Security and Authentication CS-4513, D-Term 20071 Security and Authentication (continued) CS-4513...

Security and Authentication

CS-4513, D-Term 2007 1

Security and Authentication(continued)

CS-4513D-Term 2007

(Slides include materials from Operating System Concepts, 7th ed., by Silbershatz, Galvin, & Gagne, Modern Operating Systems, 2nd ed., by Tanenbaum, and Distributed Systems: Principles & Paradigms, 2nd

ed. By Tanenbaum and Van Steen)


CS-4513, D-Term 2007 2

Review

• Authentication• How to identify someone• How to establish that they are who they say they are

• Fundamental to establishing authority in Distributed Systems

• Everything else is based on trust that the person or agent doing something has the authority to do it

• Threats• Masquerading as someone else• Intercepting / corrupting communications


CS-4513, D-Term 2007 3

Review (continued)

• Passwords• Easy to steal

• Easy to guess or “crack”

• Human frailties• Errors

• Dilemmas (“social engineering”)


CS-4513, D-Term 2007 4

Video


CS-4513, D-Term 2007 5

Reading Assignments

• Allman, Eric, “E-mail Authentication: what? Why? How?,” ACM Queue, November 2006, pp 30-34. (.pdf)

• One of– Tanenbaum, MOS, Chapter 9– Silbershatz, OSP, Chapters 14-15– Tanenbaum & van Steen, Chapter 9


CS-4513, D-Term 2007 6

Fun with Cryptography

• What is cryptography about?

• General Principles of Cryptography

• Basic Protocols– Single-key cryptography– Public-key cryptography

• A short intro to key distribution


CS-4513, D-Term 2007 7

Cryptography as a Security Tool

• Broadest security tool available– Fundamental foundation for secure storage and

communication– Basis for establishing trust– Means to constrain potential senders (sources)

and / or receivers (destinations) of messages– Means to detect/prevent intrusion or corruption

– (Cannot prevent denial of service attacks)


CS-4513, D-Term 2007 8

Principles

• Cryptography is about the exchange of messages

• The key to success is that all parties to an exchange trust that the system will both protect them from threats and accurately convey their message

• TRUST is essential


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Note

• Data storage is just another means of communication

• Writing data Sending message

• Reading data Receiving message• Perhaps much, much later!

• Integrity of data Integrity of message


CS-4513, D-Term 2007 10

Basic Premise of Cryptography

• Algorithms are (usually) public• Orders of magnitude easier to compute in forward

(normal) direction than in reverse (attack) direction

• Keys are always secret• Enough bits to prevent trying all key values

• Key management is a very big deal• Heart of all successful cryptographic systems


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Conventional Wisdom

• Algorithms must be public and verifiable

• We need to be able to estimate the risk of compromise

• The solution must practical for its users, and impractical for an attacker to break


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Public Policy Dilemma

• Algorithm intended to be a public standard must be subject to scrutiny of its users

• I.e., banks, industry, commerce, etc.

• To establish trust that it is good enough!

• Any algorithm good enough to protect billions of $$ of funds & commerce will be too hard for governments to penetrate!

• Crime, terrorism, etc.


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Ergo …

• Governments tend to use secret encryption methods and algorithms for the most secure communications

• Sometimes, confidence in such algorithms is misplaced!


CS-4513, D-Term 2007 14

History

• Most secret algorithms have been broken• Prior to computing age, at least

• Vulnerabilities• Redundancy in human languages

• Repeatability or lack of randomness in algorithm

• Repeatability or lack of randomness in keys


CS-4513, D-Term 2007 15

Guidelines

• Cryptography is always based on algorithms which are orders of magnitude easier to compute in the forward (normal) direction than in the reverse (attack) direction.

• The attacker’s problem is never harder than trying all possible keys

• The more material the attacker has the easier his task


CS-4513, D-Term 2007 16

Example

• What is 314159265358979 314159265358979?

vs.

• What are prime factors of3912571506419387090594828508241?


CS-4513, D-Term 2007 17

Caveat

• We cannot mathematically PROVE that the inverse operations are really as hard as they seem to be…It is all relative…

The Fundamental Tenet of Cryptography:

If lots of smart people have failed to solve a problem, it won’t be solved (soon)


CS-4513, D-Term 2007 18

Time marches on…

• We must assume that there will always be improvements in computational power, mathematics and algorithms.– Messages which hang around get less secure

with time!

• Increases in computing power help the good guys and hurt the bad guys for new and short-lived messages


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Two fundamental approaches

• Symmetric• Sender and receiver must share the key

• Asymmetric• Keys are paired

• Sender uses one, receiver uses its mate


CS-4513, D-Term 2007 20

Two fundamental approaches

• Symmetric• Sender and receiver must share the key there must be a secure way to get key from one to

the other

• Asymmetric• Keys are paired

• Sender uses one, receiver uses its mate there must be a secure way to get key from one to

the other


CS-4513, D-Term 2007 21

Secret key cryptography(Symmetric)

f (T,K) g (C,K)Cleartext Cleartext

K K

CyphertextT TC


CS-4513, D-Term 2007 22

Secret Key Methods

• DES (56 bit key)

• IDEA (128 bit key)• http://www.mediacrypt.com/community/index.asp

• Triple DES (three 56 bit keys)

• AES– From NIST, 2000– choice of key sizes up to 256 bits and more– Commercial implementations available

http://www.mediacrypt.com/community/index.asp


CS-4513, D-Term 2007 23

Reducing the Vulnerability

• Minimize the amount of information encrypted with shared key K

• Use K to encrypt a random number to obtain a session key

• I.e., used for one connection, conversation, exchange, etc.

• Discarded when channel is ended.


CS-4513, D-Term 2007 24

Diffie – Hellman

Alice Agree on p,g Bob

choose random A choose random BTA = gA mod p

TB = gB mod p

compute (TB)A compute (TA)B

Shared secret key for this session is gAB mod p

The shar

ed key

!


CS-4513, D-Term 2007 25

D–H Problems

• Not in itself an encryption method – we must still do a secret key encryption

• The body of the message

• Still must distribute the shared key safely

• Subject to a “man in the middle” attack• (Alice thinks she is talking to Bob, but actually

Trudy is intercepting all of the messages and substituting her own)


CS-4513, D-Term 2007 26

Questions about Secret Key Methods?


CS-4513, D-Term 2007 27

RSA Public Key Cryptography(Asymmetric Keys)

f () f ()Cleartext Cleartext

Key #1 Key #2

Cyphertext

Key #1 can be either a Public Key or a Private Key.Key #2 is then the corresponding Private Key or Public Key.

T C T


CS-4513, D-Term 2007 28

RSA Public Key Cryptography

• Rivest, Shamir and Adelman (1978)

• I can send messages that only you can read

• I can verify that you and only you could have sent a message

• I can use a trusted authority to distribute my public key – The trusted authority is for your benefit!


CS-4513, D-Term 2007 29

RSA Details

• Uses same operation to encrypt and decrypt

• To encrypt, we will use “e” as a key, to decrypt we will use “d” as a key

• e and d are inverses with respect to the chosen algorithm


CS-4513, D-Term 2007 30

RSA Details (continued)

• Based on mathematical premise that finding prime factors of large numbers is difficult computationally

• No known solution despite 100’s of years of trying!

• Note: Finding primes is also hard


CS-4513, D-Term 2007 31


• Let p and q be two large primes• 500-700 bits in length

• Let n = p qLet z = (p – 1) (q – 1)

• Choose d to be relatively prime to z

• Choose e such that d e = 1 mod z

• Publish n and either d or e (but not both!)


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• Encryption: Cyphertext = (Cleartext)e mod n• Decryption: Cleartext = (Cyphertext)d mod n

• Typical d will be on the order of 500 to 700 bits• The cost of the algorithm is between 1 and 2

the size of n, – Each operation is a giant shift and add (multiply by a

power of 2)


CS-4513, D-Term 2007 33


• References– Tanenbaum & van Steen, §9.1.3– Silbershatz, §15.4.1.2


CS-4513, D-Term 2007 34

RSA Problems

• It is computationally much more costly than typical secret-key methods

• Impractical to use for message encryption• Use RSA to encrypt a random session key

• Encrypt the message with the session key and append/prefix the RSA encrypted key

• Requires a “Public Key Infrastructure” for effective key generation and distribution

• Chain of trust thing again!


CS-4513, D-Term 2007 35

Questions about Public Key Encryption?


CS-4513, D-Term 2007 36

Authentication using Secure Channels


CS-4513, D-Term 2007 37


At this

point,

Bob knows he

is talking w

ith

Alice


CS-4513, D-Term 2007 38


At this

point,

Bob knows he

is talking w

ith

Alice

Not until this point,

does Alice know she

is talking with Bob


CS-4513, D-Term 2007 39

What is wrong with this “Optimization”?


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Subject to “Reflection Attack”

• Attacker cons Bob into encrypting RB for him

• “Reflection” attack


CS-4513, D-Term 2007 41

Reflection Attack


CS-4513, D-Term 2007 42

Key Distribution Server

• Alice requests secure channel to Bob

• KDC generates session key KA,B

• KDC sends secure messages to both Alice and Bob containing KA,B


CS-4513, D-Term 2007 43

Key Distribution Server (continued)

• Alice requests secure channel to Bob

• KDC generates session key KA,B and ticket to speak with Bob

• Alice uses ticket to contact Bob


CS-4513, D-Term 2007 44

Needham-Schroeder Protocol

• Nonce – a random number that is never re-used• E.g., RA1, RA2, and RB

• Prevents intruder from replaying old tickets


CS-4513, D-Term 2007 45

Kerberos

• Single sign-on system• One login used to generate tickets for authenticating

shared services on distributed system

• No passwords maintained by any client

• Two parts• AS – Authentication Service

• TGS – Ticket Granting Service

• Once authenticated, user may ask TGS for a ticket for a session with any service.


CS-4513, D-Term 2007 46

Kerberos (continued)


CS-4513, D-Term 2007 47

Kerberos (continued)

• With ticket, Alice can communicate securely with Bob.

• Alice knows it is Bob because only Bob could descript ticket

• Bob knows that it is Alice because TGS said it was

• Timestamp prevents replaying old sessions


CS-4513, D-Term 2007 48

Key Distribution

• Many variations– Secret (symmetric) keys– Public (asymmetric) keys

• Always based on trust

• Central part of any distributed system that requires authentication


CS-4513, D-Term 2007 49

Questions?

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