1. E-MAIL SECURITY – Chapter 15
….for authentication and confidentiality
PGP
Uses best algorithms as building blocks
General purpose
Package/source code free
Low-cost commercial version
No government

2. PGP CRYPTOGRAPHIC FUNCTIONS

3. Confidentiality Compression e-mail Segmentation PGP for…….
Authentication
Confidentiality
Compression
Segmentation

4. DIGITAL SIGNATURES (fig 15.1a)
SHA-1 with RSA  Signature
(RSA, KUa)  KRa
(H, KRa)  Signed
(alternative – DSS/SHA-1)

5. - Separate Transmission - separate log detect virus
DETACHED SIGNATURES
instead of…..
Attached Signatures
use…..
Detached Signatures
- Separate Transmission
- separate log
detect virus
many signatures – one doc

6. CONFIDENTIALITY (fig 15.1b)
CAST or IDEA or 3DES : CFB – 64
Key Distribution:
RSA/Diffie-Hellman/El Gamal
Symmetric Key used once/message
Random  128-bit key, Ks
: key sent with message

7. SYMMETRIC/PUBLIC COMBINATION
Faster than just PUBLIC
PUBLIC solves key distribution
No protocol – one-time message
No handshaking
One-time keys strengthen security
(weakest link is public)

8. CONFIDENTIALITY and AUTHENTICATION (fig 15.c)
Authentication - plaintext mess. stored
third-party can verify signature without
needing to know secret key
Compression
Confidentiality

9. COMPRESSION - why? Benefit - efficiency Why,
Signature then Compression then Confidentiality ?
Sign Uncompressed Message
- off-line storage
No need for single compression algorithm
Encryption after compression is stronger

10. COMPATIBILITY
uses ASCII
PGP(8-bit)  ASCII
Base-64: 3x8  4 x ASCII + CRC
33% Expansion !!
(fig 15.2)

11. RADIX-64 FORMAT

12. Tx and Rx of PGP Messages

13. SEGMENTATION / REASSEMBLY
Max length restriction
e.g. internet = 50,000 x 8-bits
PGP Segments automatically
but,
One session key,signature/message

14. } PGP KEYS one-time session : use random number gen. 2. public
3. private
4. passphrase-based }
key id
file of key pairs
for all users
multiple pairs

15. SESSION-KEY GENERATION
CAST / IDEA / 3DES in CFB mode 64 64
plaintext
- user key strokes K
K – user key strokes
and old session key
128 64 64 }
New Session Key

16. each public key has key ID (least 64 bits)
KEY IDENTIFIERS
Which public key?
each public key has key ID
(least 64 bits)
With high prob., no key ID collision

17. Message,m [data, filename, timestamp] signature (optional)
MESSAGE FORMAT (fig 15.3)
Message,m [data, filename, timestamp]
signature (optional)
includes digest = hash(m(data)||T)
therefore signature is:
[T, EKRa(digest),2x8(digest), KeyID]
session key (optional) [key, IDKUb]

18. MESSAGE FORMAT

19. store public/private pairs of node A Public Key Ring
KEY RINGS (fig 15.4)
Private Key Ring
store public/private pairs of node A
Public Key Ring
store public keys of all other nodes

20. KEY RINGS

21. ENCRYPTED PRIVATE KEYS on PRIVATE KEY-RING
User passphrase
System asks user for passphrase
Passphrase  160-bit hash
Ehash(private key)
subsequent access requires passphrase

22. PGP MESSAGE GENERATION

23. PGP MESSAGE RECEPTION

24. PUBLIC KEY MANAGEMENT
Problem: need tamper-resistant public-keys
(e.g. in case A thinks KUc is KUb)
Two threats: C  A (forge B’s signature)
A  B (decrypt by C)
solution:
Key-Revoking

25. PGP TRUST MODEL EXAMPLE

26. ZIP freeware (c) : UNIX, PKZIP : Windows LZ77 (Ziv,Lempel)
Repetitions  short code (on the fly)
codes re-used
algorithm MUST be reversible

27. ZIP (example)
(Fig 15.9)
char  9 bits = 1 bit + 8-bit ascii
look for repeated sequences
continue until repetition ends
e.g. the brown fox
 8-bit pointer, 4-bit length, 00
 12-bit pointer, 6-bit length, 01
then ’ jump’  ptr + length, ind
compressed to 35x9-bit + two codes
= 343 bits
Compression Ratio = 424/343 = 1.24

28. ZIP (example)

29. COMPRESSION ALGORITHM
Sliding History Buffer – last N chars
Look-Ahead Buffer – next N chars
Algorithm tries to match chars from 2. to 1.
if no match,
9 bits LAB  9 bits SHB
else if match found output:
indicator for length K string, ptr, length
K bits LAB  K bits SHB

30. COMPRESSION ALGORITHM

31. PGP RANDOM NUMBER GENERATION

32. S/MIME (Secure/Multipurpose Mail Extension) S/MIME - commercial
PGP private
S/MIME - based on MIME
(designed for RFC822)
RFC traditional text-mail
internet standard
Envelope + Contents

33. CRYPTO ALGORITHMS USED in S/MIME
(Table 15.6)
Sender/Recipients must agree on
common encryption algorithm
S/MIME secures MIME entity with
signature and/or encryption
MIME entity
entire message
subpart of message

34. SECURING a MIME ENTITY WRAPPED in PREPARE S/MIME MIME security data
PKCS OBJECT
S/MIME

35. S/MIME CERTIFICATE PROCESSING
Hybrid of X.509 certification authority
and PGP’s ”web of trust”
Configure each client
 Trusted Keys
Certification Revocation List