Dr Alejandra Flores-Mosri Security Internet Management & Security 06 Learning outcomes At the end of this session, you should be able to: –Describe the reasons for having system and network security –Relate security services to security attacks –Describe the basic operation of encryption trhough Feistel cypher structure –Compare different symmetric encryption algorithms: DES and AES –Design a simple encryption algorithm

Dr Alejandra Flores-Mosri Security Internet Management & Security 06 Introduction With the high availability of resources on the Internet, security has become a very relevant issue. The main aim is to protect the resources on the Internet so that they cannot: –Be accessed –Be changed –Be deleted by non-authorised or malicious users

Dr Alejandra Flores-Mosri Security Internet Management & Security 06 Vulnerability of systems and networks A B C D Malicious user Malicious user Malicious user Malicious user

Dr Alejandra Flores-Mosri Security Internet Management & Security 06 Passive security attacks Passive attacks –Obtain information being transmitted or stored in a system –Do not alter the contents of messages or files –Difficult to detect Solution: –Prevent the attack: data confidentiality –Encrypt information, strongly authenticate access

Dr Alejandra Flores-Mosri Security Internet Management & Security 06 Active security attacks Active attacks –Modify or delete data stored or being transmitted Masquerade Replay Modification Denial of service –Easier to detect –Harder to prevent

Dr Alejandra Flores-Mosri Security Internet Management & Security 06 Active security attacks Solution: –Authentication –Access control –Data confidentiality –Data integrity –Non-repudiation –Availability

Dr Alejandra Flores-Mosri Security Internet Management & Security 06 Conclusions Security strategies are important because they: –help maintain good performance of resources on a network –guarantee users that their information is safe and private –deter potential attacks on resources

Dr Alejandra Flores-Mosri Security Internet Management & Security 06 Resources Stallings W., Network Security Essentials, 2 nd Edition, Prentice Hall, 2002 (Chapter 1) RFC 2828 Internet Security Glossary

Dr Alejandra Flores-Mosri Security Internet Management & Security 06 Cryptography Cryptography modifies the content of a message according to a predetermined pattern only known by sender and receiver with the aim to prevent other parties from reading the content An encrypted message is: 1 Transformed 2 Transmitted 3 Unchanged The predetermined pattern or key needs to be transmitted separately

Dr Alejandra Flores-Mosri Security Internet Management & Security 06 Symmetric cryptography Message Encryption algorithm Secret key Ciphertext Decryption algorithm (the success of the process relies on the secrecy of the key, but it must be known by the sender and the receiver)

Dr Alejandra Flores-Mosri Security Internet Management & Security 06 Feistel Cipher Structure

Dr Alejandra Flores-Mosri Security Internet Management & Security 06 Data Encryption Standard (DES) It is a symmetric block cipher algorithm: 64 bit code 56 bit key Sixteen rounds (iterations), hence 16 subkeys need to be generated Advantages: No one has found a fatal weakness of the algorithm Disadvantages: It has been broken by “brute-force” (2 56 possible keys) in 1998 by the Electronic Frontier Foundation with their DES-cracker

Dr Alejandra Flores-Mosri Security Internet Management & Security 06 Triple DES The same as DES but it uses three executions of the full DES algorithm with three different keys Used in financial applications C=E K 3 [D K 2 [E K 1 [P]]] C – ciphertext P – plaintext E K [X] – encryption of X using key K D K [X] – decryption of X using key K Advantage – effective key of 128 bits Disadvantage – more processing power needed

Dr Alejandra Flores-Mosri Security Internet Management & Security 06 Advanced Encryption Standard (AES) AES – first thought of to replace 3DES with a lighter algorithm that provides the same security as 3DES Block length – 128 bits Supports keys of – 128, 192 and 256 bits (most common key is 128) NOT a Feistel structure

Dr Alejandra Flores-Mosri Security Internet Management & Security 06 AES

Dr Alejandra Flores-Mosri Security Internet Management & Security 06 AES

Dr Alejandra Flores-Mosri Security Internet Management & Security 06 AES

Dr Alejandra Flores-Mosri Security Internet Management & Security 06 AES

Dr Alejandra Flores-Mosri Security Internet Management & Security 06 Other symmetric algorithms IDEA Blowfish RC5

Dr Alejandra Flores-Mosri Security Internet Management & Security 06 Conclusions Encrypted information has more probabilities of remaining private Most common symmetrical encryption algorithms use Feistel’s cipher The length of the block and the key determine the efficiency of the cipher based algorithms AES solves the problem of processing power and key sizes The transmission of the key remains a problem

Dr Alejandra Flores-Mosri Security Internet Management & Security 06 Resources Stallings W., Network Security Essentials, 2 nd Edition, Prentice Hall, 2002 (Chapter 2) FIPS Advanced Encryption Standard, ps-197.pdf ps-197.pdf FIPS DES Data Encryption Standard,