1. Security in Computing Cryptography (Introduction) Derived from Greek words: ‘Kruptos’ (hidden) and ‘graphein’ (writing

2. I.Terms and Concepts A. Encryption: transform a message so its meaning in not obvious B. Decryption: remove the transformation C. Plaintext: unencrypted version of a message (i.e. original message) D. Ciphertext: encrypted versions of a message E. Cipher: Encryption algorithm

3. I.Terms and Concepts D. Key: object (string) that personalises an encryption algorithm (in the same way that a key personalises a physical lock) E. Types of encryption 1.Symmetric: same key performs both encryption and decryption 2.Asymmetric: distinct keys, very different keys, one for encryption only and the other for decrypting only

4. I.Terms and Concepts F. Processes 1.Cryptography: performing encryption and decryption 2.Cryptology: designing encryption algorithms 3.Cryptanalysis: analyzing algorithms and encrypted text with the objective of breaking the encryption

5. I.Terms and Concepts G. Breaking encryption 1.Determining the content of an encrypted message without the use of the key 2.Almost any encryption can be broken in theory; in practice the amount of work (time) to do so may be prohibitive 3.“Work factor” is the estimate of amount of time sufficient to break an encryption; work factor may depend on speed of computers

6. I.Terms and Concepts 4.The attacker can use any tools, techniques and approaches... ‘All is fair in love, war and cryptanalysis’!

7. II.“Secure” Encryption Characteristics A. According to Shannon(1949) characteristics of good ciphers: 1.Desired amount of secrecy should determine the amount of labour for encryption and decryption a)Low sensitivity items should not be protected under a cipher that is very difficult to implement b)For high sensitivity items, it is worth paying a price in difficulty to encrypt

8. II.“Secure” Encryption Characteristics 2.Keys and algorithms should not be complex a)Applying a complex algorithm or allowing only particular keys is error-prone b)A complex algorithm may be avoided in the field 3.The implementation of the process should be as simple as possible a)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

9. II.“Secure” Encryption Characteristics 5.Size of ciphertext should be no be larger than the size of the plaintext a)a larger ciphertext size cannot carry more information (because that depends on the plaintext) b)Longer ciphertext required more space and time to decrypt

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

11. II.“Secure” Encryption Characteristics C. More terms and concepts 1.Key distribution: problem of establishing shared secret keys between sender and receiver 2.Key management: problem of storing and changing keys over time 3.Stream Vs block ciphers a)Stream cipher: plaintext converted immediately to ciphertext b)Block cipher: plaintext queued up until a full block is ready, then encrypted as a block

12. II.“Secure” Encryption Characteristics 4.Confusion and diffusion a)Confusion: complex relationship between a plaintext unit and its ciphertext b)Diffusion: effect of changing a plaintext character to ciphertext is spread widely through the ciphertext 4. Types of Cryptanalysis 1.Ciphertext only: common case, ciphertext intercepted without context Note: The algorithm used may be known but not the key

13. II.“Secure” Encryption Characteristics 2.Known plaintext: some plaintext and corresponding (matched) ciphertext a)the goal is to figure out how the plaintext was transformed to that ciphertext b)use the same approach to break the ciphertext for which the plaintext is not available 3.Chosen plaintext: a)ability to force the system to encrypt anything (“chosen plaintext”) and see the effect b)deduce what algorithm (or key was used) c)to break other ciphertext

14. A. Building Blocks 1.Substitution – change one symbol (or unit) into another a)Achieves confusion: obscures the meaning of a symbol 2.Transposition (or permutation): move symbols (or units) around in ciphertext a)Achieves diffusion: spreads effects of encryption throughout ciphertext III.Simple (Traditional) Ciphers

15. B. Caesar Cipher (a form of substitution) 1.Shifts all letters of alphabet n positions forward. original cipher used n = 3 2.Easily implemented, can be done easily in the head, required no paper or other physical resources 3.Also easy to break nopqrstuvwxyz 13141516171819202122232425 III.Simple (Traditional) Ciphers

16. 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 abcdefghijklm 0123456789101112 III.Simple (Traditional) Ciphers

17. C. One-Time Pad (next week) D. Multiple Substitutions (next week) E. Columnar Transposition (next week) F. Other (next week) III.Simple (Traditional) Ciphers

18. A. Brute Force 1.Try all the possible keys a)E.g.: Cesar Cipher – there are only 25 possible keys to try - try all 25 possible keys and the plaintext leaps out 2.Cryptanalysis a)the art of breaking ciphers based on nature of algorithm or plaintext b)Based on letter frequency distributions of written language, e.g.: English IV.Types of Attacks