This document provides an overview of the Data Encryption Standard (DES). It begins with a review of classical cryptography techniques like monoalphabetic and polyalphabetic ciphers. It then discusses the history and design of the DES algorithm, including its Feistel cipher structure using substitution boxes and permutation functions. The document also notes controversies around the DES key size and the algorithm's adoption as a standard. While DES was widely used, concerns were raised about its 56-bit key being vulnerable to brute force attacks with increasing computational power.
The Roman number system was very cumbersome because there was no concept ... Historical pen and paper ciphers used in the past are sometimes known as ...
The Roman number system was very cumbersome because there was no concept ... Historical pen and paper ciphers used in the past are sometimes known as ...
Information and network security 12 classical substitution ciphersVaibhav Khanna
There are two basic building blocks of all encryption techniques: substitution and transposition. A substitution technique is one in which the letters of plaintext are replaced by other letters or by numbers or symbols
Information and network security 12 classical substitution ciphersVaibhav Khanna
There are two basic building blocks of all encryption techniques: substitution and transposition. A substitution technique is one in which the letters of plaintext are replaced by other letters or by numbers or symbols
Industrial Training at Shahjalal Fertilizer Company Limited (SFCL)MdTanvirMahtab2
This presentation is about the working procedure of Shahjalal Fertilizer Company Limited (SFCL). A Govt. owned Company of Bangladesh Chemical Industries Corporation under Ministry of Industries.
Cosmetic shop management system project report.pdfKamal Acharya
Buying new cosmetic products is difficult. It can even be scary for those who have sensitive skin and are prone to skin trouble. The information needed to alleviate this problem is on the back of each product, but it's thought to interpret those ingredient lists unless you have a background in chemistry.
Instead of buying and hoping for the best, we can use data science to help us predict which products may be good fits for us. It includes various function programs to do the above mentioned tasks.
Data file handling has been effectively used in the program.
The automated cosmetic shop management system should deal with the automation of general workflow and administration process of the shop. The main processes of the system focus on customer's request where the system is able to search the most appropriate products and deliver it to the customers. It should help the employees to quickly identify the list of cosmetic product that have reached the minimum quantity and also keep a track of expired date for each cosmetic product. It should help the employees to find the rack number in which the product is placed.It is also Faster and more efficient way.
Explore the innovative world of trenchless pipe repair with our comprehensive guide, "The Benefits and Techniques of Trenchless Pipe Repair." This document delves into the modern methods of repairing underground pipes without the need for extensive excavation, highlighting the numerous advantages and the latest techniques used in the industry.
Learn about the cost savings, reduced environmental impact, and minimal disruption associated with trenchless technology. Discover detailed explanations of popular techniques such as pipe bursting, cured-in-place pipe (CIPP) lining, and directional drilling. Understand how these methods can be applied to various types of infrastructure, from residential plumbing to large-scale municipal systems.
Ideal for homeowners, contractors, engineers, and anyone interested in modern plumbing solutions, this guide provides valuable insights into why trenchless pipe repair is becoming the preferred choice for pipe rehabilitation. Stay informed about the latest advancements and best practices in the field.
Immunizing Image Classifiers Against Localized Adversary Attacksgerogepatton
This paper addresses the vulnerability of deep learning models, particularly convolutional neural networks
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introduce a novel volumization algorithm, which transforms 2D images into 3D volumetric representations.
When combined with 3D convolution and deep curriculum learning optimization (CLO), itsignificantly improves
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CFD Simulation of By-pass Flow in a HRSG module by R&R Consult.pptxR&R Consult
CFD analysis is incredibly effective at solving mysteries and improving the performance of complex systems!
Here's a great example: At a large natural gas-fired power plant, where they use waste heat to generate steam and energy, they were puzzled that their boiler wasn't producing as much steam as expected.
R&R and Tetra Engineering Group Inc. were asked to solve the issue with reduced steam production.
An inspection had shown that a significant amount of hot flue gas was bypassing the boiler tubes, where the heat was supposed to be transferred.
R&R Consult conducted a CFD analysis, which revealed that 6.3% of the flue gas was bypassing the boiler tubes without transferring heat. The analysis also showed that the flue gas was instead being directed along the sides of the boiler and between the modules that were supposed to capture the heat. This was the cause of the reduced performance.
Based on our results, Tetra Engineering installed covering plates to reduce the bypass flow. This improved the boiler's performance and increased electricity production.
It is always satisfying when we can help solve complex challenges like this. Do your systems also need a check-up or optimization? Give us a call!
Work done in cooperation with James Malloy and David Moelling from Tetra Engineering.
More examples of our work https://www.r-r-consult.dk/en/cases-en/
Water scarcity is the lack of fresh water resources to meet the standard water demand. There are two type of water scarcity. One is physical. The other is economic water scarcity.
TECHNICAL TRAINING MANUAL GENERAL FAMILIARIZATION COURSEDuvanRamosGarzon1
AIRCRAFT GENERAL
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The College Bus Management system is completely developed by Visual Basic .NET Version. The application is connect with most secured database language MS SQL Server. The application is develop by using best combination of front-end and back-end languages. The application is totally design like flat user interface. This flat user interface is more attractive user interface in 2017. The application is gives more important to the system functionality. The application is to manage the student’s details, driver’s details, bus details, bus route details, bus fees details and more. The application has only one unit for admin. The admin can manage the entire application. The admin can login into the application by using username and password of the admin. The application is develop for big and small colleges. It is more user friendly for non-computer person. Even they can easily learn how to manage the application within hours. The application is more secure by the admin. The system will give an effective output for the VB.Net and SQL Server given as input to the system. The compiled java program given as input to the system, after scanning the program will generate different reports. The application generates the report for users. The admin can view and download the report of the data. The application deliver the excel format reports. Because, excel formatted reports is very easy to understand the income and expense of the college bus. This application is mainly develop for windows operating system users. In 2017, 73% of people enterprises are using windows operating system. So the application will easily install for all the windows operating system users. The application-developed size is very low. The application consumes very low space in disk. Therefore, the user can allocate very minimum local disk space for this application.
About
Indigenized remote control interface card suitable for MAFI system CCR equipment. Compatible for IDM8000 CCR. Backplane mounted serial and TCP/Ethernet communication module for CCR remote access. IDM 8000 CCR remote control on serial and TCP protocol.
• Remote control: Parallel or serial interface.
• Compatible with MAFI CCR system.
• Compatible with IDM8000 CCR.
• Compatible with Backplane mount serial communication.
• Compatible with commercial and Defence aviation CCR system.
• Remote control system for accessing CCR and allied system over serial or TCP.
• Indigenized local Support/presence in India.
• Easy in configuration using DIP switches.
Technical Specifications
Indigenized remote control interface card suitable for MAFI system CCR equipment. Compatible for IDM8000 CCR. Backplane mounted serial and TCP/Ethernet communication module for CCR remote access. IDM 8000 CCR remote control on serial and TCP protocol.
Key Features
Indigenized remote control interface card suitable for MAFI system CCR equipment. Compatible for IDM8000 CCR. Backplane mounted serial and TCP/Ethernet communication module for CCR remote access. IDM 8000 CCR remote control on serial and TCP protocol.
• Remote control: Parallel or serial interface
• Compatible with MAFI CCR system
• Copatiable with IDM8000 CCR
• Compatible with Backplane mount serial communication.
• Compatible with commercial and Defence aviation CCR system.
• Remote control system for accessing CCR and allied system over serial or TCP.
• Indigenized local Support/presence in India.
Application
• Remote control: Parallel or serial interface.
• Compatible with MAFI CCR system.
• Compatible with IDM8000 CCR.
• Compatible with Backplane mount serial communication.
• Compatible with commercial and Defence aviation CCR system.
• Remote control system for accessing CCR and allied system over serial or TCP.
• Indigenized local Support/presence in India.
• Easy in configuration using DIP switches.
Saudi Arabia stands as a titan in the global energy landscape, renowned for its abundant oil and gas resources. It's the largest exporter of petroleum and holds some of the world's most significant reserves. Let's delve into the top 10 oil and gas projects shaping Saudi Arabia's energy future in 2024.
Overview of the fundamental roles in Hydropower generation and the components involved in wider Electrical Engineering.
This paper presents the design and construction of hydroelectric dams from the hydrologist’s survey of the valley before construction, all aspects and involved disciplines, fluid dynamics, structural engineering, generation and mains frequency regulation to the very transmission of power through the network in the United Kingdom.
Author: Robbie Edward Sayers
Collaborators and co editors: Charlie Sims and Connor Healey.
(C) 2024 Robbie E. Sayers
Final project report on grocery store management system..pdfKamal Acharya
In today’s fast-changing business environment, it’s extremely important to be able to respond to client needs in the most effective and timely manner. If your customers wish to see your business online and have instant access to your products or services.
Online Grocery Store is an e-commerce website, which retails various grocery products. This project allows viewing various products available enables registered users to purchase desired products instantly using Paytm, UPI payment processor (Instant Pay) and also can place order by using Cash on Delivery (Pay Later) option. This project provides an easy access to Administrators and Managers to view orders placed using Pay Later and Instant Pay options.
In order to develop an e-commerce website, a number of Technologies must be studied and understood. These include multi-tiered architecture, server and client-side scripting techniques, implementation technologies, programming language (such as PHP, HTML, CSS, JavaScript) and MySQL relational databases. This is a project with the objective to develop a basic website where a consumer is provided with a shopping cart website and also to know about the technologies used to develop such a website.
This document will discuss each of the underlying technologies to create and implement an e- commerce website.
Final project report on grocery store management system..pdf
UNIT 2.ppt
1. CSCE 815 Network Security
Lecture 3
Data Encryption Standard (DES)
2. Outline
• Review classical cryptography
– Monoalphabetic Ciphers
– cryptanalysis using letter frequencies
– Playfair ciphers
– polyalphabetic ciphers
– transposition ciphers
– product ciphers and rotor machines
• Background on Block Ciphers
• History of DES
• DES revisited
3. Classical Cryptography
• Symmetric Encryption
– sender and recipient share a common key
• Terminology:plaintext, ciphertext, key, cryptanalysis
• Requirements
– a strong encryption algorithm
– a secret key known only to sender / receiver
Y = EK(X)
X = DK(Y)
4. Requirements
• two requirements for secure use of
symmetric encryption:
– a strong encryption algorithm
– a secret key known only to sender / receiver
Y = EK(X)
X = DK(Y)
• assume encryption algorithm is known
• implies a secure channel to distribute key
5. Security Levels
• unconditional security
– no matter how much computer power is
available, the cipher cannot be broken since
the ciphertext provides insufficient information
to uniquely determine the corresponding
plaintext
• computational security
– given limited computing resources (eg time
needed for calculations is greater than age of
universe), the cipher cannot be broken
6. Classical Substitution Ciphers
• earliest known substitution cipher by Julius
Caesar
• first attested use in military affairs
• replaces each letter by 3rd letter on
• example:
meet me after the toga party
PHHW PH DIWHU WKH WRJD SDUWB
7. Cryptanalysis of Rotation Cipher
• only have 26 possible ciphers
– A maps to A,B,..Z
• could simply try each in turn
• a brute force search
• given ciphertext, just try all shifts of letters
• do need to recognize when have plaintext
• eg. break ciphertext "GCUA VQ DTGCM"
8. Monoalphabetic Cipher
• rather than just shifting the alphabet
• could shuffle (jumble) the letters arbitrarily
• each plaintext letter maps to a different random
ciphertext letter
• hence key is 26 letters long
Plain: abcdefghijklmnopqrstuvwxyz
Cipher: DKVQFIBJWPESCXHTMYAUOLRGZN
Plaintext: ifwewishtoreplaceletters
Ciphertext: WIRFRWAJUHYFTSDVFSFUUFYA
9. Monoalphabetic Cipher Security
• now have a total of 26! = 4 x 1026 keys
• with so many keys, might think is secure
• but would be !!!WRONG!!!
• problem is language characteristics
10. Language Redundancy and
Cryptanalysis
• human languages are redundant
• eg "th lrd s m shphrd shll nt wnt"
• letters are not equally commonly used
• in English e is by far the most common letter
• then T,R,N,I,O,A,S
• other letters are fairly rare
• cf. Z,J,K,Q,X
• have tables of single, double & triple letter
frequencies
12. Use in Cryptanalysis
• key concept - monoalphabetic substitution
ciphers do not change relative letter frequencies
• discovered by Arabian scientists in 9th century
• calculate letter frequencies for ciphertext
• compare counts/plots against known values
• if Caesar cipher look for common peaks/troughs
– peaks at: A-E-I triple, NO pair, RST triple
– troughs at: JK, X-Z
• for monoalphabetic must identify each letter
– tables of common double/triple letters help
13. Example Cryptanalysis
• given ciphertext:
UZQSOVUOHXMOPVGPOZPEVSGZWSZOPF
PESXUDBMETXAIZVUEPHZHMDZSHZOWS
FPAPPDTSVPQUZWYMXUZUHSXEPYEPO
PDZSZUFPOMBZWPFUPZHMDJUDTMOHM
Q
• count relative letter frequencies
– P 13.33, Z 11.67
– S 8.33, U 8.33, O 7.50 M 6.67
– …
– C,K,L,N,R 0.00
15. Playfair Cipher
• not even the large number of keys in a
monoalphabetic cipher provides security
• one approach to improving security was to
encrypt multiple letters
• the Playfair Cipher is an example
• invented by Charles Wheatstone in 1854,
but named after his friend Baron Playfair
16. Playfair Key Matrix
• a 5X5 matrix of letters based on a keyword
• fill in letters of keyword (sans duplicates)
• fill rest of matrix with other letters
• eg. using the keyword MONARCHY
MONAR
CHYBD
EFGIK
LPQST
UVWXZ
17. Encrypting and Decrypting
• plaintext encrypted two letters at a time:
1. if a pair is a repeated letter, insert a filler like 'X',
eg. "balloon" encrypts as "ba lx lo on"
2. if both letters fall in the same row, replace each with
letter to right (wrapping back to start from end),
eg. “ar" encrypts as "RM"
3. if both letters fall in the same column, replace each
with the letter below it (again wrapping to top from
bottom), eg. “mu" encrypts to "CM"
4. otherwise each letter is replaced by the one in its
row in the column of the other letter of the pair, eg.
“hs" encrypts to "BP", and “ea" to "IM" or "JM" (as
desired)
18. Security of the Playfair Cipher
• security much improved over monoalphabetic
• since have 26 x 26 = 676 digrams
• would need a 676 entry frequency table to
analyse (verses 26 for a monoalphabetic)
• and correspondingly more ciphertext
• was widely used for many years (eg. US &
British military in WW1)
• it can be broken, given a few hundred letters
• since still has much of plaintext structure
19. Polyalphabetic Ciphers
• another approach to improving security is to use
multiple cipher alphabets
• called polyalphabetic substitution ciphers
• makes cryptanalysis harder with more alphabets
to guess and flatter frequency distribution
• use a key to select which alphabet is used for
each letter of the message
• use each alphabet in turn
• repeat from start after end of key is reached
20. Vigenère Cipher
• simplest polyalphabetic substitution cipher
is the Vigenère Cipher
• effectively multiple caesar ciphers
• key is multiple letters long K = k1 k2 ... kd
• ith letter specifies ith alphabet to use
• use each alphabet in turn
• repeat from start after d letters in message
• decryption simply works in reverse
21. Example
• write the plaintext out
• write the keyword repeated above it
• use each key letter as a caesar cipher key
• encrypt the corresponding plaintext letter
• eg using keyword deceptive
key: deceptivedeceptivedeceptive
plaintext: wearediscoveredsaveyourself
ciphertext:ZICVTWQNGRZGVTWAVZHCQYGLMGJ
22. Security of Vigenère Ciphers
• have multiple ciphertext letters for each
plaintext letter
• hence letter frequencies are obscured
• but not totally lost
• start with letter frequencies
– see if look monoalphabetic or not
• if not, then need to determine number of
alphabets, since then can attack each
23. Kasiski Attack
• method developed by Babbage / Kasiski
• repetitions in ciphertext give clues to period
• so find same plaintext an exact period apart
• which results in the same ciphertext
• of course, could also be random fluke
• eg repeated “VTW” in previous example
• suggests size of 3 or 9
• then attack each monoalphabetic cipher
individually using same techniques as before
24. Vernam Cipher (1918)
• Key as long as and indepdent of the
plaintext
• Works on binary data as opposed to
letters
• Ci = Pi (xor) Ki
• Where
– Pi – ith binary digit of plaintext
– Ki – ith digit of key
– Ci – ith digit of ciphertext
• Key needs to be long and random
25. One-Time Pad
• if a truly random key as long as the
message is used, the cipher will be secure
• called a One-Time pad
• is unbreakable since ciphertext bears no
statistical relationship to the plaintext
• since for any plaintext & any ciphertext
there exists a key mapping one to other
• can only use the key once though
• have problem of safe distribution of key
26. Transposition Ciphers
• now consider classical transposition or
permutation ciphers
• these hide the message by rearranging
the letter order
• without altering the actual letters used
• can recognise these since have the same
frequency distribution as the original text
27. Rail Fence cipher
• write message letters out diagonally over a
number of rows
• then read off cipher row by row
• eg. write message out as:
m e m a t r h t g p r y
e t e f e t e o a a t
• giving ciphertext
MEMATRHTGPRYETEFETEOAAT
28. Product Ciphers
• ciphers using substitutions or transpositions are
not secure because of language characteristics
• hence consider using several ciphers in
succession to make harder, but:
– two substitutions make a more complex substitution
– two transpositions make more complex transposition
– but a substitution followed by a transposition makes a
new much harder cipher
• this is bridge from classical to modern ciphers
29. Rotor Machines
• before modern ciphers, rotor machines were
most common product cipher
• were widely used in WW2
– German Enigma, Allied Hagelin, Japanese Purple
• implemented a very complex, varying
substitution cipher
• used a series of cylinders, each giving one
substitution, which rotated and changed after
each letter was encrypted
• with 3 cylinders have 263=17576 alphabets
30. Classical Cryptography Summary
• have considered:
– classical cipher techniques and terminology
– monoalphabetic substitution ciphers
– cryptanalysis using letter frequencies
– Playfair ciphers
– polyalphabetic ciphers
– transposition ciphers
– product ciphers and rotor machines
31. Block vs Stream Ciphers
• block ciphers process messages in into
blocks, each of which is then en/decrypted
• like a substitution on very big characters
– 64-bits or more
• stream ciphers process messages a bit or
byte at a time when en/decrypting
• many current ciphers are block ciphers
• hence are focus of course
32. Block Cipher Principles
• most symmetric block ciphers are based on a
Feistel Cipher Structure
• needed since must be able to decrypt ciphertext
to recover messages efficiently
• block ciphers look like an extremely large
substitution
• would need table of 264 entries for a 64-bit block
• instead create from smaller building blocks
• using idea of a product cipher
33. Claude Shannon and Substitution-
Permutation Ciphers
• in 1949 Claude Shannon introduced idea of
substitution-permutation (S-P) networks
– modern substitution-transposition product cipher
• these form the basis of modern block ciphers
• S-P networks are based on the two primitive
cryptographic operations we have seen before:
– substitution (S-box)
– permutation (P-box)
• provide confusion and diffusion of message
34. Confusion and Diffusion
• cipher needs to completely obscure
statistical properties of original message
• a one-time pad does this
• more practically Shannon suggested
combining elements to obtain:
• diffusion – dissipates statistical structure
of plaintext over bulk of ciphertext
• confusion – makes relationship between
ciphertext and key as complex as possible
35. Feistel Cipher Structure
• Horst Feistel devised the feistel cipher
– based on concept of invertible product cipher
• partitions input block into two halves
– process through multiple rounds which
– perform a substitution on left data half
– based on round function of right half & subkey
– then have permutation swapping halves
• implements Shannon’s substitution-
permutation network concept
37. Feistel Cipher Design Principles
• block size
– increasing size improves security, but slows cipher
• key size
– increasing size improves security, makes exhaustive key
searching harder, but may slow cipher
• number of rounds
– increasing number improves security, but slows cipher
• subkey generation
– greater complexity can make analysis harder, but slows cipher
• round function
– greater complexity can make analysis harder, but slows cipher
• fast software en/decryption & ease of analysis
– are more recent concerns for practical use and testing
39. Data Encryption Standard (DES)
• most widely used block cipher in world
• adopted in 1977 by NBS (now NIST)
– as FIPS PUB 46
• encrypts 64-bit data using 56-bit key
• has widespread use
• has been considerable controversy over
its security
40. DES History
• IBM developed Lucifer cipher
– by team led by Feistel
– used 64-bit data blocks with 128-bit key
• then redeveloped as a commercial cipher
with input from NSA and others
• in 1973 NBS issued request for proposals
for a national cipher standard
• IBM submitted their revised Lucifer which
was eventually accepted as the DES
41. DES Design Controversy
• although DES standard is public
• was considerable controversy over design
– in choice of 56-bit key (vs Lucifer 128-bit)
– and because design criteria were classified
• subsequent events and public analysis
show in fact design was appropriate
• DES has become widely used, esp in
financial applications
43. Initial Permutation IP
• first step of the data computation
• IP reorders the input data bits
• even bits to LH half, odd bits to RH half
• quite regular in structure (easy in h/w)
• see text Table 3.2
• example:
IP(675a6967 5e5a6b5a) = (ffb2194d 004df6fb)
44. DES Round Structure
• uses two 32-bit L & R halves
• as for any Feistel cipher can describe as:
Li = Ri–1
Ri = Li–1 xor F(Ri–1, Ki)
• takes 32-bit R half and 48-bit subkey and:
– expands R to 48-bits using perm E
– adds to subkey
– passes through 8 S-boxes to get 32-bit result
– finally permutes this using 32-bit perm P
46. Substitution Boxes S
• have eight S-boxes which map 6 to 4 bits
• each S-box is actually 4 little 4 bit boxes
– outer bits 1 & 6 (row bits) select one rows
– inner bits 2-5 (col bits) are substituted
– result is 8 lots of 4 bits, or 32 bits
• row selection depends on both data & key
– feature known as autoclaving (autokeying)
• example:
S(18 09 12 3d 11 17 38 39) = 5fd25e03
47. DES Key Schedule
• forms subkeys used in each round
• consists of:
– initial permutation of the key (PC1) which
selects 56-bits in two 28-bit halves
– 16 stages consisting of:
• selecting 24-bits from each half
• permuting them by PC2 for use in function f,
• rotating each half separately either 1 or 2 places
depending on the key rotation schedule K
48. DES Decryption
• decrypt must unwind steps of data computation
• with Feistel design, do encryption steps again
• using subkeys in reverse order (SK16 … SK1)
• note that IP undoes final FP step of encryption
• 1st round with SK16 undoes 16th encrypt round
• ….
• 16th round with SK1 undoes 1st encrypt round
• then final FP undoes initial encryption IP
• thus recovering original data value
49. Avalanche Effect
• key desirable property of encryption alg
• where a change of one input or key bit
results in changing approx half output bits
• making attempts to “home-in” by guessing
keys impossible
• DES exhibits strong avalanche
50. Strength of DES – Key Size
• 56-bit keys have 256 = 7.2 x 1016 values
• brute force search looks hard
• recent advances have shown is possible
– in 1997 on Internet in a few months
– in 1998 on dedicated h/w (EFF) in a few days
– in 1999 above combined in 22hrs!
• still must be able to recognize plaintext
• now considering alternatives to DES
51. Strength of DES – Timing Attacks
• attacks actual implementation of cipher
• use knowledge of consequences of
implementation to derive knowledge of
some/all subkey bits
• specifically use fact that calculations can
take varying times depending on the value
of the inputs to it
• particularly problematic on smartcards
52. Strength of DES – Analytic Attacks
• now have several analytic attacks on DES
• these utilise some deep structure of the cipher
– by gathering information about encryptions
– can eventually recover some/all of the sub-key bits
– if necessary then exhaustively search for the rest
• generally these are statistical attacks
• include
– differential cryptanalysis
– linear cryptanalysis
– related key attacks
53. Differential Cryptanalysis
• one of the most significant recent (public)
advances in cryptanalysis
• known by NSA in 70's cf DES design
• Murphy, Biham & Shamir published 1990
• powerful method to analyse block ciphers
• used to analyse most current block ciphers
with varying degrees of success
• DES reasonably resistant to it, cf Lucifer
54. Differential Cryptanalysis
• a statistical attack against Feistel ciphers
• uses cipher structure not previously used
• design of S-P networks has output of
function f influenced by both input & key
• hence cannot trace values back through
cipher without knowing values of the key
• Differential Cryptanalysis compares two
related pairs of encryptions
55. Differential Cryptanalysis
Compares Pairs of Encryptions
• with a known difference in the input
• searching for a known difference in output
• when same subkeys are used
56. Differential Cryptanalysis
• have some input difference giving some
output difference with probability p
• if find instances of some higher probability
input / output difference pairs occurring
• can infer subkey that was used in round
• then must iterate process over many
rounds (with decreasing probabilities)
58. Differential Cryptanalysis
• perform attack by repeatedly encrypting plaintext pairs
with known input XOR until obtain desired output XOR
• when found
– if intermediate rounds match required XOR have a right pair
– if not then have a wrong pair, relative ratio is S/N for attack
• can then deduce keys values for the rounds
– right pairs suggest same key bits
– wrong pairs give random values
• for large numbers of rounds, probability is so low that
more pairs are required than exist with 64-bit inputs
• Biham and Shamir have shown how a 13-round iterated
characteristic can break the full 16-round DES
59. Linear Cryptanalysis
• another recent development
• also a statistical method
• must be iterated over rounds, with
decreasing probabilities
• developed by Matsui et al in early 90's
• based on finding linear approximations
• can attack DES with 247 known plaintexts,
still in practise infeasible
60. Linear Cryptanalysis
• find linear approximations with prob p != ½
P[i1,i2,...,ia](+)C[j1,j2,...,jb] =
K[k1,k2,...,kc]
where ia,jb,kc are bit locations in P,C,K
• gives linear equation for key bits
• get one key bit using max likelihood alg
• using a large number of trial encryptions
• effectiveness given by: |p–½|
61. Block Cipher Design Principles
• basic principles still like Feistel in 1970’s
• number of rounds
– more is better, exhaustive search best attack
• function f:
– provides “confusion”, is nonlinear, avalanche
• key schedule
– complex subkey creation, key avalanche
62. Modes of Operation
• block ciphers encrypt fixed size blocks
• eg. DES encrypts 64-bit blocks, with 56-bit key
• need way to use in practise, given usually have
arbitrary amount of information to encrypt
• four were defined for DES in ANSI standard
ANSI X3.106-1983 Modes of Use
• subsequently now have 5 for DES and AES
• have block and stream modes
63. Electronic Codebook Book (ECB)
• message is broken into independent
blocks which are encrypted
• each block is a value which is substituted,
like a codebook, hence name
• each block is encoded independently of
the other blocks
Ci = DESK1 (Pi)
• uses: secure transmission of single values
65. Advantages and Limitations of ECB
• repetitions in message may show in
ciphertext
– if aligned with message block
– particularly with data such graphics
– or with messages that change very little,
which become a code-book analysis problem
• weakness due to encrypted message
blocks being independent
• main use is sending a few blocks of data
66. Cipher Block Chaining (CBC)
• message is broken into blocks
• but these are linked together in the
encryption operation
• each previous cipher blocks is chained
with current plaintext block, hence name
• use Initial Vector (IV) to start process
Ci = DESK1(Pi XOR Ci-1)
C-1 = IV
• uses: bulk data encryption, authentication
68. Advantages and Limitations of CBC
• each ciphertext block depends on all message blocks
• thus a change in the message affects all ciphertext
blocks after the change as well as the original block
• need Initial Value (IV) known to sender & receiver
– however if IV is sent in the clear, an attacker can change bits of
the first block, and change IV to compensate
– hence either IV must be a fixed value (as in EFTPOS) or it must
be sent encrypted in ECB mode before rest of message
• at end of message, handle possible last short block
– by padding either with known non-data value (eg nulls)
– or pad last block with count of pad size
• eg. [ b1 b2 b3 0 0 0 0 5] <- 3 data bytes, then 5 bytes pad+count
69. Cipher FeedBack (CFB)
• message is treated as a stream of bits
• added to the output of the block cipher
• result is feed back for next stage (hence name)
• standard allows any number of bit (1,8 or 64 or
whatever) to be feed back
– denoted CFB-1, CFB-8, CFB-64 etc
• is most efficient to use all 64 bits (CFB-64)
Ci = Pi XOR DESK1(Ci-1)
C-1 = IV
• uses: stream data encryption, authentication
71. Advantages and Limitations of CFB
• appropriate when data arrives in bits/bytes
• most common stream mode
• limitation is need to stall while do block
encryption after every n-bits
• note that the block cipher is used in
encryption mode at both ends
• errors propogate for several blocks after
the error
72. Output FeedBack (OFB)
• message is treated as a stream of bits
• output of cipher is added to message
• output is then feed back (hence name)
• feedback is independent of message
• can be computed in advance
Ci = Pi XOR Oi
Oi = DESK1(Oi-1)
O-1 = IV
• uses: stream encryption over noisy channels
74. Advantages and Limitations of OFB
• used when error feedback a problem or where need to
encryptions before message is available
• superficially similar to CFB
• but feedback is from the output of cipher and is
independent of message
• a variation of a Vernam cipher
– hence must never reuse the same sequence (key+IV)
• sender and receiver must remain in sync, and some
recovery method is needed to ensure this occurs
• originally specified with m-bit feedback in the standards
• subsequent research has shown that only OFB-64
should ever be used
75. Counter (CTR)
• a “new” mode, though proposed early on
• similar to OFB but encrypts counter value
rather than any feedback value
• must have a different key & counter value
for every plaintext block (never reused)
Ci = Pi XOR Oi
Oi = DESK1(i)
• uses: high-speed network encryptions
77. Advantages and Limitations of CTR
• efficiency
– can do parallel encryptions
– in advance of need
– good for bursty high speed links
• random access to encrypted data blocks
• provable security (good as other modes)
• but must ensure never reuse key/counter
values, otherwise could break (cf OFB)
78. Summary
• have considered:
• block cipher design principles
• DES
– details
– strength
• Differential & Linear Cryptanalysis
• Modes of Operation
– ECB, CBC, CFB, OFB, CTR