1. Tutorial on Network Security: Sep 2003
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Tutorial on Network Security: Sep 2003

2. Lecture 1: Introduction
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Tutorial on Network Security: Sep 2003

3. Tutorial on Network Security: Sep 2003
Top-level issues
Safety, security and privacy
Security policy
threats, both external and internal
economic gains
cost of securing resources
cryptographic methods vs. physical security
Information security:
nature of resources (HW, SW, information)
during storage, access and communication
limited to a single computer vs. network security
various layers (physical through application layers)
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Tutorial on Network Security: Sep 2003

4. Tutorial on Network Security: Sep 2003
Security threats
Intentional vs. accidental
Various forms of violations:
Non-destructive
Destructive
Repudiation
Denial of service
Threat techniques:
crypt-analysis
snooping
masquerading
replay attacks
virus, worms
etc.
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Tutorial on Network Security: Sep 2003

5. Tutorial on Network Security: Sep 2003
Security services
Services (or functions) vs. mechanisms
Security functions:
confidentiality
authentication
integrity
non-repudiation
access control
availability
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Tutorial on Network Security: Sep 2003

6. Tutorial on Network Security: Sep 2003
Security mechanisms
Physical controls
Audit trails
Fraud detection (data mining)
Steganography
Encryption:
private-key vs. public-key encryption
key generation, exchange, and management
certification
Firewalls
etc.
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Tutorial on Network Security: Sep 2003

7. Lecture 2: Symmetric-key encryption
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Tutorial on Network Security: Sep 2003

8. Cryptographic systems
Symmetric vs. asymmetric encryption
Number of keys used
Key lengths
Block vs. stream cipher
Crypt-analysis (assume algorithm is known)
ciphertext (only)
plaintext + ciphertext
chosen plaintext + ciphertext
chosen ciphertext + plaintext
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Tutorial on Network Security: Sep 2003

9. Symmetric cryptographic system
Symmetric encryption
Plaintext, X
Ciphertext, Y
Secret keys for encryption, decryption, K
Secret key, K
Encrypt
EK(X)
Decrypt
DK(X)
Crypt-analysis X Y K
Secure channel
Insecure channel
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Tutorial on Network Security: Sep 2003

10. Asymmetric cryptographic system
Asymmetric encryption
Plaintext, X
Ciphertext, Y
Two keys K1, and K2. One is secret, other is public
One of them (secret or public) is used to encrypt, the other for decryption
Helps with confidentiality, digital signatures
Key generation, management
Encrypt
EK(X)
Decrypt
DK(X)
Crypt-analysis X Y K1 K2
Insecure channel
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11. Tutorial on Network Security: Sep 2003
Symmetric encryption
Substitution cipher
Transposition cipher
DES
Triple DES
Blowfish, RC5, RC4, etc.
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Tutorial on Network Security: Sep 2003

12. Tutorial on Network Security: Sep 2003
Substitution cipher
Ceasar cipher
encrypt C  (p+k) mod n
decrypt p  (C-k) mod n
assumes set of n characters
easily breakable in n-1 steps
Substitute using n x n table
encrypt Ci  lookup_encrypt(pi)
decrypt pj  lookup_decrypt(Cj)
26! Different keys
may be broken using known “relative frequency” of each character
To counter:
use multiple symbols to substitute
substitute multiple symbols at a time
e.g. two letter strings at a time
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Tutorial on Network Security: Sep 2003

13. Tutorial on Network Security: Sep 2003
Transposition cipher
Transposition example:
To make it more secure:
transposition it multiple times
combine it with substitution ciphers
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14. Tutorial on Network Security: Sep 2003
DES
Combination of several substitution and transposition ops
Applied to each block of size 64 bits
Key is 56 bits
Uses portions of key at different steps
Uses techniques referred to by “diffusion and confusion”
Developed by IBM , accepted by NBS (USA) as a standard in 1977
Primarily a block cipher
Encypt
EK(X) C1 K P1
Decrypt
DK(X) P1 K C1
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Tutorial on Network Security: Sep 2003

15. DES encryption algorithm
Initial permutation
Round 1
Round 2
Round 16
32-bit swap
Inverse permute K1 K2
K16
Permuted key
Left circular shift
64-bit plaintext
64-bit ciphertext
56-bit key
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16. Tutorial on Network Security: Sep 2003
Cipher Block Chaining
Primarily a block cipher
May be used in “block chaining mode”
Encrypt
EK(X) C1 IV K + P1 C2 P2
Decrypt
DK(X) P1 IV K + C1 P2 C2
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17. Tutorial on Network Security: Sep 2003
Strength of DES
Key size of 56 bits appears to be too small
In 1993 Weiner developed HW device for $100K with 5760 search engines to break it in 35 hours
In 1997, 70,000 systems on Internet discovered the key in less than 96 days (part of plaintext is given)
Automating the process is difficult, unless plaintext is known
Perhaps breakable by studying and exploiting weakness
Differential cryptanalysis
Linear cryptanalysis
Trapdoor
US Govt changed the original design
Continues to enjoy wide acceptibility
Particularly with triple-DES (used in PGP)
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18. Tutorial on Network Security: Sep 2003
Double-DES
Two stages of encryption, using two different keys
Decrypt
EK2(X) X K2
Encypt
EK1(X) C P K1
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19. Tutorial on Network Security: Sep 2003
Double-DES
“two stages cannot be reduced to one stage”:
for given K1, K2, there is no K s.t. EK2(EK1(P)) = EK(P)
Meet-in-the-middle attack
Let C = EK2(EK1(P)), and X = EK1(P) = DK2(C)
Let known P and C
Search for K1 and K2 such that X = EK1(P) = DK2(C)
Complexity is O( ), not O(2128)
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Tutorial on Network Security: Sep 2003

20. Tutorial on Network Security: Sep 2003
Triple-DES
Three stages of encryption, using two different keys
Decrypt
EK2(X) X1 K2
Encypt
EK1(X) C P K1 X2
EK3(X) K3
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21. Tutorial on Network Security: Sep 2003
IDEA
International data encryption algorithm (IDEA)
developed in 1991, gaining ground
block cipher
better understood
US government has had no role in its design
design principle:
block size 64 bits
key length 128 bits
more emphasis on “diffusion” and “confusion”
uses three operations:
“exclusive-OR”, “addition”, “multiplication”
some effort to make HW implementation easier
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Tutorial on Network Security: Sep 2003

22. Tutorial on Network Security: Sep 2003
RC5
developed by Rivest, in 1994
suitable for HW or SW implementation on microprocessors
simple
different word length
low memory
high level of security
simpler determination of strength
variable no. of “rounds”, key length
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Tutorial on Network Security: Sep 2003

23. Tutorial on Network Security: Sep 2003
Blowfish
Developed in 1993
block cipher
up to 448 bit keys
no known attacks
simple, fast and compact
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Tutorial on Network Security: Sep 2003

24. Summary: symmetric key encryption
Since the same key is used to encrypt and decrypt, the system is also know as private-key encryption
Symmetric key encryption
uses shared secret keys
also known as “private-key” encryption
Primarily used for purpose of confidentiality
but may be used to authenticate as well, but may be “repudiated”
Key sharing or management is an issue
particularly when the no. of clients sharing the key is “large”
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Tutorial on Network Security: Sep 2003

25. Application to confidentiality
Private-key encryption may be used to provide confidentiality of messages during transfer over LANs and/or WANs
At issue:
what information:
User data vs. headers
Identity of correspondents vs. node/route identity
in what layer, and between what points
Link-layer vs. end-to-end vs. application level
Assumption: data over physical network is accessible
Wireless links
Employee of the network service provider
Your own colleagues
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Tutorial on Network Security: Sep 2003

26. Link-level vs. end-to-end confidentiality
Host A B R
Link-level enrypt/ decrypt
End-to-end enrypt/ decrypt
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Tutorial on Network Security: Sep 2003

27. Link-level vs. end-to-end confidentiality
Link-level encryption
End-to-end encryption
Security within nodes, hosts
Exposed in intermediate nodes
Exposed in end hosts
Encrypted in intermediate nodes
Encrypted/Decrypted by end hosts
Role of end devices, intermediate nodes
Intermediate nodes require encryption One key for each link
Done in hardware
Only end hosts need encryption One key per session/connection
Perhaps done in software
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Tutorial on Network Security: Sep 2003

28. Traffic confidentiality
Issues:
Identity of communicating entities
Identity of hosts, routers
Traffic volumes, patterns
Link-level encryption offers better confidentiality
Padding may be used to “hide” patterns and volumes
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Tutorial on Network Security: Sep 2003

29. Tutorial on Network Security: Sep 2003
Key distribution
Secret key must be distributed between the communicating entities, say A and B
Link level encryption requires L number of keys to be distributed, one for each device at the end of a link
Host-to-host encryption requires N*(N-1)/2 keys to be distributed
Two techniques:
Physical delivery (works only in a very limited environs)
A delivers it to B
A trusted third party C delivers the key to A and to B
Electronic delivery using an established secure connection or session
A delivers it to B after suitably encrypting it
A trusted third party C delivers the key to A and to B using secure channels to A and to B.
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Key distribution
Electronic distribution by B to A, though process initiated by A
Above:
N1 and N2 are “nonce”,
MKm is the “master key” used by A and B
KS is the new “session key”
F is a well-known function, such as ADD 1
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31. Tutorial on Network Security: Sep 2003
Key distribution
Electronic distribution by trusted third party C to A and to B
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32. Tutorial on Network Security: Sep 2003
Key distribution
Above:
KA and KB are keys used by A and B, respectively, to communicate with C
IDA identifies entity A
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33. Tutorial on Network Security: Sep 2003
Key distribution
Secure operation of these schemes, against:
Masquerade
replay attacks
Other issues:
Hierarchy of keys
Lifetime of a session key
Generation of Nonce or Random numbers
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Tutorial on Network Security: Sep 2003

34. Tutorial on Network Security: Sep 2003
Thanks
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Tutorial on Network Security: Sep 2003