Introduction to Cryptography

1. Lecture 2
Introduction to Cryptography
Dr. Shafqat Cheema
05 Oct 2021

2. Flashback
 IS deals with CIA
 Intro to Cryptography – Terminologies
 Symmetric / Asymmetric Cryptography
 Malicious Programs
 We often say a term brute force attack. So what it
actually is?
2

3. Enigma
3
 The Enigma machine was invented by the
German engineer Arthur Scherbius at the end of
World War I
 Enigma has an electromechanical rotor
mechanism that scrambles the 26 letters of the
alphabet.

4. Enigma
4
 In typical use, one person enters text on the Enigma's keyboard and
another person writes down which of 26 lights above the keyboard lights
up at each key press. If plain text is entered, the lit-up letters are the
encoded ciphertext. Entering ciphertext transforms it back into readable
plaintext.
 The rotor mechanism changes the electrical connections between the
keys and the lights with each keypress.
 The security of the system depends on a set of machine settings that
were generally changed daily during the war, based on secret key lists
distributed in advance, and on other settings that were changed for each
message.

5. Enigma
5
The receiving station has to know and use the exact settings
employed by the transmitting station to successfully decrypt
a message.
While Germany introduced a series of improvements to
Enigma over the years, and these hampered decryption
efforts to varying degrees, they did not ultimately prevent
Britain and its allies from exploiting Enigma-encoded
messages as a major source of intelligence during the war.

6. Malware Countermeasures

7. Signatures: A Malware Countermeasure
 Scan compare the analyzed object with a database of
signatures
 A signature is a virus fingerprint
– E.g.,a string with a sequence of instructions specific
for each virus
– Different from a digital signature
 A file is infected if there is a signature inside its code
– Fast pattern matching techniques to search for
signatures
 All the signatures together create the malware database
that usually is proprietary
7

8. White/Black Listing
 Maintain database of cryptographic hashes for
– Operating system files
– Popular applications
– Known infected files
 Compute hash of each file in hard drives
 Look up into database to compare
 Needs to protect the integrity of the database
 Example: TripWire software
8

9. Heuristic Analysis
 Useful to identify new and “zero day” malware
 Code analysis
– Based on the instructions, the antivirus can determine
whether or not the program is malicious, i.e., program
contains instruction to delete system files,
 Execution emulation
– Run code in isolated emulation environment
• Such as in Virtual Machine
– Monitor actions that target file takes
– If the actions are harmful, mark as virus
 Heuristic methods can trigger false alarms
9

10. Shield vs. On-demand
 Shield
– Background process
(service/daemon)
– Scans each time a file
is touched (open, copy,
execute, etc.)
On-demand
• Scan on explicit user
request or according to
regular schedule
• On a suspicious file,
directory, drive, etc.
10

11. Online vs Offline Anti Virus Software
Online
 Free browser plug-in
 Authentication through third
party certificate (i.e. VeriSign)
 No shielding
 Software and signatures
update at each scan
 Poorly configurable
 Scan needs internet
connection
 Report collected by the
company that offers the
service
Offline
 Paid annual subscription
 Installed on the OS
 Software distributed securely by
the vendor online or a retailer
 System shielding
 Scheduled software and signatures
updates
 Easily configurable
 Scan without internet connection
 Report collected locally and may
be sent to vendor
11

12. Quarantine
 A suspicious file can be isolated in a folder called
quarantine:
– E.g,. if the result of the heuristic analysis is positive and you are
waiting for db signatures update
 The suspicious file is not deleted but made harmless: the
user can decide when to remove it or eventually restore for
a false positive
– Interacting with a file in quarantine it is possible only through the
antivirus program
 The file in quarantine is harmless because it is encrypted
 Usually the quarantine technique is proprietary and the
details are kept secret
12

13. Static vs. Dynamic Analysis
Static Analysis
 Checks the code without
trying to execute it
 Quick scan in white list
 Filtering: scan with different
antivirus and check if they
return same result with
different name
 Weeding: remove the correct
part of files as junk to better
identify the virus
 Code analysis: check binary
code to understand if it is an
executable, e.g., PE
 Disassembling: check if the
byte code shows something
unusual
Dynamic Analysis
 Check the execution of
codes inside a virtual
sandbox
 Monitor
– File changes
– Registry changes
– Processes and threads
– Networks ports
13

14. Cryptographic Algorithms

15. Types of Cryptography
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 Symmetric Key Cryptography
 Asymmetric Key Cryptography
 Hash Functions

16. Symmetric Key Cryptography - Examples
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Data Encryption Standard (DES)
The Data Encryption Standard was published in 1977 by the US National
Bureau of Standards.
It uses 56 bit key and maps a 64 bit input block of plaintext onto a 64 bit
block of ciphertext. 56 bits is a rather small key for today's computing power.

17. Problems with Conventional Cryptography
17
Key Management
Symmetric-key systems are simpler and faster; their main drawback is
that the two parties must somehow exchange the key in a secure way
and keep it secure after that.
Key Management caused nightmare for the parties using the symmetric key
cryptography. They were worried about how to get the keys safely and securely
across to all users. This gave the chance for third parties to intercept the keys
in transit to decode the top-secret messages. Thus, if the key was
compromised, the entire coding system was compromised and a “Secret”
would no longer remain a “Secret”.
This is why the “Public Key Cryptography” came into existence.

18. Asymmetric Key Cryptography
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Asymmetric cryptography , also known as Public-key cryptography, refers
to a cryptographic algorithm which requires two separate keys, one of
which is private and one of which is public. The public key is used to
encrypt the message and the private one is used to decrypt the message.

19. Asymmetric Key Cryptography
19
Public Key Cryptography is a very advanced form of cryptography.
Officially, it was invented by Whitfield Diffie and Martin Hellman in 1975.
The basic technique of public key cryptography was first discovered in 1973
by the British Clifford Cocks of Communications-Electronics Security Group
(CESG) of (Government Communications Headquarters - GCHQ) but this
was a secret until 1997.

20. Comparison
20

21. Hash Functions
21
A cryptographic hash function is a function that takes an arbitrary block of
data and returns a fixed-size bit string, the cryptographic hash value, such
that any (accidental or intentional) change to the data will (with very high
probability) change the hash value.
The data to be encoded are often called
the message, and the hash value is
sometimes called the message digest or
simply digest.

22. Hash Functions
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An ideal cryptographic hash function has four main properties:
 it is deterministic, same message always results in the same hash
 it is easy to compute the hash value for any given message
 it is infeasible to generate a message that has a given hash
 it is infeasible to modify a message without changing the hash
 it is infeasible to find two different messages with the same hash.

23. Hash Functions
23
Collusion Discovery
In March 2005 Xiaoyun Wang and Hongbo Yu of Shandong University in
China created a pair of files that share the same MD5 checksum hence
prove that there is a collusion when using MD5
$ md5sum file1.dat
MD5 Sum = a4c0d35c95a63a805915367dcfe6b751
$ md5sum file2.dat
MD5 Sum = a4c0d35c95a63a805915367dcfe6b751
Visit the following websites for more information
http://www.mscs.dal.ca/~selinger/md5collision/
http://www.x-ways.net/md5collision.html

24. Block vs Stream Ciphers
 Block ciphers process messages in 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
– better analyzed
– broader range of applications

25. Block vs Stream Ciphers

26. Block Cipher Principles
 Most symmetric block ciphers are based on a Feistel
Cipher Structure
 Must be able to decrypt ciphertext to recover messages
efficiently
 Block ciphers look like an extremely large substitution
 264 entries for a 64-bit block
 instead create from smaller building blocks
 using idea of a product cipher

27. Claude Shannon and Substitution – Permutation Ciphers
 Claude Shannon introduced idea of substitution-permutation (S-P)
networks in 1949 paper
 form basis of modern block ciphers
 S-P networks are based on the two primitive cryptographic
operations:
 substitution (S-box)
 permutation (P-box)
 provide confusion & diffusion of message & key

28. Confusion and Diffusion
 cipher needs to completely obscure statistical properties
of original message
 a one-time pad does this
 more practically Shannon suggested combining S & P
elements to obtain:
 diffusion – dissipates statistical structure of plaintext over
bulk of ciphertext
 confusion – makes relationship between ciphertext and
key as complex as possible

29. 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 S-P net concept

30. Feistel Cipher Structure

31. Feistel Cipher Structure

32. Feistel Cipher Design Elements
 block size
 key size
 number of rounds
 subkey generation algorithm
 round function
 fast software en/decryption
 ease of analysis

33. Data Encryption Standard
(DES)

34. Data Encryption Standard (DES)
 most widely used block cipher in world till it
became obsolete
 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

35. DES History
 IBM developed Lucifer cipher
– by team led by Feistel in late 60’s
– 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

36. 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
 use of DES had flourished
– especially in financial applications
– still standardised for legacy application use

37. DES Round Structure
 uses two 32-bit L & R halves
 as for any Feistel cipher can describe as:
Li = Ri–1
Ri = Li–1  F(Ri–1, Ki)
 F takes 32-bit R half and 48-bit subkey:
– expands R to 48-bits using permutation
– adds to subkey using XOR
– passes through 8 S-boxes to get 32-bit result
– finally permutes using 32-bit permutation

38. DES – Round Structure

39. DES – One Round

40. 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

41. DES Round Structure

42. 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 row of 4
 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)

43. DES Key Schedule
 forms subkeys used in each round
 initial permutation of the key (PC1) which selects 56-
bits in two 28-bit halves
 16 stages consisting of:
• rotating each half separately either 1 or 2 places
depending on the key rotation schedule K
• selecting 24-bits from each half & permuting them by to
use in round function F

44. DES Decryption
 decrypt must unwind steps of data computation
 with Feistel design, do encryption steps again
using subkeys in reverse order (SK16 … SK1)
– 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

45. DES Decryption

46. Avalanche Effect
46
• key desirable property of any encryption algorithm
• want a change of one input or key bit to result in changing
approx half output bits
• making attempts to “home-in” by guessing keys impossible
• DES exhibits strong avalanche

47. Strength of DES
47
•56-bit keys have 256 = 7.2 x 1016 values
•brute force search looks hard
•past advances have shown it 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!
–Nowadays … can do it on a desktop!
•still, it must be possible to recognize plaintext
•forced consideration of alternatives to DES

48. Attacks on DES – Timing Attack
48
•attacks actual implementation of cipher
•use knowledge of consequences of implementation to
derive information about 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

49. Attacks on DES – Analytic Attacks
49
• 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
–differential cryptanalysis
–linear cryptanalysis
–related key attacks

50. Attacks on DES – Analytic Attacks
50
Differential Cryptanalysis
• one of the most significant (public) advances in
cryptanalysis
• known by NSA in 70's … influenced DES
• Murphy, Biham & Shamir published in 90’s
• powerful method to analyse block ciphers
• used to analyse most current block ciphers with varying
degrees of success
• DES reasonably resistant to it.

51. Attacks on DES – Analytic Attacks
51
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 243 known plaintexts
• easier but still in practise infeasible

52. DES Design Criteria
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•as reported by Coppersmith in 1994
•7 criteria for S-boxes provide for
–non-linearity
–resistance to differential cryptanalysis
–good confusion
•3 criteria for permutation P provide for
–increased diffusion

53. Block Cipher Design
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Many principles from Feistel in 70s still hold
• number of rounds
–more is better, make exhaustive search the best attack
option
• function f:
–provides “confusion”, is nonlinear, avalanche
–issues of how S-boxes are selected
• key schedule
–complex subkey creation, key avalanche

54. 54
Q & A