• Announcement
• Authentication
– Cryptography
– Authentication
• Quiz #3 at the end of today’s class

• You must do your demo before 4:30pm, April 29, 2003
• Name server: program1
• Prefix table
Logical Prefix Server IP Server Port Remote directory
/ 128.186.120.34 1281 /tmp/XXX/cop5611-DFS
/program1 128.186.120.53 1282 /tmp/XXX/cop5611-DFS
/linprog1 128.186.120.33 1280 /tmp/liux/cop5611-DFS
/program3 128.186.120.55 1285 /tmp/XXX/cop5611-DFS
• You need to have your servers running before you come to my office
– I will test your system through your client program and based on your report
April 16, 2020 COP 5611 - Operating Systems 2

• The fundamental problem to security in distributed systems is the use of cryptographic techniques
– Access matrix model can be used to prevent unauthorized accesses if the users that claimed to be are true
– However, in distributed systems, the user authentication becomes a big problem
April 16, 2020 COP 5611 - Operating Systems 3

– cont.
April 16, 2020 COP 5611 - Operating Systems 4

• A threat to a system in which an intruder can have access to only the ciphertext is called a ciphertextonly attack
• A threat to a system in which an intruder can have access to both ciphertext and a considerable amount of corresponding plaintext is called a known-plaintext attack
• A threat to a system in which an intruder can obtain ciphertext corresponding to plaintext of his choice is referred to as a chosen-plaintext attack
April 16, 2020 COP 5611 - Operating Systems 5

• Shannon’s principle
– Shannon’s principle of diffusion – Spread the correlation and dependencies among key-string variables over substrings as much as possible
– Shannon’s principle of confusion – Change a piece of information so that the output has no obvious relation to the input
• Exhaustive search principle
– The determination of the key requires an exhaustive search of the an extremely large space
April 16, 2020 COP 5611 - Operating Systems 6

• Data encryption standard (DES)
– It is a block cipher that crypts 64-bit data blocks using a 56-bit key
– Two basic operations
• Permutation
• Substitution
– Three stages
• Initial permutation stage
• Complex transformation stage
• Final permutation stage
April 16, 2020 COP 5611 - Operating Systems 7

– cont.
April 16, 2020 COP 5611 - Operating Systems 8

– cont.
April 16, 2020 COP 5611 - Operating Systems 9

– cont.
April 16, 2020 COP 5611 - Operating Systems 10

• Private key cryptography and conventional cryptographic techniques require the distribution of secret keys
– Known as the key distribution problem
• Public key cryptography solves the key distribution problem by making the encryption procedure and the associated key available in the public domain
April 16, 2020 COP 5611 - Operating Systems 11

– cont.
• Now it is possible for two users to have a secure communication even they have not communicated before
• Implementation issues
– One-way functions
April 16, 2020 COP 5611 - Operating Systems 12

• The encryption key is a pair (e, n)
• The decryption key is a pair (d, n)
April 16, 2020 COP 5611 - Operating Systems 13

• Generating the private and public key requires four steps
– Choose two very large prime numbers, p and q
– Compute n = p x q and z = (p – 1) x (q – 1)
– Choose a number d that is relatively prime to z
– Compute the number e such that e x d = 1 mod z
14 April 16, 2020 COP 5611 - Operating Systems

• In distributed systems, authentication means verifying the identity of communicating entities to each other
– The assumption is that the communication network is not secure in that an intruder can copy and play back a message on the network
– The textbook called it “interactive secure connections”
April 16, 2020 COP 5611 - Operating Systems 15

– cont.
• Authentication based on a shared secret key.
April 16, 2020 COP 5611 - Operating Systems 16

– cont.
• Authentication based on a shared secret key, but using three instead of five messages.
April 16, 2020 COP 5611 - Operating Systems 17

– cont.
• The reflection attack.
April 16, 2020 COP 5611 - Operating Systems 18

Authentication Using a Key Distribution Center
• The principle of using a KDC.
April 16, 2020 COP 5611 - Operating Systems 19

Authentication Using a Key Distribution Center – cont.
• Using a ticket and letting Alice set up a connection to Bob.
April 16, 2020 COP 5611 - Operating Systems 20

Authentication Using a Key Distribution Center – cont.
• The Needham-Schroeder authentication protocol.
April 16, 2020 COP 5611 - Operating Systems 21

Authentication Using a Key Distribution Center – cont.
• Protection against malicious reuse of a previously generated session key in the Needham-Schroeder protocol.
April 16, 2020 COP 5611 - Operating Systems 22

Authentication Using Public-Key Cryptography
• Mutual authentication in a public-key cryptosystem.
April 16, 2020 COP 5611 - Operating Systems 23

Message Integrity and Confidentiality
• Message integrity means that messages are protected against modification
• Confidentiality ensures that messages cannot be intercepted and read by eavesdroppers
• Digital signatures
– A user cannot forge the signature of other users
– A sender of a signed message cannot deny the validity of his signature on the message
– A recipient of a signed message cannot modify the signature in the message
April 16, 2020 COP 5611 - Operating Systems 24

• Digital signing a message using public-key cryptography.
April 16, 2020 COP 5611 - Operating Systems 25

– cont.
April 16, 2020 COP 5611 - Operating Systems 26

– cont.
• Digitally signing a message using a message digest.
April 16, 2020 COP 5611 - Operating Systems 27

• The principle of Diffie-Hellman key exchange.
April 16, 2020 COP 5611 - Operating Systems 28

April 16, 2020 COP 5611 - Operating Systems 29

– cont.
April 16, 2020 COP 5611 - Operating Systems 30

April 16, 2020 COP 5611 - Operating Systems 31

– cont.
• Setting up a secure channel in Kerberos.
April 16, 2020 COP 5611 - Operating Systems 32

•
Payment systems based on direct payment between customer and merchant.
a) Paying in cash.
b) Using a check.
c) Using a credit card.
April 16, 2020 COP 5611 - Operating Systems 33

– cont.
• Payment systems based on money transfer between banks.
a) Payment by money order.
b) Payment through debit order.
April 16, 2020 COP 5611 - Operating Systems 34

April 16, 2020 COP 5611 - Operating Systems 35

April 16, 2020 COP 5611 - Operating Systems 36

Summary
• Cryptography is a fundamental problem in security of distributed systems
– Based on private keys
– Based on public keys
• Authentication in distributed systems
• There are still a lot of challenges and research issues in this area
April 16, 2020 COP 5611 - Operating Systems 37