1. Chapters 8 Network Security
Professor Rick Han
University of Colorado at Boulder

2. Prof. Rick Han, University of Colorado at Boulder
Announcements
Programming Assignment #3 due May 2
HW #4 handed back today, solutions on Web tonight
In Chapter 8, read all sections.
Need a volunteer for FCQ’s.
Next, Network Security
Prof. Rick Han, University of Colorado at Boulder

3. Recap of Previous Lecture
Samba/SMB
Enables file-sharing between UNIX and Windows
SMB is basis of Windows’ Network Neighborhood
Originally built on top of NETBIOS, now directly above TCP/UDP
Samba server on Linux emulates SMB protocol, enables Windows client to see Linux files
Security
Confidentiality
Authentication
Authorization
Integrity
Non-repudiation
Availability
Prof. Rick Han, University of Colorado at Boulder

4. Recap of Previous Lecture (2)
Cryptography
Plaintext -> encryption -> ciphertext -> decryption -> plaintext
Modern encryption algorithms use secret keys
Algorithm itself can be known
Cryptanalysis attacks:
Brute force
Cipher-text only
Plaintext-only
Chosen-plaintext
Adaptive Chosen-plaintext = differential cryptanalysis
Prof. Rick Han, University of Colorado at Boulder

5. Prof. Rick Han, University of Colorado at Boulder
Cryptography
plaintext
Encryption
ciphertext
Decryption
plaintext
Encryption algorithm also called a cipher
Cryptography has evolved so that modern encryption and decryption use secret keys
Only have to protect the keys! => Key distribution problem
Cryptographic algorithms can be openly published
Encryption
Decryption
plaintext
ciphertext
Key KA
Key KB
Prof. Rick Han, University of Colorado at Boulder

6. Principles of Confusion and Diffusion
Encryption
Decryption
plaintext
ciphertext
Key KA
Key KB
Terms courtesy of Claude Shannon, father of Information Theory
“Confusion” = Substitution
a -> b
Caesar cipher
“Diffusion” = Transposition or Permutation
abcd -> dacb
DES
Prof. Rick Han, University of Colorado at Boulder

7. Principles of Confusion and Diffusion (2)
“Confusion” : a classical Substitution Cipher
Courtesy:
Andreas
Steffen
Modern substitution ciphers take in N bits and substitute N bits using lookup table: called S-Boxes
Prof. Rick Han, University of Colorado at Boulder

8. Principles of Confusion and Diffusion (3)
“Diffusion” : a classical Transposition cipher
Courtesy:
Andreas
Steffen
modern Transposition ciphers take in N bits and permute using lookup table : called P-Boxes
Prof. Rick Han, University of Colorado at Boulder

9. Symmetric-Key Cryptography
Encryption
Decryption
plaintext
ciphertext
Key KA
Key KB=KA
Secure Key Distribution
Both sender and receiver keys are the same: KA=KB
The keys must be kept secret and securely distributed – we’ll study this later
Thus, also called “Secret Key Cryptography”
Data Encryption Standard (DES)
Prof. Rick Han, University of Colorado at Boulder

10. Symmetric-Key Cryptography (2)
DES
64-bit input is permuted
16 stages of identical operation
differ in the 48-bit key extracted from 56-bit key - complex
R2= R1 is encrypted with K1 and XOR’d with L1
L2=R1, …
Final inverse permutation stage
Prof. Rick Han, University of Colorado at Boulder

11. Symmetric-Key Cryptography (3)
Data Encryption Standard (DES)
Encodes plaintext in 64-bit chunks using a 64-bit key (56 bits + 8 bits parity)
Uses a combination of diffusion and confusion to achieve security
Was cracked in 1997
Parallel attack – exhaustively search key space
Triple-DES: put the output of DES back as input into DES again with a different key, loop again: 3*56 = 168 bit key
Decryption in DES – it’s symmetric! Use KA again as input and then the same keys except in reverse order
Advanced Encryption Standard (AES) successor
Prof. Rick Han, University of Colorado at Boulder

12. Symmetric-Key Cryptography (4)
DES is an example of a block cipher
Divide input bit stream into n-bit sections, encrypt only that section, no dependency/history between sections
Courtesy:
Andreas
Steffen
In a good block cipher, each output bit is a function of all n input bits and all k key bits
Prof. Rick Han, University of Colorado at Boulder

13. Symmetric-Key Cryptography (5)
Electronic Code Book (ECB) mode for block ciphers of a long digital sequence
Vulnerable to replay attacks: if an attacker thinks block C2 corresponds to $ amount, then substitute another Ck
Attacker can also build a codebook of <Ck, guessed Pk> pairs
Prof. Rick Han, University of Colorado at Boulder

14. Symmetric-Key Cryptography (6)
Cipher Block Chaining (CBC) mode for block ciphers
Inhibits replay attacks and codebook building: identical input plaintext Pi =Pk won’t result in same output code due to memory-based chaining
IV = Initialization Vector – use only once
Prof. Rick Han, University of Colorado at Boulder

15. Symmetric-Key Cryptography (7)
Stream ciphers
Rather than divide bit stream into discrete blocks, as block ciphers do, XOR each bit of your plaintext continuous stream with a bit from a pseudo-random sequence
At receiver, use same symmetric key, XOR again to extract plaintext
Prof. Rick Han, University of Colorado at Boulder

16. Symmetric-Key Cryptography (8)
RC4 stream cipher by Ron Rivest of RSA Data Security Inc. – used in b’s security
Block ciphers vs. stream ciphers
Stream ciphers work at bit-level and were originally implemented in hardware => fast!
Block ciphers work at word-level and were originally implemented in software => not as fast
Error in a stream cipher only affects one bit
Error in a block cipher in CBC mode affects two blocks
Distinction is blurring:
Stream ciphers can be efficiently implemented in software
Block ciphers getting faster
Prof. Rick Han, University of Colorado at Boulder

17. Symmetric-Key Cryptography (9)
Symmetric key is propagated to both endpoints A & B via Diffie-Hellman key exchange algorithm
A & B agree on a large prime modulus n, a “primitive element” g, and a one-way function f(x)=gx mod n
n and g are publicly known
A chooses a large random int a and sends B AA=ga mod n
B chooses a large random int b and sends A BB= gb mod n
A & B compute secret key S = gba mod n
Since x=f-1(y) is difficult to compute, then observer who knows AA, BB, n, g and f will not be able to deduce the product ab and hence S is secure
Prof. Rick Han, University of Colorado at Boulder

18. Public-Key Cryptography
Encryption
Decryption
plaintext
ciphertext
Key KPUBLIC
Key KPRIVATE
For more than 2000 years, from the time of Caesar up to the 1970s, encrypted communication required that both sides shared a common secret key
Diffie and Hellman in 1976 invented asymmetric public key cryptography – elegant, revolutionary!
No longer need both sides to share a secret key
Can be used for authentication and digital signatures in addition to encryption!
Prof. Rick Han, University of Colorado at Boulder

19. Public-Key Cryptography (2)
Encryption
Decryption
plaintext
ciphertext
Key KPUBLIC
Key KPRIVATE
Public Key Distribution
Secure Key
Host who wants data sent to it advertises a public encryption key Kpublic
Decryption algorithm has the property that only a private key Kprivate can decrypt the ciphertext, even though attacker knows the public key Kpublic and the encryption algorithm
Prof. Rick Han, University of Colorado at Boulder

20. Public-Key Cryptography (3)
Decryption algorithm has the property that only a private key Kprivate can decrypt the ciphertext
Based on the difficulty of factoring the product of two prime #’s
Example: RSA algorithm (Rivest, Shamir, Adleman)
Choose 2 large prime #’s p and q
n=p*q should be about 1024 bits long
z=(p-1)*(q-1)
Choose e<n with no common factors with z
Find d such that (e*d) mod z = 1
Public key is (n,e), private key is (n,d)
Message m is encrypted to c = me mod n
Ciphertext c is decrypted m = cd mod n
Prof. Rick Han, University of Colorado at Boulder

21. RSA example: A host chooses p=5, q=7. Then n=35, z=24.
e=5 (so e, z relatively prime).
d=29 (so ed-1 exactly divisible by z. e
c = m mod n e
letter m m
encrypt:
“L” 12 17 c d
m = c mod n d c
letter
decrypt: 17 12
“L”
Prof. Rick Han, University of Colorado at Boulder

22. Public-Key Cryptography (4)
Provides security because:
There are no known algorithms for quickly factoring n=p*q, the product of two large prime #’s
If we could factor n into p and q, then it would be easy to break the algorithm: have n, p, q, e, then just iterate to find decryption key d.
Incredibly useful property of public-key cryptography:
m = cd mod n = (me)d mod n = (md)e mod n
Thus, can swap the order in which the keys are used. Example: can use private key for encryption and a public key for decryption – will see how it is useful in authentication!
Prof. Rick Han, University of Colorado at Boulder

23. Public-Key Cryptography (5)
Public-key cryptography is slow because of the exponentiation:
m = cd mod n = (me)d mod n = (md)e mod n
From kbps (1024-bit value for n)
So, don’t use it for time-sensitive applications and/or use only for small amounts of data – we’ll see how SSL makes use of this
A 512 bit number (155 decimals) was factored into two primes in 1999 using one Cray and 300 workstations
1024 bit keys still safe
Prof. Rick Han, University of Colorado at Boulder

24. Authentication (1)
Both sender and receiver need to verify the identity of the other party in a communication:
Goal: Bob wants Alice to “prove” her identity to him
Protocol ap1.0: Alice says “I am Alice”
Failure scenario??
Trudy says “I am Alice”
Prof. Rick Han, University of Colorado at Boulder

25. Prof. Rick Han, University of Colorado at Boulder
Authentication (2)
Protocol ap2.0: Alice says “I am Alice” and sends her IP
address along to “prove” it.
Failure scenario??
Trudy says “I am Alice”, Alice’s IP address
IP spoofing is easy. Some router’s don’t forward if IP src addr doesn’t match src LAN, but not all
Prof. Rick Han, University of Colorado at Boulder

26. Prof. Rick Han, University of Colorado at Boulder
Authentication (3)
Protocol ap3.0: Alice says “I am Alice” and sends her
secret password to “prove” it.
Failure scenario?
Trudy says “I am Alice”, Alice’s password
Telnet sents passwords in the clear
Prof. Rick Han, University of Colorado at Boulder

27. Prof. Rick Han, University of Colorado at Boulder
Authentication (4)
Protocol ap3.1: Alice says “I am Alice” and sends her
encrypted secret password to “prove” it.
I am Alice
encrypt(password)
Failure scenario?
Trudy says “I am Alice”, Alice’s encrypted password
Replay or playback attack: Trudy replays encrypted password without needing to know actual password
Prof. Rick Han, University of Colorado at Boulder