted’s certificate Catbert’s certificate

13. Key Legitimacy • Points for key legitimacy assigned based on trust level • Full = 1 point • Partial = ½ point • None = 0 points • A certificate must be fully trusted before use

14. Key Legitimacy • Each signer adds to the trust level • A certificate signed by 2 partially trusted providers is fully trusted Ted’s certificate Ted’s certificate

15. Alternative Trust Measures • User can directly assign certificate full trust if: • Signed by recognized CA • Provided in face to face meeting • Provided over phone • Must be able to recognize voice! • Difficult in practice since keys are long • Sent over email and digest verified over phone • Sender and recipient create short digest of key • 16 byte MD5  8 ASCII chars • 20 byte SHA-1  10 ASCII chars • Chars in digest read over phone

16. PGP “Web of Trust” • “Networks” of trusted entities can form rapidly • The more providers you trust, the easier it is to build larger lists of trusted certificates