07security

04/09/23 ICSS420 - Security 1

Security

• In most systems security is an important concern– Communications should be secure against

eavesdropping and tampering– Servers/clients should be able to verify the

identity of their clients/servers– The originator of a message should be

verifiable after the message has been delivered


Policy vs. Mechanism

• Security policies– Who can access what resource– Defines the appropriate levels of security

• Security Mechanisms– Techniques used to implement the security

policies


Principal

• The agents accessing the information or resources– Human beings

– Servers

– Applications

• Principals with the same access rights are often collected together in groups

• Each principal has a unique user identifier associated with it


Threats

• Security threats common to computer systems fall into four broad classes– Leakage

• Acquisition of information by unauthorized parties

– Tampering• The unauthorized alteration of information

– Resource Stealing– Vandalism


Methods of Attack

• Some common methods of attack include– Eavesdropping

• Information in transit• Information in storage

– Masquerading• Sending/receiving messages using the identity of another user

– Message Tampering– Replaying

• Storing messages and sending them at a later date

– Denial of Service


Infiltration

• Attacker must have access to the system in order to attack– Password cracking– Virus

• Attaches itself to an existing program

– Worm• Standalone program• Not always nasty!!

– Trojan Horse


Morris Worm

Target System

Grappling hook

Worm

Target System

Worm

rsh attack

finger attack

sendmail attack

Request for worm

Worm sent


Security in a Network

• In a networked system– The principal threats to security come from the

openness of communication channels– Potential violators are not easily identifiable, so

we must not assume trust. Assume untrustworthy until proven otherwise

– The mechanisms used to implement security must be validated to a high standard


Techniques

• Security mechanisms are based on three techniques– Cryptography

• Used to conceal information

• Used in support of authentication

• Used to implement digital signatures

– Authentication• Validate the identity of the sender

– Access Control• Allow resources to access only by authorized individuals


Cryptography

• Information can be encoded using a key when it is written (or transferred)– encryption

• It is then decoded using a key when it is read (or received)– decryption

• Very widely used for secure network transmission


plaintext ciphertext

encryption

decryption

More on Cryptography


plaintext plaintextEncryptEncrypt DecryptDecrypt

Ke Kd

C = EKe(plaintext)



plaintext EncryptEncrypt DecryptDecrypt

Ke Kd

C = EKe(plaintext)

InvaderInvaderSide information plaintext

plaintext


Cryptanalysis


Cryptographic Systems

Cryptographic Systems

Conventional Systems Modern Systems

Private Key Public Key

•Ke and Kd are essentially the same

•Ke and Kd are private

•Ke is public•Kd is private


Private Key Systems

• In private key systems, such as the US Federal Data Encryption Standard (DES), a single key is used for both encryption and decryption

• This means that both parties must know the key(s) before communication can take place– write it down ahead of time

– have some sort of physical key

– exchange key(s) via secure channels


Block Ciphers

• Many commonly used ciphers are block ciphers. – This means that they take a fixed-size block of

data (usually 64 bits)– Transform it to another 64 bit block using a

function selected by the key.


Block Cipher Modes

• If the same block is encrypted twice with the same key, the resulting ciphertext blocks are the same– It is desirable to make identical plaintext blocks encrypt

to different ciphertext blocks.

• Two methods are commonly used for this:– CFB mode: a ciphertext block is obtained by

encrypting the previous ciphertext block, and xoring the resulting value with the plaintext.

– CBC mode: a ciphertext block is obtained by first xoring the plaintext block with the previous ciphertext block, and encrypting the resulting value.


Secret Key Systems

• DES– Developed in the 1970s adopted as a standard by the

US government

– DES is a block cipher with 64-bit block size. It uses 56-bit keys.

– This makes it fairly easy to break with modern computers or

– A variant of DES, Triple-DES or 3DES is based on using DES three times (normally in an encrypt-decrypt-encrypt sequence with three different, unrelated keys).


Secret Key Systems

• Blowfish– An algorithm developed by Bruce Schneier.– It is a block cipher with 64-bit block size and variable

length keys (up to 448 bits).– No attacks are known against it.

• IDEA (International Data Encryption Algorithm)– Developed at ETH Zurich in Switzerland. – Uses a 128 bit key, and is considered to be very secure. – No practical attacks on it have been published despite

numerous attempts to analyze it.


Secret Key Systems

• RC4– The algorithm is very fast. – Its security is unknown, but breaking it does not seem

trivial either.

• SAFER– Developed by J. L. Massey (a developer of IDEA). – It is claimed to provide secure/fast encryption

• Enigma– The cipher used by the Germans in World War II. – This cipher is used by the unix crypt(1) program


Public Key Systems

• In public key cryptosystems, everyone has two related complementary keys, a publicly revealed key and a secret key

• Each key unlocks the code that the other key makes. Knowing the public key does not help you deduce the corresponding secret key

• The public key can be published and widely disseminated across a communications network

• This protocol provides security without the need to reveal the private key


plaintext EncryptEncrypt DecryptDecrypt

Kpublic Kprivate

C = EKpublic(plaintext)

Public KeyDatabase

Public KeyDatabase

plaintext

Public Key Systems


RSA

• Rivest, Shamir and Adelman (RSA)– To find a key pair e and d:

• Chose two large prime numbers, P and Q (each greater than 10100), and form

– N = P x Q

– Z = (P-1) x (Q-1)

• For d chose any number relatively prime to Z

• To find e solve the equation– e x d = 1 mod Z


Comparison

• Secret and public key systems– With suitable keys both are secure enough– Public-key systems are more convenient to

implement because they do not require a secure channel to exchange keys

– Secret-key systems are faster


Establishing a Shared Key

• In order for a symmetrical system to work, both parties need to know a shared key

• Is it possible for two parties to safely use the network to agree on a shared key?– To put this another way, can two machines

agree on a common number such that anyone who listens to that conversation can determine the number?


Diffie-Hellman Key Exchange

A B

n, g, gx mod n

gy mod n

n and g, both are prime, public and special. A picks x in private, B picks y in private

Compute (gy mod n)x mod n = gxy mod n

Compute (gx mod n)y mod n = gxy mod n


It Works!!

• n=47, g=3• I’ll pick a small x, you pick a small y • I send to you

– (47, 3, 9)

• You send to me– 3y mod 47 (call it z)

• I compute zx mod 47• You compute 9y mod 47


To Break it

• You know– n = 47, g = 3

• You also know– gx mod n = 9– gy mod n = z

• You need to solve the equation– zx mod 47 = 9y mod 47


Bucket Brigade

A BX

n, g, gx mod n

gy mod n

gq mod n

n, g, gq mod n

Session key S Session key R

Also known as the person in the middle attack


Key Distribution Center

• With the previous example, you would need n different keys to talk to n different people– Perhaps the same key could be used for an entire

session

• An alternative approach is to use a key distribution center (KDC)– The KDC stores a single key for each user

– Authentication and session key management goes through the KDC


KDC

A

BKDC

A, KA(B,KS) KB(A,KS)

KS(message1)

A, KA(C,KS)

CKS(message2)

KC(A,KS)


Analysis

• Authentication comes for free– The KDC knows the message came from A– B knows the first message came from the KDC– B knows the third message came from A


Replay Attack

A B

KDCA, KA(B,KS)

KB(A,KS)

KS(message)

CKB(A,KS)

KS(message)


Solutions

• Timestamp messages– Obsolete messages are discarded– Clocks cannot be perfectly synchronized– So timestamps are valid for an interval

• Unique message numbers (nonce)– Each party remembers all previous nonces– Messages with used nonces are rejected– Nonces have to be remembered forever


Needham-Schroeder

A B

KDC

RA, A, B

KA(RA, B, KS, KB(A,KS))

KB(A,KS), KS(RA2)

KS(RA2-1), RB

KS(RB-1)

Not a replay Ticket

KS(message)

Who the ticket is for

Challenge B

Must be B, Challenge A

Must be A


Attack

X BKB(A,KS), KS(RA2)

KS(RA2-1), RB

KS(RB-1)

KS(message)

Challenge B

Must be B, Challenge A

Must be A

Obtains an old session key

Replays old message (RA2 could be different)


Otway-Rees

A BKDC

A, B, R, KA(A,B,R,RA)

A, KA(A,B,R,RA),B, KB(A,B,R,RB)

KB(RB,KS)

KA(RA,KS)


Kerberos

Client

Server

AuthenticationServer

Trusted server, repository of keys, protected by a nasty three-headed dog (Kerberos of Greek mytholodgy)


Kerberos

Client

Server

Client ID

Session Key

Session Key

Encrypted for clientEncrypted for server

Ticket


After message arrives, user is prompted for password which is used to decrypt the message


Kerberos

Client

Server


Session Key


Client ID

Session Key

Ticket


Kerberos

Client

Server


Client ID

Session Key

Ticket

Session Key



Kerberos

Client

Server


Session Key

Client ID

Session Key



Kerberos

Client

Server



Message

Encrypted for session


Authentication

• User/process authentication– Is this user/process who it claims to be?

• Passwords

• More sophisticated mechanisms

• Authentication in networks– Is this computer who it claims to be?

• File downloading

• Obtaining network services


Public-Key Authentication

A B

PublicB(A,RA)

PublicA(RA,RB, KS)

KS(RB)

Must be B

Must be A


Challenge Response

A B

A

RB

KAB(RB)

KAB(RA)

RA

KAB(Message)


Challenge Response

A B

A, RA

RB, KAB(RA)

KAB(RB)


Reflection Attack

X B

A, RX

RB, KAB(RX)

KAB(RB)

A, RB

RB2, KAB(RB)

Needs KAB(RB)


The Lesson

• Designing a correct authentication protocol is harder than it looks

• General rules– Have the initiator prove who they are before the

responder has to– Have the initiator and responder use different

keys for proof– Have the initiator and challenger draw their

challenges from different sets


Digital Signatures

• Public key systems can also be used to provide message authentication:– The sender’s secret key can be used to encrypt a

message, thereby signing it

– This creates a digital signature of a message, which the recipient (or anyone else) can check by using the sender's public key to decrypt it.

– This proves that the sender was the true originator of the message, and that the message has not been subsequently altered by anyone else


Secure Shell

• Secure Shell (ssh/ssh2) is a tool for improving Internet security by providing– Strong authentication– All communications are automatically and

transparently encrypted– X11 connection forwarding provides secure X11

sessions– Arbitrary TCP/IP ports can be redirected over the

encrypted channel in both directions. – The client RSA-authenticates the server machine in the

beginning of every connection


How It Works

• SSH uses both authentication and encryption– Authentication is done using RSA

public/private keys– Encryption can be done using a variety of

algorithms• IDEA (default)

• DES• 3DES• Blowfish


SSH in Action

Public/private keys stored on mordor

Public key for mordor available on laptop

Random string encrypted with public key for mordor

Mordor returns unencrypted stringIf the string matches what was sent, mordor has been authenticated

Both hosts authenticate themselves!!


User Authentication

• User name and password sent to remote host encrypted with host’s public key

• Host sends random session key encrypted with user’s public key

• Session key is decrypted• User is authenticated• Rest of conversation is encrypted using

IDEA

Date post:	29-Nov-2014
Category:	Technology
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