Chapter 29 Internet Security

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

29.1 INTRODUCTION The McGraw-Hill Companies, Inc., 2000

Introduction Security involves four aspects Privacy (confidentiality) Message authentication Message integrity Nonrepudiation

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

Authentication Receiver is sure of the sender’s identity

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

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

Figure 29-1 Aspects of Security The McGraw-Hill Companies, Inc., 2000

29.2 PRIVACY The McGraw-Hill Companies, Inc., 2000

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

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)

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

Secret-Key Encryption Figure 29-2 Secret-Key Encryption The McGraw-Hill Companies, Inc., 2000

In secret-key encryption, the same key is used by the sender (for encryption) and the receiver (for decryption). The key is shared. The McGraw-Hill Companies, Inc., 2000

Secret-key encryption is often called symmetric encryption because the same key can be used in both directions. The McGraw-Hill Companies, Inc., 2000

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

Secret-key encryption is often used for long messages. The McGraw-Hill Companies, Inc., 2000

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

We discuss one secret-key algorithm in Appendix E. The McGraw-Hill Companies, Inc., 2000

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)

KDC can solve the problem of secret-key distribution. The McGraw-Hill Companies, Inc., 2000

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

Public-Key Encryption Figure 29-3 Public-Key Encryption The McGraw-Hill Companies, Inc., 2000

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

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 email to B B must be sure that the public key really belongs to A and nobody else Solution: Certification Authority (CA)

Public-key algorithms are more efficient for short messages. The McGraw-Hill Companies, Inc., 2000

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

A CA can certify the binding between a public key and the owner. The McGraw-Hill Companies, Inc., 2000

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

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

Figure 29-4 Combination The McGraw-Hill Companies, Inc., 2000

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. The McGraw-Hill Companies, Inc., 2000

29.3 DIGITAL SIGNATURE The McGraw-Hill Companies, Inc., 2000

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

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

Signing the Whole Document Figure 29-5 Signing the Whole Document The McGraw-Hill Companies, Inc., 2000

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

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

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. The McGraw-Hill Companies, Inc., 2000

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

Figure 29-6 Signing the Digest The McGraw-Hill Companies, Inc., 2000

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

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

Figure 29-7 Sender Site The McGraw-Hill Companies, Inc., 2000

Figure 29-8 Receiver Site The McGraw-Hill Companies, Inc., 2000

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

SECURITY IN THE INTERNET 29.4 SECURITY IN THE INTERNET The McGraw-Hill Companies, Inc., 2000

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

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

APPLICTION LAYER SECURITY 29.5 APPLICTION LAYER SECURITY The McGraw-Hill Companies, Inc., 2000

Application Layer Security Simpler since only involves two parties For example, email or TELNET Two well-known protocols PGP SSH

Pretty Good Privacy (PGP) Provide all four aspects of security in the sending of email 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

PGP at The Sender Site Figure 29-9 The McGraw-Hill Companies, Inc., 2000

PGP at The Receiver Ssite Figure 29-10 PGP at The Receiver Ssite The McGraw-Hill Companies, Inc., 2000

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

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

29.6 TRANSPORT LAYER SECURITY (TLS) The McGraw-Hill Companies, Inc., 2000

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

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

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

Figure 29-11 Position of TLS The McGraw-Hill Companies, Inc., 2000

Transport Layer Security: TLS (Cont.) Two protocols Handshake protocol Data exchange protocol

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

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

Figure 29-12 Handshake Protocol The McGraw-Hill Companies, Inc., 2000

Data Exchange Protocol Use the secret key to Encrypt the data for secrecy Encrypt the message digest for integrity

SECURITY AT THE IP LAYER (IPSec) 29.7 SECURITY AT THE IP LAYER (IPSec) The McGraw-Hill Companies, Inc., 2000

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

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

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

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

Figure 29-13 Authentication The McGraw-Hill Companies, Inc., 2000

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

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

Figure 29-14 Header Format The McGraw-Hill Companies, Inc., 2000

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

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

Figure 29-15 ESP The McGraw-Hill Companies, Inc., 2000

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

Figure 29-16 ESP Format The McGraw-Hill Companies, Inc., 2000

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

29.8 FIREWALLS The McGraw-Hill Companies, Inc., 2000

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

Figure 29-17 Firewall The McGraw-Hill Companies, Inc., 2000

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)

Packet-Filter Firewall Figure 29-18 Packet-Filter Firewall The McGraw-Hill Companies, Inc., 2000

Packet-Filter Firewall (Cont.) According to the above table Incoming packets from network 131.34.0.0 are blocked The * means “any” Incoming packets destined for any internal TELNET server (port 23) are blocked Incoming packets destined for internal host 194.78.20.8 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

A packet-filter firewall filters at the network or transport layer. The McGraw-Hill Companies, Inc., 2000

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

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

Figure 29-19 Proxy Firewall The McGraw-Hill Companies, Inc., 2000

A proxy firewall filters at the application layer. The McGraw-Hill Companies, Inc., 2000