1. By: Nasoor Bagheri (na_bagheri@yahoo.com)
In the name of god
Network Security
By: Nasoor Bagheri

2. Introduction In most systems security is an important concern
Communications should be secure against eavesdropping and tampering
Servers/clients should be able to verify the identity of their clients/servers
The originator of a message should be verifiable after the message has been delivered

3. Security Requirements
Confidentiality
Integrity
Availability
Authenticity

4. Security Levels unconditional security computational security
no matter how much computer power is available, the cipher cannot be broken since the ciphertext provides insufficient information to uniquely determine the corresponding plaintext
computational security
given limited computing resources (eg time needed for calculations is greater than age of universe), the cipher cannot be broken
Unconditional security would be nice, but the only known such cipher is the one-time pad (later). For all reasonable encryption algorithms, have to assume computational security where it either takes too long, or is too expensive, to bother breaking the cipher.

5. Evolution of System Security
Current
Platforms
Multi-user
timesharing
computers
Distributed systems
based on local
networks
The Internet, wide-
area services
The Internet + mobile
devices
Shared
resources
Memory, files
Local services (e.g.
NFS), local networks
, web sites,
Internet commerce
Distributed objects,
mobile code
Security
requirements
User identification and
authentication
Protection of services
Strong security for
commercial
transactions
Access control for
individual objects,
secure mobile code
management
environment
Single authority,
single authorization
database (e.g. /etc/
passwd)
delegation, repli-
cated authorization
databases (e.g. NIS)
Many authorities,
no network-wide
authorities
Per-activity
authorities, groups
with shared
responsibilities

6. Techniques Security mechanisms are based on three techniques
Cryptography
Used to conceal information
Used in support of authentication
Used to implement digital signatures
Authentication
Validate the identity of the sender
Access Control
Allow resources to be accessed only by authorized individuals

7. Goals Authentication: Are you who you claim to be?
Authorization: Are you authorized to do this?
Integrity: Is the message what the user has sent?
Confidentiality: Can one read this?
Availability (Denial-of-service attacks)
Accountability (Non-repudiation): How can we prove that the message was indeed sent by the sender?

8. Tools Cryptography: Symmetric and asymmetric
Symmetric: A single secret key for encryption and decryption
Asymmetric: A key pair: Public key and private key; sender encrypts using key owner’s public key; key owner decrypts using corresponding private key

9. Tools (contd.) Encryption/Decryption algorithms
Digital signatures: Sender uses private key to generate a special tag and appends to the message. Receiver cross checks the integrity by rechecking with the sender’s public key. (Third party can resolve a dispute)
Message authentication code (MAC): Secret key, when applied on a message, gives MAC. The receiver can regenerate the MAC and ensure the integrity of the message
Cryptographic hash functions

10. Security Persons managing the security of a valued resource consider
Prevention: taking measures that prevent damage.
E.g., one-time passwords (s/key)
Detection: measures that allow detection of when an asset has been damaged, altered, or copied.
E.g., intrusion detection, network forensics
Risk assessment: the value of a resource should determine how much effort (or money) is spent protecting it.
E.g., If you have nothing in your house of value do you need to lock your doors other than to protect the house itself?
If you have an $16,000,000 artwork, you might consider a security guard.

11. Threats and Form of Attacks
Security threats common to computer systems fall into four broad classes
Leakage
Acquisition of information by unauthorized parties
Tampering
The unauthorized alteration of information
Resource Stealing
use of facilities without authorization
Vandalism
interference with proper operation
2 main categories of attack: passive and active.

12. Passive Attacks Eavesdropping on transmissions to obtain information
Release of message contents
Outsider learns content of transmission
Traffic analysis
By monitoring frequency and length of messages, even encrypted, nature of communication may be guessed
Difficult to detect
Can be prevented

13. Active Attacks Masquerade Replay Modification of messages
Pretending to be a different entity
Replay
Modification of messages
Denial of service
Easy to detect
Detection may lead to deterrent
Hard to prevent

14. Methods of attack Eavesdropping Replaying DoOperation . (continue)
GetRequest .
execute
SendReply
replay
eaves-
drop

15. Methods of attack cont. Masquerading GetRequest . execute SendReply
client
imposter
DoOperation .
(continue)
server
imposter

16. Cryptography
The most widely used tool for securing information and services is cryptography.
Cryptography relies on ciphers: mathematical function used for encryption and decryption of a message.
Encryption: the process of disguising a message in such a way as to hide its substance.
Ciphertext: an encrypted message
Decryption: the process of returning an encrypted message back into plaintext.
Encryption
Decryption
Plaintext
Ciphertext
Original

17. Example Ciphers
Caesar cipher: each plaintext characters is replaced by a character k to the right.
“Watch out for Brutus!” => “Jngpu bhg sbe Oehghf!”
Only 25 choices! Not hard to break by brute force.
Substitution Cipher: each character in plaintext is replaced by a corresponding character of ciphertext.
E.g., cryptograms in newspapers.
plaintext code: a b c d e f g h i j k l m n o p q r s t u v w x y z
ciphertext code: m n b v c x z a s d f g h j k l p o i u y t r e w q
26! Possible pairs.

18. Ciphers
For some message M, let’s denote the encryption of that message into cipher text as
Ek(M) = C
Similarly, the decryption into plain text as
Dk(C) = M
Notice,
Dk(Ek(M)) = M symmetric key algorithms.
Some algorithms use different keys for each operation:
Dk1(Ek2(M))= M public-key algorithms.

19. Figure 16.1 Simplified Model of Symmetric Encryption

20. Ingredients Plain text Encryption algorithm Secret key Cipher text
Decryption algorithm

21. Requirements for Security
Strong encryption algorithm
Even if known, should not be able to decrypt or work out key
Even if a number of cipher texts are available together with plain texts of them
Sender and receiver must obtain secret key securely
Once key is known, all communication using this key is readable

22. Attacking Encryption Cryptanalysis Brute force
Relay on nature of algorithm plus some knowledge of general characteristics of plain text
Attempt to deduce plain text or key
Brute force
Try every possible key until plain text is achieved

23. Timing Attacks developed in mid-1990’s
exploit timing variations in operations
eg. multiplying by small vs large number
or IF's varying which instructions executed
infer operand size based on time taken
RSA exploits time taken in exponentiation
countermeasures
use constant exponentiation time
add random delays
blind values used in calculations

24. Cryptanalysis Ke Kd C = EKe(plaintext) Encrypt Decrypt plaintext
Invader
Side information
plaintext
Cryptanalysis

25. Cryptanalysis
Cryptanalysis is the science of recovering the plaintext of a message without access to the key.
Doesn’t have to discover the key necessarily.
The loss of a key without cryptanalysis is called a compromise.
Ciphertext-only attack
The attacker has to recover the plaintext from only the ciphertext.
Known-plaintext attack
Portions of the cipher are known as plaintext. The rest may be easier to recover
Chosen-plaintext attack
The attacker can choose what plaintext to encrypt, again making it easier to recover other ciphertext.

26. Encryption Algorithms
Block cipher
Process plain text in fixed block sizes producing block of cipher text of equal size
Data encryption standard (DES)
Triple DES (TDES)
Advanced Encryption Standard

27. Simple Block Cipher
Plaintext message B3 B2 B1 B0
encrypt B0 B1 B2 B3

28. Problem
If the same block is encrypted twice with the same key, the resulting ciphertext blocks are the same
It is desirable to make identical plaintext blocks encrypt to different ciphertext blocks.
Two methods are commonly used for this:
CBC mode: a ciphertext block is obtained by first xoring the plaintext block with the previous ciphertext block, and encrypting the resulting value.
CFB mode: a ciphertext block is obtained by encrypting the previous ciphertext block, and xoring the resulting value with the plaintext.

29. CBC
XOR
Bn+4
Bn+3
Bn+2
Bn+1
encrypt
Bn-3
Bn-2
Bn-1 Bn

30. Stream Ciphers
For some applications encryption in blocks will not work
Telephone conversation
Radio Broadcast …
White noise…

31. Stream Cipher encrypt K3 K2 K1 K0 buffer keystream XOR
number
generator
encrypt K3 K2 K1 K0
buffer
keystream
XOR
Plaintext stream
Encrypted stream

32. Data Encryption Standard
US standard
64 bit plain text blocks
56 bit key
Broken in 1998 by Electronic Frontier Foundation
Special purpose machine
Less than three days
DES now worthless

33. Triple DEA ANSI X9.17 (1985) Incorporated in DEA standard 1999
Uses 3 keys and 3 executions of DEA algorithm
Effective key length 112 or 168 bit
Slow
Block size (64 bit) too small

34. Advanced Encryption Standard
National Institute of Standards and Technology (NIST) in 1997 issued call for Advanced Encryption Standard (AES)
Security strength equal to or better than 3DES
Improved efficiency
Symmetric block cipher
Block length 128 bits
Key lengths 128, 192, and 256 bits
Evaluation include security, computational efficiency, memory requirements, hardware and software suitability, and flexibility
2001, AES issued as federal information processing standard (FIPS 197)

35. AES Description Assume key length 128 bits
Input is single 128-bit block
Depicted as square matrix of bytes
Block copied into State array
Modified at each stage
After final stage, State copied to output matrix
128-bit key depicted as square matrix of bytes
Expanded into array of key schedule words
Each four bytes
Total key schedule 44 words for 128-bit key
Byte ordering by column
First four bytes of 128-bit plaintext input occupy first column of in matrix
First four bytes of expanded key occupy first column of w matrix

36. Location of Encryption Devices Figure: Encryption Across a Packet Switching Network

37. Link Encryption Each communication link equipped at both ends
All traffic secure
High level of security
Requires lots of encryption devices
Message must be decrypted at each switch to read address (virtual circuit number)
Security vulnerable at switches
Particularly on public switched network

38. End to End Encryption Encryption done at ends of system
Data in encrypted form crosses network unaltered
Destination shares key with source to decrypt
Host can only encrypt user data
Otherwise switching nodes could not read header or route packet
Traffic pattern not secure
Use both link and end to end

39. Key Distribution
Question: How to deliver a shared key to 2 parties that wish to exchange data without others to see the key?
Key selected by A and delivered to B
Third party selects key and delivers to A and B
Use old key to encrypt and transmit new key from A to B
Use old key to transmit new key from third party to A and B

40. Message Authentication
Protection against active attacks
Falsification of data
Eavesdropping
Message is authentic if it is genuine and comes from the alleged source
Authentication allows receiver to verify that message is authentic
Message has not altered
Message is from authentic source
Message timeline

41. Authentication Using Encryption
Assumes sender and receiver are only entities that know key
Message includes:
error detection code
sequence number
time stamp

42. Authentication Without Encryption
Authentication tag generated and appended to each message
Message not encrypted
Useful for:
Messages broadcast to multiple destinations
Have one destination responsible for authentication
One side heavily loaded
Encryption adds to workload
Can authenticate random messages
Programs authenticated without encryption can be executed without decoding

43. Message Authentication Code
Generate authentication code based on shared key and message
Common key shared between A and B
If only sender and receiver know key and code matches:
Receiver assured message has not altered
Receiver assured message is from alleged sender
If message has sequence number, receiver assured of proper sequence

44. Figure : Message Authentication Using a Message Authentication Code

45. One Way Hash Function
Accepts variable size message and produces fixed size tag (message digest)
Advantages of authentication without encryption
Encryption is slow
Encryption hardware expensive
Encryption hardware optimized to large data
Algorithms covered by patents
Algorithms subject to export controls (from USA)

46. Secure Hash Functions Hash function must have following properties:
Can be applied to any size data block
Produce fixed length output
Easy to compute
Not feasible to reverse
Not feasible to find two message that give the same hash

47. SHA-1 Secure Hash Algorithm 1 Input message less than 264 bits
Processed in 512 bit blocks
Output 160 bit digest

48. Public Key Encryption Based on mathematical algorithms Asymmetric
Use two separate keys
Ingredients
Plain text
Encryption algorithm
Public and private key
Cipher text
Decryption algorithm

49. Figure 16.9 Public-Key Cryptography

50. Public Key Encryption - Operation
One key made public
Used for encryption
Other kept private
Used for decryption
Infeasible to determine decryption key given encryption key and algorithm
Either key can be used for encryption, the other for decryption

51. Digital Signature Sender encrypts message with their private key
Receiver can decrypt using senders public key
This authenticates sender, who is only person who has the matching key
Does not give privacy of data
Decrypt key is public

52. Signatures Handwritten signatures can verify that a document is
Authentic
The signature is mine and has not been altered
Unforgettable
Proves that I signed the document
Non-repudiation
I cannot deny that I signed the document

53. Digital Signatures
Public key systems can also be used to provide message authentication:
The sender’s secret key can be used to encrypt a message, thereby signing it
This creates a digital signature of a message, which the recipient (or anyone else) can check by using the sender's public key to decrypt it.
This proves that the sender was the true originator of the message, and that the message has not been subsequently altered by anyone else

54. Digital Properties
The properties of digital documents are different from paper documents
We need to be able to bind a signature to the entire sequence of bits that make up the document
How do I prevent someone from revealing their private key and then claiming they never signed something?

55. Last Slide ?
Thank you