Cryptography and Network Security Chapter 18

1. Cryptography and Network SecurityChapter 18 Fifth Edition by William Stallings Lecture slides by Lawrie Brown

2. Despite the refusal of VADM Poindexter and LtCol North to appear, the Board's access to other sources of information filled much of this gap. The FBI provided documents taken from the files of the National Security Advisor and relevant NSC staff members, including messages from the PROF system between VADM Poindexter and LtCol North. The PROF messages were conversations by computer, written at the time events occurred and presumed by the writers to be protected from disclosure. In this sense, they provide a first-hand, contemporaneous account of events. —The Tower Commission Report to President Reagan on the Iran-Contra Affair, 1987 Chapter 15 – Electronic Mail Security

3. email is one of the most widely used and regarded network services currently message contents are not secure may be inspected either in transit or by suitably privileged users on destination system Email Security

4. confidentiality protection from disclosure authentication of sender of message message integrity protection from modification non-repudiation of origin protection from denial by sender Email Security Enhancements

5. widely used de facto secure email program developed by Phil Zimmermann selected best available crypto algs to use integrated into a single program on Unix, PC, Macintosh and other systems originally free, now also have commercial versions available Pretty Good Privacy (PGP)

6. sender creates message make SHA-1160-bit hash of message attached RSA signed hash to message receiver decrypts & recovers hash code receiver verifies received message hash PGP Operation – Authentication

7. sender forms 128-bit random session key encrypts message with session key attaches session key encrypted with RSA receiver decrypts & recovers session key session key is used to decrypt message PGP Operation – Confidentiality

8. can use both services on same message create signature & attach to message encrypt both message & signature attach RSA/ElGamal encrypted session key PGP Operation – Confidentiality & Authentication

9. by default PGP compresses message after signing but before encrypting so can store uncompressed message & signature for later verification & because compression is non deterministic uses ZIP compression algorithm PGP Operation – Compression

10. when using PGP will have binary data to send (encrypted message etc) however email was designed only for ASCII text hence PGP must encode raw binary data into printable ASCII characters uses radix-64 algorithm maps 3 bytes to 4 printable chars also appends a CRC(A cyclic redundancy check is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data) PGP also segments messages if too big PGP Operation – Email Compatibility

11. PGP Operation – Summary

12. PGP makes use of four types of keys one-time session symmetric keys, public keys, private keys, and passphrase-based symmetric keys. need a session key for each message of varying sizes: 56-bit DES, 128-bit CAST or IDEA, 168-bit Triple-DES Random numbers generated using ANSI X12.17 mode uses random inputs taken from previous uses and from keystroke timing of user PGP Session Keys

13. since many public/private keys may be in use, need to identify which is actually used to encrypt session key in a message could send full public-key with every message but this is inefficient rather use a key identifier based on key is least significant 64-bits of the key will very likely be unique also use key ID in signatures PGP Public & Private Keys

14. PGP Message Format

15. A message consists of three components: • the message component, • a signature (optional), • and a session key component (optional). • The message component includes the actual data to be stored or transmitted, as well as a filename and a timestamp that specifies the time of creation.

16. The signature component includes a • timestamp, • encrypted SHA-1 message digest, • leading two digest octets for verification, • and the Key ID of the sender’s public key. • The session key component includes the session key and the identifier of the recipient's public key that was used by the sender to encrypt the session key. The entire block is usually encoded with radix-64 encoding.

17. each PGP user has a pair of keyrings: public-key ring contains all the public-keys of other PGP users known to this user, indexed by key ID private-key ring contains the public/private key pair(s) for this user, indexed by key ID & encrypted keyed from a hashed passphrase security of private keys thus depends on the pass-phrase security PGP Key Rings

18. PGP Key Rings

19. PGP Message Generation

20. The sending PGP entity performs the following steps: • 1. Signing the message: • a. PGP retrieves the sender's private key from the private-key ring using your_userid as an index. If your_userid was not provided in the command, the first private key on the ring is retrieved. • b. PGP prompts the user for the passphrase to recover the unencrypted private key. • c. The signature component of the message is constructed.

21. 2. Encrypting the message: • a. PGP generates a session key and encrypts the message. • b. PGP retrieves the recipient's public key from the public-key ring using her_userid as an index. • c. The session key component of the message is constructed.

22. PGP Message Reception

23. PGP crypto services (again ignoring compression and radix-64 conversion for simplicity). The receiving PGP entity performs the following steps: • 1. Decrypting the message: • a. PGP retrieves the receiver's private key from the private-key ring, using the Key ID field in the session key component of the message as an index. • b. PGP prompts the user for the passphrase to recover the unencrypted private key.

24. c. PGP then recovers the session key and decrypts the message. • 2. Authenticating the message: • a. PGP retrieves the sender's public key from the public-key ring, using the Key ID field in the signature key component of the message as an index. • b. PGP recovers the transmitted message digest. • c. PGP computes the message digest for the received message and compares it to the transmitted message digest to authenticate.

25. rather than relying on certificate authorities in PGP every user is own CA can sign keys for users they know directly forms a “web of trust” trust keys have signed can trust keys others have signed if have a chain of signatures to them key ring includes trust indicators users can also revoke their keys PGP Key Management

26. PGP Trust Model Example

27. security enhancement to MIME email original Internet RFC822 email was text only MIME provided support for varying content types and multi-part messages with encoding of binary data to textual form S/MIME added security enhancements have S/MIME support in many mail agents eg MS Outlook, Mozilla, Mac Mail etc S/MIME (Secure/Multipurpose Internet Mail Extensions)

28. enveloped data encrypted content and associated keys signed data encoded message + signed digest clear-signed data cleartext message + encoded signed digest signed & enveloped data nesting of signed & encrypted entities S/MIME Functions

29. digital signatures: DSS & RSA hash functions: SHA-1 & MD5 session key encryption: ElGamal & RSA message encryption: AES, Triple-DES, RC2/40 and others MAC: HMAC with SHA-1 have process to decide which algs to use S/MIME Cryptographic Algorithms

30. S/MIME secures a MIME entity with a signature, encryption, or both forming a MIME wrapped PKCS(public-key cryptography specifications) object have a range of content-types: enveloped data signed data clear-signed data registration request certificate only message S/MIME Messages

31. S/MIME uses X.509 v3 certificates managed using a hybrid of a strict X.509 CA hierarchy & PGP’s web of trust each client has a list of trusted CA’s certs and own public/private key pairs & certs certificates must be signed by trusted CA’s S/MIME Certificate Processing

32. have several well-known CA’s Verisign one of most widely used Verisign issues several types of Digital IDs increasing levels of checks & hence trust Class Identity Checks Usage 1 name/email check web browsing/email 2 + enroll/addr check email, subs, s/w validate 3 + ID documents e-banking/service access Certificate Authorities

33. S/MIME Enhanced Security Services • 3 proposed enhanced security services: • signed receipts • security labels • secure mailing lists

34. Domain Keys Identified Mail a specification for cryptographically signing email messages so signing domain claims responsibility recipients / agents can verify signature proposed Internet Standard RFC 4871 has been widely adopted

35. Internet Mail Architecture

36. Email Threats • see RFC 4684- Analysis of Threats Motivating DomainKeys Identified Mail • describes the problem space in terms of: • range: low end, spammers, fraudsters • capabilities in terms of where submitted, signed, volume, routing naming etc • outside located attackers

37. DKIM Strategy • transparent to user • MSA sign • MDA verify • for pragmatic reasons

38. DCIM Functional Flow

39. have considered: secure email PGP S/MIME domain-keys identified email Summary