1. Chapter 29 Internet Security

2. Outline INTRODUCTION PRIVACY DIGITAL SIGNATURE
SECURITY IN THE INTERNET
APPLICATION LAYER SECURITY
TRANSPORT LAYER SECURITY: TLS
SECURITY AT THE IP LAYER: IPSEC
FIREWALLS

3. 29.1
INTRODUCTION
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4. Introduction Security involves four aspects Privacy (confidentiality)
Message authentication
Message integrity
Nonrepudiation

5. Privacy The sender and the receiver expect confidentiality
The transmitted message must make sense to only the intended receiver
To all others, the message must be unintelligible

6. Authentication
Receiver is sure of the sender’s identity

7. Integrity Data must arrive at the receiver exactly as it was sent
There must be no changes during the transmission
Either accidental or malicious

8. Nonrepudiation
A receiver must be able to prove that a received message came from a specific sender
The sender must not be able to deny sending a message that he, in fact, did send
The burden of proof falls on the receiver
For example, when a customer sends a message to transfer money from one account to another
The bank must proof that the customer actually requested this transaction

9. Figure 29-1
Aspects of Security
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10. 29.2
PRIVACY
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11. Privacy To achieve privacy Sender: Receiver
Message must be encrypted
Sender:
Plaintext -> encrypted -> ciphertext
Receiver
Ciphertext -> decrypted -> plaintext
Two categories of encryption/decryption methods
The secret-key methods and the public-key methods

12. Secret-Key Encryption/Decryption
The same key is used by both parties
Thus, often referred to as symmetric encryption
Well-known algorithm
DES (Data encryption standard)

13. Secret-Key Encryption/Decryption
The algorithm used for decryption is the inverse of the algorithm used for encryption
For example
If the encryption algorithm uses a combination of addition and multiplication
The decryption algorithm uses a combination of division and subtraction

14. Secret-Key Encryption
Figure 29-2
Secret-Key Encryption
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15. In secret-key encryption, the same key is used by the sender (for encryption) and the receiver (for decryption). The key is shared.
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16. Secret-key encryption is often called symmetric encryption because the same key can be used in both directions.
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17. Advantages Secret-key algorithms are efficient
It takes less time to encrypt a message than using a public-key algorithm
Because the key is usually smaller
Thus, secret-key algorithms are used to encrypt and decrypt long message

18. Secret-key encryption is often used for long messages.
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19. Disadvantages Each pair of users must have a secret key
If N people want to use this method
There needs to be N(N-1)/2 secret key
The distribution of the keys between two parties can be difficult

20. We discuss one secret-key algorithm in Appendix E.
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21. Key Distribution Center (KDC)
In secret-key encryption
Two parties must agree on a shared secret key
However, these two parties may never be in contact with each other
One acceptable solution
For both to trust a third party, a key distribution center (KDC)

22. KDC can solve the problem of secret-key distribution.
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23. Public-Key Encryption
There are two keys
A private key and a public key
The private key is kept by the receiver
The public key is announced to be public
For example, in next slide, A wants to send a message to B
A uses the public key to encrypt the message
B use the private key to decrypt the message
Well-known algorithm: RSA

24. Public-Key Encryption
Figure 29-3
Public-Key Encryption
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25. Advantages
The whole idea behind public-key encryption is to remove the restriction of a shared key between two parties
The number of keys needed is reduced tremendously

26. Disadvantage The complexity of the algorithm
Public-key encryption is not recommended for large amounts of text
The association between an entity and it public key must be verified
For example, if A sends its public key via an to B
B must be sure that the public key really belongs to A and nobody else
Solution: Certification Authority (CA)

27. Public-key algorithms are more efficient for short messages.
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28. Certification Authority (CA)
Public-key encryption needs the owner of the public key must be verified
Thus, a Certification Authority (CA) is an agency that binds a public key and an entity and issues a certificate

29. A CA can certify the binding between a public key and the owner.
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30. Using the Combination We can combine both Method
The advantage of the secret-key method: efficiency
The advantage of the public-key method: easy distribution of keys
Method
The public key is used to encrypt the secret key
The secret key is used to encrypt the message

31. Using the Combination (Cont.)
The procedures is as follows
The sender chooses a secret key
The sender uses the public key of the receiver to encrypt the secret key and sends the encrypted secret key to the receiver
Since public-key method is good for short message
A secret key is a short text message
The receiver uses its private key to decrypt the secret key
The sender uses the shared secret key to encrypt the actual message

32. Figure 29-4
Combination
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33. To have the advantages of both secret-key and public-key encryption, we can encrypt the secret key using the public key and encrypt the message using the secret key.
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34. 29.3
DIGITAL SIGNATURE
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35. Digital Signature Privacy has been achieved above
The other three methods can be achieved using digital signature
Authentication, integrity, and nonrepudication
Two choices
Sign the entire document
Sign the digest (condensed version) of the document

36. Signing the Whole Document
Public-key encryption can be used to sign a document
However, the role of public and private keys are different here
Sender uses her private key to encrypt (sign) the message
Receiver uses the public key to decrypt the message

37. Signing the Whole Document
Figure 29-5
Signing the Whole Document
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38. Signing the Whole Document (Cont.)
Digital signature can provide the
Integrity
If an intruder intercepts the message and changes it
The decrypted message would be unreadable
Authentication
If an intruder X sends a message pretending that it is coming from user G
However, X must use her private key for encryption
But the receiver will decrypt the message using G’s public key. As a result, the message would be unreadable

39. Signing the Whole Document (Cont.)
Nonrepudiation
If the sender denies sending the message, however, it does sent before
We can test the message using her private key and pubic key

40. Digital signature does not provide privacy
Digital signature does not provide privacy. If there is a need for privacy, another layer of encryption/decryption must be applied.
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41. Signing the Digest
Public-key encryption is efficient if the message is short
Thus, sign the whole document using a public key is very inefficient
Solution
Sign a digest of the document instead of the whole document

42. Figure 29-6
Signing the Digest
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43. Signing the Digest (Cont.)
How to create a digest of the message
Use a hash function that creates a fixed-size digest from a variable-length message
Common hash function
MD5 (Message Digest 5)
Produce a 120-bit digest
SHA-1 (Secure Hash Algorithm 1)
Produce a 160-bit digest

44. Signing the Digest (Cont.)
The hash function must has two properties
Hashing is one-way
The digest can only be created from the message, not vice versa
Hashing is a one-to-one function
There is little probability that two messages will create the same digest
The actions perform in the sender and receiver are shown in the two next slides

45. Figure 29-7
Sender Site
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46. Figure 29-8
Receiver Site
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47. Signing the Digest (Cont.)
According to the previous discussion
Digest is secure in terms of integrity, authentication, and nonrepudiation
But, how about the message itself ?
Integrity
Authentication
Nonrepudiation
Verify by yourself

48. SECURITY IN THE INTERNET
29.4
SECURITY IN THE INTERNET
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49. Security in the Internet
Security measures can be applied to the application layer, transport layer, and the IP layer
At the application layer
Each application is responsible for providing security
The implementation of security at this level is the simplest
It only concerns two entities: client and server

50. Security in the Internet (Cont.)
At the transport layer
Security is more complicated
Implementation methods
Modify the transport layer for security
Glue a new layer to the transport layer to provide security on behalf of the transport layer
At the IP layer
Implementation of security features is very complicated
Since every device must be able to handle it

51. APPLICTION LAYER SECURITY
29.5
APPLICTION LAYER SECURITY
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52. Application Layer Security
Simpler since only involves two parties
For example, or TELNET
Two well-known protocols
PGP
SSH

53. Pretty Good Privacy (PGP)
Provide all four aspects of security in the sending of
PGP uses one hash function, one secret key, and two private-public key pairs
PGP uses
Digital signature, a combination of hashing and public-key encryption, to provide integrity, authentication, and nonrepudiation
A combination of secret-key and public-key encryption to provide privacy

54. PGP at The Sender Site Figure 29-9
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55. PGP at The Receiver Ssite
Figure
PGP at The Receiver Ssite
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56. Secure Shell (SSH) A client-server program that provides security
A secure form of the rlogin client-server application program
Authentication
When a user logs into the system, the authentication test must be passed
SSH uses public-key encryption to provide authentication
Authorization
SSH uses sophisticated authorization to allow access to files

57. Secure Shell (SSH) (Cont.)
Privacy
Data exchanged between the user and the system are encrypted to provide privacy
Integrity
SSH guarantees the integrity of the message in both directions
Tunneling
SSH uses application level tunneling other applications inside itself

58. 29.6 TRANSPORT LAYER SECURITY (TLS)
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59. Transport Layer Security: TLS
TLS was designed to provide security at the transport layer
Derived from the Secure Sockets Layer (SSL)
Designed by Netscape to provide security on the WWW
A nonproprietary version of SSL designed by IETF

60. Transport Layer Security: TLS (Cont.)
A browser needs the following
The customer needs to be sure that the server belongs to the actual vender
A customer does not want an imposter to make charges on her credit card
The server must be authenticated
The customers needs to be sure that the contents of the message are not modified during transition
A bill for $100 must not be changed to $1000
The integrity of the message must be preserved

61. Transport Layer Security: TLS (Cont.)
The customer needs to be sure that an imposter does not intercept sensitive information (credit card number)
There is a need for privacy

62. Figure
Position of TLS
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63. Transport Layer Security: TLS (Cont.)
Two protocols
Handshake protocol
Data exchange protocol

64. Handshake Protocol The browser sends a hello message
The server sends a certificate message
Include the public key of the server
The public key is certified by some CA
Then the browser decrypts the certificate and finds the server public key
Brower has a list of CAs and their public key
Thus, browser also authenticates the server

65. Handshake Protocol (Cont.)
The browser generates a secret key, encrypts it with the server public key and sends it to the server
The browser sends a message, encrypted by the secret key, to inform the server that handshaking is terminating from the browser side
Finally, the server
Decrypt the secret key using its private key
Decrypt the message using the secret key
Send a message, encrypted by the secret key, to inform the browser that handshaking is terminating

66. Figure
Handshake Protocol
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67. Data Exchange Protocol
Use the secret key to
Encrypt the data for secrecy
Encrypt the message digest for integrity

68. SECURITY AT THE IP LAYER (IPSec)
29.7
SECURITY AT THE IP LAYER (IPSec)
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69. Security at the IP Layer: IPSEC
IPSec is a collection of protocols designed by the IETF
To provide security for a packet carried on the Internet
IPSec does not define the use of any specific encryption or authentication method
It provide a framework and a mechanism
It leaves the selection of the encryption/authentication and hashing methods to the user

70. Security at the IP Layer: IPSEC (Cont.)
IPSec defines two protocols to be used
Authentication Header (AH) protocol
Encapsulating Security Payload protocol

71. Authentication Header (AH) Protocol
Designed to provide integrity
Involve a digital signature using a hashing function

72. Authentication Header (AH) Protocol
Addition of an AH header follows the steps
An AH header is added to the payload with the authentication data field set to zero
The AH header and the payload are hashed to create the authentication data
The authentication data are inserted into the AH header
The IP header is added after changing the value of the protocol field to 51
The original value of the protocol field is copied to a field in AH header

73. Figure
Authentication
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74. Header Format Next Header: 8-bit Payload Length: 8-bit
Define the type of the payload carried by the IP datagram (TCP, UDP, ICMP, OSPF)
Copy the value of the protocol field in the IP datagram
The value of the protocol field in IP is changed to 51
Payload Length: 8-bit
Define the length of the AH header in multiples of 4 bytes
Does not define the length of the payload

75. Header Format (Cont.) Security Parameter Index: 32-bit
Define the security method used in creating the authentication data
Sequence Number: 32-bit
Provide ordering information
Authentication Data
The result of applying a hash function to the entire IP datagram, except for the fields that are changed during transmit, e.g., time-to-live

76. Figure
Header Format
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77. Encapsulating Security Payload
The AH protocol does not provide privacy
Only provides integrity and message authentication (digital signature)
IPSec thus defines another protocol called Encapsulating Security Payload (ESP)
Provide privacy and a combination of integrity and message authentication

78. Encapsulating Security Payload (Cont.)
ESP procedures
An ESP trailer is added to the payload
The payload and the trailer are encrypted
The ESP header is added
The ESP header, payload, and the ESP trailer are used to create authentication data
The authentication data are added at the end of the ESP trailer
The IP header is added after changing the protocol field to 50

79. Figure
ESP
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80. Format of Header and Trailer
ESP header
Security Parameter Index: 32-bit
Define the security method used in creating the authentication data
Sequence Number: 32-bit
ESP trailer
Padding: a variable length field
For alignment or make the length of data to be encrypted a multiple of some predefined value
Pad Length: 8-bit
Define the number of padding bytes
Next header: 8-bit
Copy the value in the protocol field in the IP datagram
The protocol value in the IP datagram is changed to 50

81. Figure
ESP Format
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82. Format of Header and Trailer (Cont.)
ESP Auth.
Authentication Data:
The result of applying an authentication scheme to parts of the datagram

83. 29.8
FIREWALLS
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84. Firewalls
A router attached between the internal network of an organization and the rest of the Internet
Firewalls are normally used for two purposes
As a packet-filter firewall
As a proxy-based firewall

85. Figure
Firewall
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86. Packet-Filter Firewall
Forward or block packets based on the information in the network layer and transport layer headers
Source and destination IP addresses
Source and destination port addresses
Type of protocol (TCP or UDP)

87. Packet-Filter Firewall
Figure
Packet-Filter Firewall
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88. Packet-Filter Firewall (Cont.)
According to the above table
Incoming packets from network are blocked
The * means “any”
Incoming packets destined for any internal TELNET server (port 23) are blocked
Incoming packets destined for internal host are blocked
Organization wants this host for internal use only
Outgoing packet destined for an HTTP server (port 80) are blocked
The organization does not want employees to browse the Internet

89. A packet-filter firewall filters at the network or transport layer.
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90. Proxy Firewall
Packet-filter firewall is based on the information available on the network layer and transport layer headers
However, we may need to filter a message based on the information available in the message itself
At the application layer

91. Proxy Firewall (Cont.) Solution: A proxy computer
Sometimes called an application gateway
Look at the packet in the application level

92. Figure
Proxy Firewall
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93. A proxy firewall filters at the application layer.
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