Unknown Blessings: Health and Security

1. نعمتان مجهولتان: الصحة و الامان Chapter 15 Electronic Mail Security – Part I: PGP Data & Network Security Spring 2006 Dr. Jalili

2. Agenda • In this session, we’ll study PGP. In the next session, other email security standards will be studied. • PGP History • PGP Building Blocks • Web of Trust • PGP Key Management

3. New lords, new laws… • PGP (Pretty Good Privacy) was Phil R. Zimmermann’s (PRZ) answer to the US government 1991 Senate Bill 266: …force manufacturers of secure communications equipment to insert special "trap doors" in their products…

4. He that never climbed never fell… • PRZ gave PGP to some friends of his, and they gave it to their friends, and so on. Kelly Goen posted it on several BBS networks… • Somehow, PGP leaked outside the US. • The USG began investigating PRZ for alleged aiding with ITAR violation. (ITAR: International Traffic in Arms Regulations)

5. The wind cannot be caught in a net… • The USG investigation of Zimmermann went on for 5 years, but was eventually dropped: • PRZ was not the one who uploaded PGP to BBS’. • People were nervous about their privacy.

6. Necessity knows no law… • PGP was using RSA & IDEA encryption algorithms. • The company holding the RSA patent, RSA Security, Inc., alleged that PGP's use of the RSA algorithm infringed on its patent. • PRZ has the same problem with IDEA’s patent.

7. Feel PGP…

8. Base64? • Described in RFC 3548, Base64 is a way of encoding any stream of bits into a set of alphanumeric characters (a-z, A-Z, 0-9 and two/three other characters). • Example: 111100100011010001010110 8jRW • Good for RFC 822 compliance, but increases e-mail size by about 33%.

9. PGP Services

10. What makes PGP “pretty good” • The best available cryptographic algorithms as building blocks. • A general-purpose application that is independent of operating system and processor and that is based on a small set of easy-to-use commands. • Well-documented, free, open-source, widely available. • Good support, low-cost commercial versions available, too.

11. Digging a bit deeper • PRZ’s art was not to use the best building blocks, but also to “glue” them in the best way. • Let’s see how sticky his glue was!

12. Compression • Compensates for 33% increase of Base64 encoding. • Statistical analysis shows that, on average, ZIP compression ratio is 50%. • Typical messages are shrunk by 66% (ZIP+Base64).

13. Compression (cont’d) • Why sign before compression? • People are likely to store uncompressed messages; • Re-compression not deterministic. • Why use compression before encryption? • To reduce encryption time; • To remove redundancy, thus making cryptanalysis harder. • Bruce Schneier’s “Decryption Oracle”

14. Encryption Process • The sender generates a message and a random 128/168-bit number to be used as a session key for this message only. The random number generation is based on ANSI X12.17. • The message is encrypted, using CAST-128 (or IDEA or 3DES) with the session key. Encryption is done in 64-bit CFB (Cipher Feedback) mode.

15. Encryption Process (cont’d) • The session key is encrypted with RSA/ElGamal, using the recipient's public key, and is appended to the message. • The receiver uses RSA/ElGamal with its private key to decrypt and recover the session key. • The session key is used to decrypt the message.

16. Types of Keys • How many keys are used in PGP? • one-time session conventional keys; • public keys; • private keys; • passphrase-based conventional keys (more on this later).

17. Key Requirements • A means of generating unpredictable session key. • A user must be allowed to have multiple public-key/private-key pairs. • Why? • Each PGP entity must maintain a file of its own public/private key pairs as well as a file of public keys of correspondents.

18. Session Keys • One-to-one relationship between messages & session keys. • Session keys are random numbers seeded with user’s keystrokes. • Both keystroke timing & the actual keys struck are used. • The conventional algorithm itself is used to derive the random number from input.

19. A PGP message Differs from that of the book.

20. Key Rings • Key management has received a large amount of attention in PGP as it is the Achilles heel of all security systems. • PGP provides two types of data structures for key management called Key Ring: Public Key Ring, and Private Key Ring.

21. Private Key Ring Passphrase is used here Usually email address Must be unique

22. Public Key Ring Trust

23. Public Key Management: Trust • The most important issue in public-key systems is how to distribute public keys, preventing man-in-the-middle / masquerade attacks. • This issue becomes a real pain in the neck when the parties don’t know each other, and don’t have physical access to each other. • Traditional approach: Using CAs.

24. Trust (cont’d) • PRZ didn’t believe in the USG, so he didn’t trust in the USG’s agents (CAs) either. • He introduced a new, government-agnostic approach, called Web of Trust. • While, IMHO, WoT is the most elegant feature of PGP today, it’s also the source of every sin!

25. Trust (cont’d) • To better understand the WoT, think you are lost in a small city, and want to find somewhere. Note that not all people can be trusted, and you’ll be in danger if you choose the wrong path  • As a starting point, suppose you know a few people in the city, each of which knows a few of other citizens, but isn’t informative enough by his own to give you some hints.

26. Trust (cont’d)

27. Trust (cont’d) • A Trust model consists of two parts: • A set of rule, e.g. : • I will trust everyone’s signature! • I will trust everyone trusted by two distinct persons already trusted by me. • A set of information: • Ali, Mohsen, and Reza are trusted by me; • Mohsen trusts Hassan; • Ali & Reza both trust Hossein.

28. PGP Trust Model • Each entry in the public-key ring is a public key certificate. • Associated with each entry are zero or more signatures that the key ring owner has collected that sign this certificate. • In turn, each signature has associated with it a signature trust field that indicates the degree to which this PGP user trusts the signer to certify public keys.

29. PGP Trust Model (cont’d) • An owner trust field indicates the degree to which this public key is trusted to sign other public-key certificates; this level of trust is assigned by the user. • The signature trust can be thought of the cached copy of this field.

30. PGP Trust Model (cont’d) • The key legitimacy field indicates the extent to which PGP will trust that this is a valid public key for this user; the higher the level of trust, the stronger is the binding of this user ID to this key. This field is computed by PGP. • The key legitimacy field is derived from the collection of signature trust fields in the entry.

31. Fields & Values

32. Revoking Public Keys • A user may wish to revoke his or her current public key either because compromise is suspected or simply to avoid the use of the same key for an extended period. • The convention for revoking a public key is for the owner to issue a key revocation certificate, signed by the owner. • Any Problem with this approach?

33. If privacy is outlawed, only outlaws will have privacy. ---Phil Zimmermann