Network Security – Part 1Spring 2005

V.T. Raja, Ph.D.,

Oregon State University

BA483: Network SecurityPart 1 - Outline

• Introduction– Identify characteristics of a secure communication

• Confidentiality• Message Integrity• Non-repudiation• Authentication• Availability and Access Control

– Explanation of these characteristics– Cryptography

Confidentiality

• Alice wants the following to be confidential:– The fact that she is communicating with Bob– Timing of communication– Frequency of communication

• Only Alice and Bob should be able to understand the contents of the transmitted message; Should not be understood by eavesdropper Trudy.

Confidentiality

• Alice and Bob could represent two real users, or a client and a server, or 2 DNS servers, or 2 routers etc.

• Confidentiality often relies on cryptographic techniques.

Message Integrity

• Content of communication is not altered maliciously or by accident

• Message integrity relies on cryptographic techniques

Non-repudiation

• Assume sender transmitted document “D”. • Non-repudiation:

– Sender unable to successfully deny in court having transmitted document D.

• Non-repudiation also relies on cryptography techniques

Authentication

• Both sender and receiver should be able to confirm identity of other party involved in communication

• Confirm that the other party is indeed who/what they claim to be

• Authentication relies on authentication techniques, several of which rely on cryptographic techniques

Availability and Access Control

– Can communication occur in first place?• Detect breaches and respond to attacks

– Are entities seeking to gain access to resources allowed to do so only if they have the appropriate access rights, and perform their access in a well-defined manner?

• Firewalls provide access control based on a per-packet basis, and on a per-service basis.

• Provide a degree of isolation and protection from those outside of one’s network

Cryptography

• Symmetric Key and Public Key Cryptography• Basic Terminology

– Plain Text • Original data – not disguised

– Cipher (Encrypted) Text• Disguised data – looks unintelligible to intruder• Data disguised using encryption algorithm

– Key• A string of #s or characters used as input to encryption

algorithm to disguise plain text

Symmetric Key Cryptography

• Symmetric Key: – Alice and Bob use same key to encrypt and

decrypt text

• Symmetric Key Cryptography Techniques– Caesar Cipher– Monoalphabetic Cipher– Polyalphabetic Cipher– Data Encryption Standard (DES)– Triple DES (3DES)– Advanced Encryption Standard (AES)

Symmetric Key Cryptography

• Caesar Cipher– Each letter in plaintext is substituted with letter that is

K letters later– Wrap around is allowed (i.e., z followed by letter a)– If K = 3, a in plaintext becomes d in cipher text

b in plaintext becomes e in cipher text

Participation Exercise:

Once it is known that Caesar cipher is being used, it is easy to break the code (only 25 possible key values).

Symmetric Key Cryptography

• Monoalphabetic Cipher– Improvement on Caesar Cipher– Rather than substituting according to a regular pattern – any

letter can be substituted for any other letter, as long as each letter has a unique substitute letter, and vice versa.

– Example of a monoalphabetic cipher

Plain Text: 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

Cipher Text: 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 pairings of letters – so breaking code is not as easy as

in the case of Caesar cipher. Usually statistical analysis of plain text language (in this case English language), and some basic knowledge of intruder can help in breaking the code faster

Symmetric Key Cryptography

• Polyalphabetic Encryption– Use multiple monoalphabetic/Caesar ciphers– Use a specific monoalphabetic/Caesar cipher to

encode a letter in a specific position in the plain text message

– This implies that same letter appearing in different positions in the plaintext might be encoded differently.

Example: 2 Caesar ciphers; K = 5, K = 19

For every 5 bits in the plain text use the 2 Caesar ciphers in the following pattern: C1, C2, C2, C1, C2

Symmetric Key Cryptography

• Data Encryption Standard (DES)– Published in 1977, and updated in 1993– Used for commercial and non-classified U.S. Govt.

use– DES encodes plaintext in 64-bit chunks using 64-bit

key– Actually eight of the 64 bit-key are odd parity bits; So

actual key is perceived as 56-bits. – Objective: Scramble data and key so that every bit of

the cipher text depends on every bit of the data and every bit of the key

– Algorithm: Complex (beyond the scope of the course); Decryption works by reversing the algorithm’s operations.

How well does DES work?

• In 1997 RSA Data Security Inc., ( A network security company) launched a DES challenge contest to crack a short phrase (“strong cryptography makes the world a safer place”) it had encrypted using a 56-bit DES.

• Winning team took 4 months to decode. It had volunteers throughout the Internet to systematically explore key space. Claimed 10K cash prize after testing only a quarter of the key space (about 18 quadrillion keys)

• In 1999, RSA launched another DES challenge. • Message was decrypted in little over 22 hours by a

network of volunteers and a special purpose computer called “Deep Crack”. Claimed 250 K cash prize. Not bad for a day’s work?

Symmetric Key Cryptography

• Triple DES (3 DES)– If 56-bit DES is considered to be insecure, one can simply run

the algorithm multiple times, using a different key each time– DES run three times (with a different 56-bit key at beginning of

each time DES is run).

• Advanced Encryption Standard (AES)– NIST – in Nov 2001 announced successor to DES. – AES is also a symmetric key algorithm that processes data in

128-bit blocks– AES can operate with 128-bit keys, 192-bit keys, and 256-bit

keys– NIST estimated that a machine that could crack a 56-bit DES in

one second (i.e. Try 255 keys per second) would take approximately 149 trillion years to crack a 128-bit AES key

Symmetric Key Distribution

• Disadvantage of Symmetric Key Cryptography: – 2 communicating parties have to agree upon their

secret key ahead of time in a secure manner.

• Since sender and receiver do not meet face to face in the networking world , they need a trusted intermediary

• Trusted Intermediaries for symmetric key distribution:

• Key Distribution Center (KDC)• Kerberos

Key Distribution Center (KDC)

• A server that shares a different secret symmetric key with each registered user.

• This key might be manually installed at the server when a user first registers.

• KDC knows the secret key of each user, and each user can communicate securely with KDC using this key.

KDC for Alice and Bob

• Assume Alice and Bob use KDC for their communication.

• Assume Alice’s secret key known to Alice and KDC is KA-KDC; Assume Bob’s secret key known to Bob and KDC is KB-KDC.

• Assume Alice wishes to send an encrypted message to Bob while using KDC as the trusted intermediary.

Example: Alice and BOB using KDC

1. Using her key, Alice sends a message to KDC saying that she (A) wants to communicate with Bob (B). We denote this message as:

KA-KDC(A, B). 2. a. KDC decrypts KA-KDC(A, B).

b. KDC generates a random number R1, which is a “nonce” that will be used as symmetric key by Alice and Bob during their communication.

c. KDC sends Alice R1, and a pair of values A and R1 encrypted using Bob’s key. We denote this message sent to Alice by KDC as:

KA-KDC(R1, KB-KDC(A, R1)).

Example: Alice and BOB using KDC

3. Alice decrypts message and extracts symmetric key R1. Alice extracts and forwards (although she cannot decrypt)

KB-KDC(A, R1) to Bob.

4. Bob decrypts and understands that he is to use R1 as symmetric key to converse with Alice.

Kerberos

• Authentication service developed by MIT• Very similar to KDC• Has additional functions such as:

– Time stamp for validity of nonce R1. – Has info about which users have access privileges to

which services on which network servers. • The authentication server in Kerberos parlance,

is referred to as the Ticket Granting Server• Sending KB-KDC(A, R1) to A in the previous

example is referred in Kerberos as granting a ticket to Bob’s services.