1. Cryptography
Week-6

2. Cryptography Introduction
Cryptography, secret writing, is the strongest tool for controlling against many kinds of security threats.
Derived from Greek words: ‘Kruptos’ (hidden) and ‘graphein’ (writing)
Well-disguised data cannot be read, modified, or fabricated easily.
Cryptography is rooted in higher mathematics: group and field theory, computational complexity, and even real analysis, not to mention probability and statistics.
Fortunately, it is not necessary to understand the underlying mathematics to be able to use cryptography.

3. Cryptography Terms and Concept
Encryption: is the process of encoding a message so that its meaning is not obvious.
Decryption: is the reverse process, transforming an encrypted message back into its normal, original form.
Alternatively, the terms encode and decode or encipher and decipher are used instead of encrypt and decrypt.
Plaintext: unencrypted version of a message (i.e. original message)
Ciphertext: encrypted versions of a message
Cipher: Encryption algorithm
A system for encryption and decryption is called a Cryptosystem.

4. Cryptography Terms and Concept
Key: object (string) that personalises an encryption algorithm (in the same way that a key personalises a physical lock)
Types of Encryption
Symmetric: same key performs both encryption and decryption

5. Cryptography Terms and Concept Types of Encryption
Asymmetric: distinct keys, very different keys, one for encryption only and the other for decrypting only

6. Cryptography Terms and Concept Process
Cryptography: performing encryption and decryption
Cryptology: designing encryption algorithms
Cryptanalysis: analyzing algorithms and encrypted text with the objective of breaking the encryption

7. ‘All is fair in love, war and cryptanalysis’!
Cryptography
Terms and Concept
Breaking Encryption
Determining the content of an encrypted message without the use of the key
Almost any encryption can be broken in theory; in practice the amount of work (time) to do so may be prohibitive
“Work factor” is the estimate of amount of time sufficient to break an encryption; work factor may depend on speed of computers
The attacker can use any tools, techniques and approaches
‘All is fair in love, war and cryptanalysis’!

8. Cryptography Secure Encryption Characteristics
According to Shannon(1949) characteristics of good ciphers:
1. Desired amount of secrecy should determine the amount of labour for encryption and decryption
Low sensitivity items should not be protected under a cipher that is very difficult to implement
For high sensitivity items, it is worth paying a price in difficulty to encrypt
2. Keys and algorithms should not be complex
Applying a complex algorithm or allowing only particular keys is error-prone
A complex algorithm may be avoided in the field

9. Cryptography Secure Encryption Characteristics
According to Shannon(1949) characteristics of good ciphers:
3. The implementation of the process should be as simple as possible
Hand implementation or complex computer programs are error-prone
4. Error in ciphering should not propagate and corrupt further cipher text – e.g. acknowledges the possibility of error
5. Size of ciphertext should be no be larger than the size of the plaintext
a larger ciphertext size cannot carry more information (because that depends on the plaintext)
Longer ciphertext required more space and time to decrypt

10. Cryptography Secure Encryption Characteristics
Properties of trustworthy encryption
1. Based on sound mathematics
Has a strong theoretical underpinning to justify its strength
2. Analysed by competent experts
Has been scrutinized (examine carefully) objectively
3. Has stood the test of time
Has been used without problem and continues to undergo expert review

11. Cryptography Secure Encryption Characteristics More terms and concept
Key distribution: problem of establishing shared secret keys between sender and receiver
Key management: problem of storing and changing keys over time
Stream Vs block ciphers
Stream cipher: plaintext converted immediately to ciphertext
Block cipher: plaintext queued up until a full block is ready, then encrypted as a block

12. Cryptography Secure Encryption Characteristics Confusion and Diffusion
Confusion: complex relationship between a plaintext unit and its ciphertext
Diffusion: effect of changing a plaintext character to ciphertext is spread widely through the ciphertext
Types of Cryptanalysis
Ciphertext only: common case, ciphertext intercepted without context
Note: The algorithm used may be known but not the key

13. Cryptography Secure Encryption Characteristics Types of Cryptanalysis
Known plaintext: some plaintext and corresponding (matched) ciphertext
the goal is to figure out how the plaintext was transformed to that ciphertext
use the same approach to break the ciphertext for which the plaintext is not available
Chosen plaintext:
ability to force the system to encrypt anything (“chosen plaintext”) and see the effect
deduce what algorithm (or key was used)
To break other ciphertext

14. Cryptography Simple Traditional Ciphers Building Blocks
Substitution – change one symbol (or unit) into another
Achieves confusion: obscures the meaning of a symbol
Transposition (or permutation): move symbols (or units) around in ciphertext
Achieves diffusion: spreads effects of encryption throughout ciphertext

15. Cryptography Simple Traditional Ciphers
Caesar Cipher (a form of substitution)
Shifts all letters of alphabet n positions forward. original cipher used n = 3
Easily implemented, can be done easily in the head, required no paper or other physical resources
Also easy to break n o p q r s t u v w x y z 13 14 15 16 17 18 19 20 21 22 23 24 25

16. Cryptography Simple Traditional Ciphers
Caesar Cipher (a form of substitution)
For each plaintext letter p, substitute the ciphertext letter C:
C = E(3, p) = (p + 3) mod 26
P = D(k, C) = (C – 3) mod 26 a b c d e f g h i j k l m 1 2 3 4 5 6 7 8 9 10 11 12

17. Cryptography Simple Traditional Ciphers
Caesar Cipher (a form of substitution)
1. Use Caeser cipher to encrypt the the text “The World is not Enough” using the shift of 7.
2. Decrypt the ciphertext “wklv lv d vhfuhw phvvdjh”. Initially find out the SHIFT KEY used.
One-Time Pad (next week)
Multiple Substitutions (next week)
Columnar Transposition (next week)
Other (next week)

18. Cryptography Types of Attacks Brute Force Try all the possible keys
E.g.: Cesar Cipher – there are only 25 possible keys to try - try all 25 possible keys and the plaintext leaps out
Cryptanalysis
the art of breaking ciphers based on nature of algorithm or plaintext
Based on letter frequency distributions of written language, e.g.: English

19. Thank You !