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Review For Exam 3
© Abdou Illia – Spring 2014
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.
4
Your knowledge about Cryptography
1) Which of the following do cryptographic systems protect?
a) Data stored on local storage media (like hard drives) from access by unauthorized users.
b) Data being transmitted from point A to point B in a network
c) Both a and b
#$%^@#$5hh&*9(?>/@$#)>
5
Your knowledge about Cryptography
2) 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
6
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
7
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 decryptionParty 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
8
Your knowledge about Cryptography
3) 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
4) 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
9
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 Every additional bit in the length of the key doubles the requirements in terms of
minimum processor’s speed to crack the key.
10
Your knowledge about Cryptography
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?
________________
11
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 (512 bit 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
12
Your knowledge about Cryptography
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?
Answer: change the key very often
Symmetric Key Encryption
14
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
15
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
16
DES-Cipher Block Chaining
First64-Bit Plaintext Block
DES EncryptionProcess
Second64-Bit Plaintext Block
First64-Bit Ciphertext Block
InitializationVector (IV)
DES EncryptionProcess
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
17
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
18
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
19
Your knowledge about Cryptography
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
20
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
22
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)
23
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
24
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: MD5, Secure Hash Algorithm.
Hash: Fixed-length output of the hashing
25
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
26
Hashing & Public Key for authentication
Asymmetric Key Encryption is also used for authentication Usually used along with hashing
Confidentiality Authentication
Public KeyEncryption
Sender encrypts with receiver’s public key. Receiver decrypts with the receiver’s own private key.
Sender (supplicant) encrypts with own private key. Receiver (verifier) decrypts with the public key of the true party, usually obtained from a Certificate Authority.
Hashing Used in MS-CHAP for initial authentication and in HMACs for message-by-message authentication
Hashing and Public Key for authentication very used in cryptographic systems like SSL/TLS or IPSec
27
Cryptographic Systems
Packaged set of cryptographic countermeasures used for protecting dialogues
Example: Secure Socket Layer/Transport Layer Security –SSL/TLS used in secured webservice
Each cryptographic system includes a variety of security standards (algorithms, hashing methods, security parameters) that comm. partners needs to “agree” on.
Typical Process:
Handshaking stages
Ongoing communication stage: Message-by-Message authentication
28
Cryptographic Systems (cont.)
Packaged set of cryptographic countermeasures used for protecting dialogues
Handshaking Stage 1:Initial Negotiation of Security Parameters
Handshaking Stage 2:Initial Authentication
(Usually mutual)
Handshaking Stage 3:Keying
(Secure exchange of keys and other secrets)
Ongoing Communication Stagewith Message-by-Message
Confidentiality, Authentication,and Message Integrity
ServerClient PC
Time
Encrypted for Confidentiality
PlaintextElectronic Signature(Authentication, Integrity)
29
MS-CHAP* Hashing for Authentication
* Microsoft’s version of Challenge Handshake Authentication Protocol
CHAP is an authentication hashing scheme used by Point to Point Protocol (PPP) servers to validate the identity of remote clients
1) After the completion of the link establishment phase, the server sends a "challenge" message to the client.
2) The client responds with a value calculated using a one-way hash function, such as an MD5 or SHA (Secure Hash Algorithm).
3) The server checks the response against its own calculation of the expected hash value. If the values match, the server acknowledges the authentication; otherwise it should terminate the connection.
4) At random intervals the server sends a new challenge to the peer and repeats steps 1 through 3.
Shared secret
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
30
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
31
Application Security: General apps &Web service
33
General Applications Security Issues
34
Which of the following is true about Application Security?A. If a server application (or service) is no longer needed, it
should be turned off
B. Fewer applications on a computer means fewer attack opportunities
C. Use good security baselines to install and configure apps
D. Do not install application centrally using group policies
E. Add application layer authentication by requiring users to provide credentials to run application programs
F. Implement cryptographic authentication for sensitive apps
G. If a server application (or service) is no longer needed, it should be removed
H. Do not turn on each applications’ automatic update checking
35
Applications and Buffer Overflow
Buffers are RAM areas where data is stored temporarily Buffer overflow occurs when data spill from one buffer to
the next Buffer Overflow is the biggest issue in application coding
If an attacker sends more data than the programmer had allocated to a buffer, a buffer might overflow, overwriting an adjacent section of RAM
Buffer overflow attacks
RAMBuffer1 Buffer2
Buffer7Buffer3 Buffer4 Buffer6Buffer5
OUTDO OR
36
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 contain malware, the malware could run and creates a DoS
Example of input data: ABCDEF LET JOHN IN WITHOUT PASSWORD
36
Buffer Instructions
1 2 3 4 5 6
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
37
Preventing Buffer Overflow
Use Languages/tools that provide automatic bounds checking such as Perl, Python, and Java instead lower level language (C or 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.
Key Principle: Never Trust User Input
38
Web service security
39
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
40
Web sites’ directory browsing
Web server with Directory Browsing disabled User cannot get access to list of files in the directory by
knowing or guessing directory names
41
Web site with directory browsing
Web server with Directory Browsing enabled User can get access to the list of files in the directory by
knowing or guessing directory names
42
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
43
Traversal Attacks (Cont.) Preventing traversal attacks
Companies filter out ../ and ..\ using URL scanning software
Attackers respond with hexadecimal and UNICODE representations for ../ and ..\
ASCII Character Chart with Decimal, Binary and Hexadecimal Conversions
Name Character Code Decimal Binary HexNull NUL Ctrl @ 0 00000000 00
Start of Heading SOH Ctrl A 1 00000001 01
Space 32 00100000 20
Exclamation Point ! Shift 1 33 00100010 22
Plus + Shift = 43 00101011 2B
Period . . 46 00101110 2E
Forward Slash / / 47 00101111 2F
Tilde ~ Shift’ 126 01111110 7E
44
IIS IPP Buffer Overflow
The Internet Printing Protocol (IPP) service included in IIS 5.0 and earlier versions is vulnerable to buffer overflow 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.
45
IIS IPP Buffer Overflow (cont.)
Link to jill.c code
Code compilable using gcc jill.c –o jill on Linux
Precompiled version (jill-win32.c) and executable (jill-win32.exe) available at ftp://ftp.technotronic.com/
newfiles/jill-win32.exe. This executable file is ready to run on a Windows machine.
46
IIS IPP Buffer Overflow (cont.)
Source: http://puna.net.nz/archives/Hacking/David_Sheridan_GCIH.doc
47
Login Screen Bypass Attack
Website user gets a login screen
Instead of logging in, user enters a URL to bypass the login screen and gain access without authorization.
48
Browser Attacks
Malicious links User must click on them to execute (but not
always)
Common extensions are hidden by default in some operating systems.
attack.txt.exe seems to be attack.txt