review for exam 3 © abdou illia – spring 2012. the elements of cryptography

Review For Exam 3

© Abdou Illia – Spring 2012

The Elements of Cryptography

3

Cryptography?

Traditionally, cryptography refers to The practice and the study of encryption Transforming information in order to prevent

unauthorized people to read it.

Today, cryptography goes beyond encryption/decryption to include Techniques for making sure that encrypted

messages are not modified en route Techniques for secure

identification/authentication of communication partners.

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Your knowledge about Cryptography

Which of the following security issues is addressed by cryptographic systems?

a) Confidentiality; i.e. protection against eavesdroppingb) Authentication; i.e. assurance parties involved in a

communication are who they claim to bec) Message integrity; i.e. assurance that messages are not

altered en routed) Availability; i.e. making sure that communication systems

are not shut down by intruders.e) All of the above

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Basic Terminology 1

Network

Plaintext“Hello”

Ciphertext “11011101”

Ciphertext “11011101”Plaintext“Hello”

DecryptionAlgorithm

Interceptor

Party A

Party B

Plaintext: original message to be sent. Could be text, audio, image, etc. Encryption/Decryption Algorithm: mathematical tool (software) used to

encrypt or decrypt Key: A string of bits used by to encrypt the plaintext or decrypt the ciphertext Ciphertext: encrypted message. Looks like a random stream of bits

+ Decryption key

EncryptionAlgorithm

+ Encryption key

HelloHello

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Basic Terminology 2 Encryption:

Converting plaintext into ciphertext using algorithms and keys The size of the ciphertext is proportional to the size of the plaintext Ciphertext is reversible to plaintext

Symmetric Key Encryption: Same key is used both for encryption and decryption Keys are usually identical or trivially identical*

Asymmetric Key Encryption: Also called Public/Private Key Encryption Two different keys are used: one for encryption, one for decryption

Party A Party B

Party AParty B

* Trivially identical means simple transformation could lead from one key to the another.

Flexcrypt: http://www.flexcrypt.com/flexcryptfree.html

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1) Based on how symmetric encryption systems work, which of the following is the worst thing to happen?

a) An attacker gets a copy of the encryption and decryption algorithms

b) An attacker gets the decryption key

c) a and b are equally damaging

2) Which of the following presents more challenge for exchanging keys between partners?a) Asymmetric encryption

b) Symmetric encryption

c) A and b are equally challenging

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Exhaustive search and Key length

Key Length in bits Number of possible keys (2key length in bits)

1 2

2 4

4 16

8 256

16 65536

56 72057594037927900

112 5192296858534830000000000000000000 or 5.1923E+33

168 3.74144E+50

256 1.15792E+77

512 1.3408E+154

Attacker could use the right algorithm and do an exhaustive search (i.e. try all possible keys) in order to decrypt the ciphertext

Most attacks require the capture of large amount of ciphertext Every additional bit in the length of the key doubles the search time

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4) If you increase the key length from 56 bits to 66 bits. How much more key combinations an attacker who captures enough ciphertext will have to try in order to decipher the captured ciphertext using the appropriate algorithm?

_______________________________________

5) Assuming that it takes 7 days to try all possible combinations of a 56 bit key, how much time it would take to try all possible combinations when the key length is increased to 58 bits?

________________

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Weak vs. Strong Keys Symmetric Key Encryption

Usually used for customer e-business Keys with lengths of less than 100 bits are considered weak

today. Keys with lengths of more than 100 bits are considered

strong today.

Asymmetric Key Encryption Usually used for B2B financial e-business Key pairs must be much longer (1024 bits and more)

because of the disastrous consequences of breaking the decryption key Key Length in bits Number of possible keys (2key length in bits)

1 2

2 4

16 65536

56 72057594037927900

112 5192296858534830000000000000000000 or 5.1923E+33

168 3.74144E+50

256 1.15792E+77

512 1.3408E+154

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6) Most attacks require the capture of large amount of ciphertext, which can take a certain amount of time. Beside using strong keys what else can be done to make it harder to crack the key?

Symmetric Key Encryption

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Symmetric Key Encryption methods Two categories of methods

Stream cipher: algorithm operates on individual bits (or bytes); one at a time Block cipher: operates on fixed-length groups of bits called blocks

Only a few symmetric methods are used today

Methods Year approved Comments

Data Encryption Standard - DES 1977 1998: Electronic Frontier Foundation’s Deep Crack breaks a DES key in 56 hours

DES-Cipher Block Chaining

Triple DES – TDES or 3DES 1999

Advanced Encryption Standard – AES 2001 Its versions among the most used today

Other symmetric encryption methods

IDEA (International Data Encryption Algorithm), RC5 (Rivest Cipher 5), CAST (Carlisle Adams Stafford Tavares), Blowfish

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Data Encryption Standard (DES)

DES EncryptionProcess

64-Bit CiphertextBlock

64-Bit DES Symmetric Key(56 bits + 8 redundant bits)64-Bit Plaintext

Block

DES is a block encryption method, i.e. uses block cipher DES uses a 64 bit key; actually 56 bits + 8 bits computable

from the other 56 bits Problem: same input plaintext gives same output ciphertext

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DES-Cipher Block Chaining

First64-Bit Plaintext Block


Second64-Bit Plaintext Block

First64-Bit Ciphertext Block

InitializationVector (IV)


Second64-Bit Ciphertext Block

DES Key

DES Key

DES-CBC uses ciphertext from previous block as input making decryption by attackers even harder

An 64-bit initialization vector is used for first block

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Triple DES (3DES)

Sender Receiver

Encrypts original plaintext with the1st key

Decrypts ciphertext withthe 3d key

Decrypts output of firststep with the 2nd key

Encrypts output of thefirst step with the 2nd key

Encrypts output of secondstep with the 3d key; givesthe ciphertext to be sent

Decrypts output of secondstep with the 1st key; givesthe original plaintext

168-Bit Encryption with Three 56-Bit Keys

1st

2nd

3rd

3rd

2nd

1st

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Triple DES (3DES)

Sender Receiver

Encrypts plaintext with the1st key

Decrypts ciphertext withthe 1st key

Decrypts output with the 2nd key

Encrypts output with the2nd key

Encrypts output with the1st key

Decrypts output with the1st key

112-Bit Encryption With Two 56-Bit Keys

1st

2nd

1st

1st

2nd

1st

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7) Based on the way DES and 3DES work, which of the following is true?a) 3DES requires more processing time than DES

b) Compared 3DES, DES requires more RAM

c) Both a and b

8) Given the increasing use of hand-held devices, 3DES will be more practical than DES.

a) True

b) False

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Advanced Encryption Standard - AES

Developed by two Belgian cryptographers, Joan Daemen and Vincent Rijmen, and submitted to the AES selection process under the name "Rijndael", a portmanteau of the names of the inventors

Offers key lengths of 128 bit, 192 bit, and 256 bit Efficient in terms of processing power and RAM

requirements compared to 3DES Can be used on a wide variety of devices including:

Cellular phones PDAs Etc.

Asymmetric Key Encryption

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Public Key Encryption For confidentiality

Party A Party BDecrypt with

Party A’s Private KeyEncrypt with

Party A’s Public Key

Encrypt withParty B’s Public Key

Decrypt withParty B’s Private Key

EncryptedMessage

EncryptedMessage

Each Party uses other party’s public key for encryption Each Party uses own private key for decryption No need to exchange private key, but key needs to be very

strong (512+ bit. Most today’s key are at least 1024 bit)

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Public Key Encryption methods

Asymmetric encryption methods are used both for Encryption in order to provide confidentiality Digital signature in order to provide partners’ authentication

Methods Year proposed Comments

RSA by Ron Rivest, Adi Shamir, and Leonard Adleman

1977 1995: First attack in lab conditions was reported

Elliptic Curve Cryptosystem - ECC 1985 Becoming widely used

Other symmetric encryption methods:

Dieffe-Hellman, El-Gamal

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Basic Terminology 3 Hashing:

Mathematical process for converting inputs into fixed-length outputs Hash function:

Algorithm that does the hashing. Uses an input + a shared secret or password. Example: Message Digest 5 (MD5), Secure Hash Algorithm (SHA).

Hash: Fixed-length output of the hashing

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Encryption Versus Hashing

Encryption

Uses a key as aninput to an encryption method

Output is similar inlength to input

Reversible; ciphertextcan be decryptedback to plaintext

Use of Key

Length of Result

Reversibility

Hashing

Password is usually addedto text; the two arecombined, and thecombination is hashed

Output is of a fixedshort length, regardless of input

One-way function; hashcannot be “de-hashed” back to the original string

MD5 (Message-Digest algorithm 5)

A widely used cryptographic hash function used to hash inputs (typed texts or files) in order to generate hash values (called checksums, message digest, or output)

An MD5 hash value is typically expressed as a 16-hexadecimal number like 912df11644fccac439b6fc5f80af5cdb

Each hex number is 8 bits MD5 generates a 128-bit hash value regardless of the

input length. Commonly used to check the integrity of files like

downloaded software programs

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SHA1 (Secure Hash Algorithm 1)

A widely used cryptographic hash function used to hash inputs (typed texts or files) in order to generate hash values (called checksums, message digest, or output)

A SHA1 hash value is typically expressed as a 20-hexadecimal number like 79054025255fb1a26e4bc422aef54eb4

SHA1 generates a 160-bit hash value regardless of the input length

Commonly used to check the integrity of files like downloaded software programs

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Application Security: General apps &Web service

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Applications and Buffer Overflow

Buffer Overflow is the biggest issue in application coding

Buffer overflow leads to Buffer Overflow Attacks

Buffers are RAM areas where data is stored temporarily

If an attacker sends more data than the programmer had allocated to a buffer, a buffer might overflow, overwriting an adjacent section of RAM

RAMBuffer1 Buffer2

Buffer7Buffer3 Buffer4 Buffer6Buffer5

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Buffer Overflow Attack

Occurs when ill-written programs allow data destined to a memory buffer to overwrite instructions in adjacent memory register that contains instructions.

If the data contains malware, the malware could run and creates a DoS

Example of input data: ABCDEF LET JOHN IN WITHOUT PASSWORD

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Buffer Instructions

1 2 3 4 5 6

Print

Run Program

Accept input

Buffer Instructions

1 2 3 4 5 6

A B C D E F LET JOHN IN WITHOUT PASSWORD

Run Program

Accept input

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Stack entry: data buffer & Return address registry

Stack Entry and Buffer Overflow

ReturnAddress

1. Write ReturnAddress

2. Add Datato Buffer

Data Buffer5. Start of

Attacker data

3. Direction ofData Writing

4. OverwriteReturn Address

When a program must put one subprogram on hold to call another, it writes the return address in RAM areas called stack entries

The called subprogram may add data to the buffer to the point it overwrites the return address

If the added buffer data is Attack code, this will be a buffer overflow attack

http://www.metacafe.com/watch/1452134/buffer_overflow_attacks_explained_with_beer/

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Preventing Buffer Overflow Use Language tools that provide automatic bounds checking

such as Perl, Python, and Java instead lower level language (C, C++, Assembly, etc). However, this is usually not possible or practical because

almost all modern OS are written in the C language. Eliminate The Use Of Flawed Library Functions like gets(),

strcpy, and strcmp that fail to check the length or bounds of their arguments.

Design And Build Security Within Code

Use Source Code Scanning Tools. Example: PurifyPlus Software Suite can perform a

dynamic analysis of Java, C, or C++ source code.

// replace le following line

Strcpy (buffer2, strng2);

// by

Strcpy (buffer2, string2, 8)

For instance, this simple change informs strcpy() that it only has an eight byte destination buffer and that it must discontinue raw

copy at eight bytes.

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Web service security

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Webservice & E-Commerce apps

Web applications could be the target of many types of attacks like: Directory browsing

Traversal attacks

Web defacement

Using HTTP proxy to manipulate interaction between client and server

IIS IPP Buffer Overflow

Browser attacks

Time configuration

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Web sites’ directory browsing

If Directory Browsing is disabled on a web server: User cannot get access to list of files in the directory by

knowing or guessing directory names

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Directory browsing

If Directory Browsing enabled on a web server: User can get access to the list of files in the directory by

knowing or guessing directory names

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Traversal Attack

Normally, paths start at the WWW root directory

Adding ../ (Windows) or ..\ (Unix) in an HTTP request might take the attacker up a level, out of the WWW root directory.

Example: http://website.com ../../

Example: http://castle.eiu.edu/~a_illia/BUS3500/Brief/Case1/../

If attacker traverses to Command Prompt directory in Windows 2000 or NT, he can execute any command with system privileges

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IIS IPP Buffer Overflow

The Internet Printing Protocol (IPP) service included in IIS 5.0 and earlier versions is vulnerable to buffer overflow and traversal attacks

The jill.c program was developed to launch the attack using:

GET NULL.printer HTTP/1.0

Host: 420-byte jill.c code to launch the command shell

IIS server responds launching the command shell (C:\WINNT\SYSTEM32\>) giving the attacker SYSTEM privileges.

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Browser Attacks

Malicious links User must click on them to execute (but not

always)

Common file extensions are hidden by default in some operating systems.

attack.txt.exe seems to be attack.txt

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Browser Attacks (Cont.) Common Attacks

Redirection to unwanted webpage Scripts might change the registry, home page Some scripts might “trojanize” your DNS error-

handling routine when you mistype a URL Pop-up windows Web bugs; i.e. links that are nearly invisible,

can be used to track users at a website

Domain names that are common misspellings of popular domain names

Microsoff.com, www.whitehouse.com (a porn site)

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Host Hardening

Computer Hardware & Software

Computer Hardware

Operating System

Web service software (IIS, Apache, ...)Web browser

Productivity Software

Client & server

application programs

Your knowledge about Host hardening

Which of the following is most likely to make a computer system unable to perform any kind of work or provide any service?

a) Client application programs get hacked

b) Server application programs (web service software, database service, network service, etc.) get hacked

c) The operating system get hacked

d) The connection to the network/Internet get shut down

OS market share

OS Vulnerability test2010 by omnired.com

OS tested: Win XP, Win Server 2003, Win Vista Ultimate, Mac OS Classic, OS X 10.4 Server, OS X 10.4 Tiger FreeBSD 6.2, Solaris 10, Fedora Core 6, Slackware 11.0, Suse Enterprise

10, Ubuntu 6.10 Tools used to test vulnerabilities:

Scanning tools (Track, Nessus) Network mapping (Nmap command) All host with OS installation defaults

Results Microsoft's Windows and Apple's OS X are ripe with remotely accessible

vulnerabilities and allow for executing malicious code The UNIX and Linux variants present a much more robust exterior to the

outside Once patched, however, both Windows and Apple’s OS are secure.

Your knowledge about Host hardening

You performed an Out-of-the-box installation of Windows XP and Linux FreeBSD 6.2 on two different computers. Which computer is more likely to be secure ?

a) Windows XPb) Linux FreeBSD 6.2c) They will have the same level of security

What needs to be done, first, in order to prevent a hacker from taking over a server with OS installation defaults that has to be connected to the Internet?

a) Lock the server roomb) Configure the firewall to deny all inbound traffic to the serverc) Download and install patches for known vulnerabilities

Security Baseline

Because it’s easy to overlook something in the hardening process, businesses need to adopt a standard hardening methodology: standard security baseline

Need to have different security baseline for different kind of host; i.e. Different security baselines for different OS and

versions Different security baselines for different types of server

applications (web service, email service, etc.) Different security baselines for different types of client

applications.

Options for Security Baselines

Organization could use different standards OS vendors’ baselines and tools

e.g. Follow MS Installation procedure and use Microsoft Baseline Security Analyzer (MBSA)

Standards Agencies baselines e.g. CobiT* Security Baseline

Company’s own security baselines Security Baseline to be implemented by

Server administrators known as systems admin

* Control Objectives for Information and Related Technology

Elements of Hardening

Physical security Secure installation and configuration Fix known vulnerabilities Remove/Turn off unnecessary services (applications) Harden all remaining applications Manage users and groups Manage access permissions

For individual files and directories, assign access permissions to specific users and groups

Back up the server regularly Advanced protections

According to

baseline

Example of Security Baseline for Win XP Clients OS Installation

Create a single partition on HDD Format disk using NTFS file system Install Win XP and Service Pack 3

Fixing OS vulnerabilities Download and install latest patches Turn on Windows’ Automatic Updates checking

Configure Windows Firewall Block incoming connections except KeyAccess and Remote

Assistance Turn off unnecessary services

Turn off Alerter, Network Dynamic Data Exchange, telnet Application Installation

Centrally assign applications using group policies Fixing applications’ vulnerabilities

Turn on each application’s automatic update checking

Hardening servers The 5 ‘P’ s of security and compliance: Proper Planning Prevents Poor

Performance Plan the installation

Identify The purpose of the server. Example: provides easy & fast access to Internet

services The services provided on the server Network service software (client and server) The users or types of users of the server

Determine Privileges for each category of users If and how users will authenticate How appropriate access rights will be enforced Which OS and server applications meet the requirements The security baseline(s) for installation & deployment

Install, configure, and secure the OS according to the security baseline Install, configure, and secure server software according to sec. baseline Test the security Add network defences Monitor and Maintain

Hardening servers (cont.)

Choose the OS that provides the following: Ability to restrict admin access (Administrator vs. Administrators) Granular control of data access Ability to disable services Ability to control executables Ability to log activities Host-based firewall Support for strong authentication and encryption

Disable or remove unnecessary services or applications If no longer needed, remove rather than disable to prevent re-enabling Additional services increases the attack vector More services can increase host load and decrease performance Reducing services reduces logs and makes detection of intrusion easier

Hardening servers (cont.)

Configure user authentication Remove or disable unnecessary accounts

(e.g. Guest account) Change names and passwords for default accounts Disable inactive accounts Assign rights to groups not individual users Don't permit shared accounts if possible Configure time sync Enforce appropriate password policy Use 2-factor authentication when necessary Always use encrypted authentication

UNIX / Linux Hardening

Many versions of UNIX No standards guideline for hardening

User can select the user interface Graphic User Interface (GUI) Command-Line Interfaces (CLIs) or shells

CLIs are case-sensitive with commands in lowercase except for file names


Three ways to start services

Start a service manually (a) through the GUI, (b) by typing its name in the CLI, or (c) by executing a batch file that does so

Using the inetd program to start services when requests come in from users

Using the rc scripts to start services automatically at boot up

Inetd = Internet daemon; i.e. a computer program that runs in the background


Program A

Program B

Program C

Program D

inetd

Port 23 Program APort 80 Program BPort 123 Program CPort 1510 Program D

1. Client RequestTo Port 123

4. Start and Process

This Request

3. Program C 2. Port 123

/etc/inetd.config

Starting services upon client requests Services not frequently used are dormant Requests do not go directly to the service Requests are sent to the inetd program which is started at server boot up


Turning On/Off unnecessary Services In UNIX

Identifying services running at any moment

ps command (process status), usually with –aux parameters, lists running programs

Shows process name and process ID (PID)

netstat tells what services are running on what ports

Turning Off Services In UNIX kill PID command is used to kill a particular process

kill 47 (If PID=47)

Advanced Server Hardening Techniques

File Integrity Checker

Creates snapshot of files: a hashed signature (message digest) for each file

After an attack, compares post-hack signature with snapshot

This allows systems administrator to determine which files were changed

Tripwire is a file integrity checker for Linux/UNIX, Windows, etc.: www.tripwire.com (ftp://coast.cs.purdue.edu/pub/tools/unix)

Advanced Server Hardening Techniques

File 1File 2…Other Files inPolicy List

File 1File 2…Other Files inPolicy List

File 1 SignatureFile 2 Signature……

File 1 SignatureFile 2 Signature……

Tripwire

Tripwire

1.EarlierTime

2.After

Attack

Post-Attack Signatures

3. Comparison to Find Changed Files

Reference Base

File Integrity problem: many files change for legitimate reasons. So it is difficult to know which ones the attacker changed.

Other types of host that can be Hardened Internetwork Operating System (IOS)

For Cisco Routers, Some Switches, Firewalls Even cable modems with web-based

management interfaces

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Which of the following is true about Application Security? If a server application (or service) is no longer needed, it

should be turned off Fewer applications on a computer, fewer attack

opportunities Use good security baselines to install and configure apps Do not install application centrally using group policies Add application layer authentication by requiring users to

provide credentials to run application programs Implement cryptographic authentication for sensitive

apps If a server application (or service) is no longer needed, it

should be removed Do not turn on each applications’ automatic update checking

review for exam 3 © abdou illia – spring 2012. the elements of cryptography

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