1. Information Systems Security
Introduction to Cryptography

2. What is Cryptography It is an applied branch of mathematics
It is used to provide
Confidentiality
Integrity
Authentication
Authorization
Non-repudiation

3. Why Cryptography Encrypting data against disclosure, modification
Signing data against modification, repudiation
To provide security for eCommerce

4. Application area Storing data encrypted Transmitting data securely
Even access would not lead to disclosure
Transmitting data securely
Prevent eavesdropping
Identifying your partner
Prevent man in the middle attack
Proof of identity
Avoiding impersonation

5. Terms used Encryption Decryption Plaintext Cipher text
The process of encoding a message so that the meaning is not obvious
Decryption
The reverse process of encryption
Plaintext
The original form of the message
Cipher text
The disguised (encrypted) text

6. Terms used C = E(P) P = D(C) P = D(E(P)) P – plaintext C – cipher text
E – encryption algorithm
D – decryption algorithm
C = E(P)
P = D(C)
P = D(E(P))

7. Terms used C = EK(P) P = DK(C) P = DK(EK(P))
The encryption process involves
An algorithm – mostly public
A key – must be private
C = EK(P)
P = DK(C)
P = DK(EK(P))

8. Software components
Hash functions: handling the whole document takes too long
Encryption/decryption: same algorithm for symmetric but different for asymmetric and signature
Signature: combine a document with a private key
Key agreement: creating a shared secret
Key generation: creating secure keys

9. Classification of Cryptographic Systems
The way the plaintext is processed
Block cipher
Stream cipher
Type of operations performed
Substitution
Transposition
Number of keys used
Symmetric
Asymmetric

10. Block encryption
Data divided into fixed size blocks and symmetric encryption worked on them one at a time (e.g. 64 bits in 64 bits out)
Main method is substitution and permutation by using S-boxes
Early block cipher: Playfair
Present block cipher: DES, AES

11. Stream encryption
Symmetric encryption done on the bit stream (1 bit in, 1 bit out)
The usual method is to use symmetric encryption in chain mode (cipher block chaining) where the previous cipher block is XOR to next plaintext block
Early stream cipher: Vigerene
Present stream cipher: RC4

12. Classical techniques - Substitution
Substitute a character, digit or symbol for each character in plaintext
Examples
Mono alphabetic cipher
Caesar cipher
Atbash cipher
Poly alphabetic cipher
Playfair cipher

13. The Caesar cipher - Cryptanalysis
Try all 25 possible keys
Use the nature of the plain text
Single character occurrences
Digrams
Trigrams

14. The Caesar cipher – Cryptanalysis Frequency Table of single letters

15. The Caesar cipher – Cryptanalysis Digrams and Trigrams
an, re, er, nt, th, on, in, am, is, to, be, he, we, no, of
Trigrams
ent, ion, and, the, are, you, she, not

16. Playfair cipher Use a 5 x 5 matrix Use a keyword
Use 2 characters at a time

17. Playfair cipher - rules
Repeating plaintext letters are separated with a filling letter e.g. X
Plaintext letters on the same row is replaced by letters right to it
Plaintext letters on the same column is replaced by letters beneath it
Else, replace plaintext by the corner letters of the rectangle formed by the 2 letters

18. Playfair example Key: PLAYFAIR EXAMPLE P L A Y F 1 R E X M B C D G H JN O S T U V W Z

19. Playfair example Plain text Hide the gold in the tree stump
Change into capital letters
HI DE TH EG OL DI NT HE TR EE ST UM P
Check for repeating letters
HI DE TH EG OL DI NT HE TR EX ES TU MP
Encrypt

20. Playfair example What is the cipher text?
BM ND ZB XD KY BE JV DM UI XM MN UV IF

21. Transposition Change the location of a character Examples
Rail fence cipher
Columnar transposition
Enigma machine

22. Rail fence cipher Rail fence cipher of 3 rails Cipher text Plain text
we are discovered flee at once
Rail fence cipher of 3 rails
W..R..I..O..R..F..E..O..E
.E..E..S..V..E..L..A..N..
..A..D..C..E..D..E..T..C.
Cipher text
WRIORFEOEEESVELANADCEDETC

23. Symmetric encryption
Based on a shared secret by the participants and an algorithm
The secret is used for both encryption and decryption key
To protect the confidentiality of the data
Are usually efficient and fast
Main weakness is the need for the shared secret

24. Symmetric encryption

25. Asymmetric encryption
Designed to overcome issues relating to key distribution
Also offers authenticity
2 keys
Public key – known by everyone
Private key – known only by owner
Keys operate as inverse, one key can decrypt message encrypted by the other

26. Asymmetric encryption

27. Symmetric vs Asymmetric
Number of keys 1 2
Protection
Must be secret
Public & Private
Key distribution
Out of band
Used to exchange other keys
Speed
Fast
10,000 times slower
Usage
Security & integrity of data
Key exchange, authentication

28. Hash
A hash is a cryptographic one way function that produces a record smaller than the plaintext
The plaintext cannot be recovered from the hash and for a good hash function it is impossible for 2 plaintexts to produce the same hash (collision)

29. Hash
A hash encrypted by the document signer’s private key can be used as a signature for a document
Used to produce Message Authentication Codes (MAC) to verify the integrity of a message

30. Digital signature

31. Algorithms Symmetric Asymmetric Hash Others DES, 3DES, AES
RSA, DSA (only for signature)
Hash
Sha-1, MD5
Others
Diffie-Hellman for key agreement

32. PGP (Pretty Good Privacy)
Designed by Phil Zimmermann for providing cryptographic protection of and file storage
Uses the strong cryptographic algorithm
Offers
Authentication using digital signatures
Confidentiality with use of encryption
Bytes conversion to ASCII for

33. PGP design philosophy
Written for individual technically skilled end users
Every user creates and manages their own keys
Every user has a freedom to choose whom to trust
No administrative organisation or government involved in operation

34. Sending a PGP message

35. Receiving a PGP message