1 Serbia 2003 INTRODUCTION to CRYPTOGRAPHY Fred Piper Codes & Ciphers Ltd 12 Duncan Road, Richmond Surrey, TW9 2JD ENGLAND Royal Holloway, University of London Egham Hill, Egham Surrey TW20 0EX ENGLAND

2 Serbia 2003 Sender lAm I happy that the whole world sees this ? lWhat am I prepared to do to stop them ? lWhat am I allowed to do to stop them ? Recipient lDo I have confidence in : uthe originator uthe message contents and message stream uno future repudiation. Network Manager lDo I allow this user on to the network ? lHow do I control their privileges ? Some Security Issues

3 Serbia 2003 Cipher System cryptogram c Enciphering Algorithm Deciphering Algorithm Key k(E)Key k(D) message m message m Interceptor

4 Serbia 2003 The Attacker’s Perspective Deciphering Algorithm Unknown Key k(D) Known c Wants m Note: k(E) is not needed unless it helps determine k(D)

5 Serbia 2003 Two Types of Cipher System Conventional or Symmetric k(D) easily obtained from k(E) Public or Asymmetric Computationally infeasible to determine k(D) from k(E)

6 Serbia 2003 Mortice Lock. If you can lock it, then you can unlock it. Bevelled Sprung Lock. Anyone can lock it, only keyholder can unlock it.

7 Serbia 2003 Types of Attack Ciphertext only Known plaintext Chosen ciphertext

8 Serbia 2003 Assumptions About Attacker (1) Military/Government: Try to keep details of system (including algorithm) secret Worst Case Conditions: Commercial: Assume he knows: lSystem (including algorithm) lAll ciphertext lSome corresponding plaintext/ ciphertext

9 Serbia 2003 Warning THE FACT THAT AN ALGORITHM HAS BEEN PUBLISHED SAYS NOTHING ABOUT ITS STRENGTH.

10 Serbia 2003 Breaking Algorithm Finding a method of determining message from cryptogram without being given deciphering key.

11 Serbia 2003 Exhaustive Key Search Attacks The security depends on the secrecy of the deciphering key. One potential attack, if the algorithm is known, is to try all possible deciphering keys and to eliminate all incorrect ones. To withstand this type of attack a large key space is required.

12 Serbia 2003 Exhaustive Key Searches Estimating time required for key search requires assumptions about the attacker’ resources

13 Serbia 2003 Saints or Sinners ? Receiver Interceptor Sender Who are the ‘good’ guys ?

14 Serbia 2003 If Someone Wants Your Plaintext Give it to them Give them the decryption key They may break algorithm They may ‘find’ plaintext in system They may ‘find’ key in system

15 Serbia 2003 Practical Considerations when Implementing Encryption Key management is the difficult part Keys need to be generated, distributed, stored, changed securely History shows that most cryptanalytic attacks exploit poor implementation and/or key management  Example: Enigma in World War 2

16 Serbia 2003 Cryptography is used to provide: 1. Confidentiality 2. Data Integrity 3. Entity/Origin Verification 4. Non-Repudiation 5. Access Control

17 Serbia 2003 Choosing an Algorithm The choice of algorithm depends upon the application. Applications of encryption include : Data confidentiality Data integrity Digital Signatures.

18 Serbia 2003 Misuse of Encryption Grade Good studentxxxxx Bad studentxxxxx Grades can be changed

19 Serbia 2003 Classification of Techniques Bit / Block operation Positional dependence/independence Message dependence/independence

20 Serbia 2003 Vernam Cipher Random sequence k 1,k 2,…,k n Message m 1,m 2,…,m n + Ciphertext k 1  m 1,k 2  m 2,…,k n  m n The message and key are bit strings

21 Serbia 2003 Stream Cipher Plaintext data Keystream sequence Ciphertext Key Sequence Generator XOR

22 Serbia 2003 Stream Cipher Enciphers bit by bit Positional dependence Security depends on properties of the keystream

23 Serbia 2003 Stream Ciphers Applications Widely used for military and paramilitary applications for both data and digitised speech The main reason for their wide use is that military communications are often over poor channels and error propagation is unacceptable

24 Serbia 2003 Symmetric Block Cipher System Key dependent permutation on s-bit blocks s-bit plaintext block s-bit ciphertext block Key

25 Serbia 2003 Block Ciphers : Key Sizes Depends on security requirement Key searches on size 2 90 are currently considered infeasible

26 Serbia 2003 DES: Key Search on Internet (1997)  DES has 2 56 keys  DES key found  Search took 140 days  Search used over 10,000 computers  Peak rate: keys/sec  ‘Might’ have taken 32 days

27 Serbia 2003 DES Breaker (1998) Electronic Frontier Foundation Design cost $ 80,000 Manufacturing cost$130,000 Test key found in 56 hours Complete search in 220 hours 90 Billion keys per second Design details published

28 Serbia 2003 DES : Double Length Key l k = (k1,k2) l k1,k2 DES keys l E k (m) = E k1 (D k2 (E k1 (m))) l key is 112 bits l key search with trials is infeasible.

29 Serbia 2003 Advanced Encryption Standard (AES) Block ciphers Block size 128 bits Key lengths 128, 192, 256 bits Must be faster than triple DES

30 Serbia 2003 AES (Continued) June 1998:15 candidates August 1998:11 April 1999:5 Decision October 2000 Rijndael

31 Serbia 2003 The following slides will not be discussed but are included for completeness

32 Serbia 2003 Applications Access Control Authentication

33 Serbia 2003 The Challenge / Response Principle Key Random number ChallengePIN-Controlled A A Response A - Encipher or OWF Y/N= ?

34 Serbia 2003 Digital Signatures According to ISO, the term Digital Signature is used: ‘to indicate a particular authentication technique used to establish the origin of a message in order to settle disputes of what message (if any) was sent’.

35 Serbia 2003 Digital Signatures A signature on a message is some data that validates a message and verifies its origin a receiver can keep as evidence a third party can use to resolve disputes. It depends on the message a secret parameter only available to the sender It should be easy to compute (by one person only) easy to verify difficult to forge

36 Serbia 2003 Principle of Digital Signatures There is a (secret) number which: Only one person can use Is used to identify that person ‘Anyone’ can verify that it has been used NB:Anyone who knows the value of a number can use that number.

37 Serbia 2003 Certification Authority Aim : To guarantee the authenticity of public keys. Method : The Certification Authority guarantees the authenticity by signing a certificate containing user’s identity and public key with its secret key. Requirement : All users must have an authentic copy of the Certification Authority’s public key.

38 Serbia 2003 Certification Process Verifies credentials Creates Certificate Receives (and checks) Certificate Presents Public Key and credentials Generates Key Set Distribution Centre Owner

39 Serbia 2003 How Does it Work? The Certificate can accompany all Fred’s messages The recipient must directly or indirectly: Trust the CA Validate the certificate The CA certifies that Fred Piper’s public key is……….. Electronically signed by the CA

40 Serbia 2003 Fundamental Requirement Internal infrastructure to support secure technological implementation