Advanced Computer Networks

1. Advanced Computer Networks Lecture 4 Security

2. 1 Motivation • More vital/secret data handled by distributed components within network systems and Internet. • Security: protecting data stored in and transferred between distributed components from unauthorised access. • Insecurity effect: Confidential Data may be stolen, e.g.: • corporate plans. • new product designs. • medical/financial records (e.g. Access bills…) • Insecurity effect: Data may be altered, e.g.: • finances made to seem better than they are. • results of tests, e.g. on drugs, altered. • examination results amended (up or down).

3. 1 Why are Distributed Systems Insecure? • Distributed component rely on messages sent and received from network. • Is network (especially WANs and wireless networks) secure? • Is client component secure? • Is client component who it claims to be? • Are users of calling components really who they claim to be?

4. 1 Consequences of Insecurity • Loss of confidence: above effects may reduce confidence in computerized systems. • Claims for damages: legal developments may allow someone to sue if data on computer has not been guarded according to best practice. • Loss of privacy: data legally stored on a computer may well be private to the person concerned (e.g. medical/personnel) record.

5. 2 Security Threats • Categorization of attacks (and goals of attacks) that may be made on system. • Three broad classes: • leakage: information leaving system. • tampering: unauthorised information altering. • vandalism: disturbing correct system operation. • Used to specify what the system is proof, or secure, against. • Attacks: Passive and Active

6. 2 Passive Attacks • Eavesdropping on transmissions • Receive message copies without authority. • To obtain information • Release of message contents • Outsider learns content of transmission • Traffic analysis • By monitoring frequency and length of messages, even encrypted, nature of communication may be guessed • Difficult to detect • Can be prevented

7. 2 Active Attacks • Masquerading: Pretending to be a different entity without authority. • Message tampering: Intercepting and altering messages. • Replaying: Storing messages and sending them later. • Denial of service: Flooding server resources with messages in order to deny other’s access. • Hard to prevent • Easy to detect • Detection may lead to deterrent

8. 3.1 Introduction • Cryptography: encode message data so that it can only be understood by intended recipient. • Romans used it in military communication. • Given knowledge of encryption algorithm, a brute force attempt is to try every possible decoding until a valid message is produced. • Computers are good at this! • Modern schemes must be computationally hard to solve to remain secure.

9. 3.2 Cryptographic Terminology • Plain text: the message before encoding (by encryption). • Cipher text: the message after encoding. • Key: information needed to convert from plain text to cipher text (or vice-versa). • Function: the encryption or decryption algorithm used, in conjunction with key, to encode or decode message. • Key distribution service: trusted service which hands out keys.

10. 3.3 Encryption • Encrypting data prevents unauthorised access to the data (i.e. prevents eavesdropping). • If encrypted data can only be encrypted with a matching key, this can be used to prove sender’s identity (i.e. prevents masquerading). • Likewise, it can be used to ensure that only intended recipients can use the data. • Two main ways: secret key and public key.

11. 3.4 Secret Keys • One key is used to both encrypt and decrypt data • Encryption and decryption functions are often chosen to be the same type • Security should not be compromised by making function well-known as security comes from secret keys

12. 3.4 Enigma Encryption Machine

13. 3.4 Using Secret Keys • Sender and recipient exchange keys through some secure, trusted, non-network based means. • Sender encodes message using encryption function and sends the message, knowing that only the holder of key (the intended recipient) can make sense of it. • Recipient decodes message, and knows that only intended sender could generate it. • Message can be captured but is of no use.

14. 3.4 Simplified Model of Symmetric (Secret Key) Encryption

15. 3.4 Secret Key Encryption Algorithms • Historical cipher • Caesar Cipher • Monoalphabetic Cipher • Polyalphabetic Cipher

16. 3.4.1 Caesar Cipher • Each letter in plaintext is substituted with letter that is K letters later • Wrap around is allowed (i.e., z followed by letter a) • If K = 3, a in plaintext becomes d in cipher text b in plaintext becomes e in cipher text • Exercise: “Dolfh, L oryh brx. Ere” • Once it is known that Caesar cipher is being used, it is easy to break the code (only 25 possible key values).

17. 3.4.1 Attacking Caesar Cipher: Brute-Force • Brute-Force attack is based on the making sense feature of the data by trying every possible key. • Suppose that the cipher “mrsxocoexsfobcsdi” is the result of a Caesar shift cipher. • Since the key space is {0, 1, …, 25}, then the possible plaintexts are: mrsxocoexsfobcsdi, nstypdpfytgpcdtej, otuzqeqgzuhqdeufk, puvarfrhavirefvgl, qvwbsgsibwjsfgwhm, rwxcthtjcxktghxin, sxyduiukdyluhiyjo, tyzevjvlezmvijzkp, uzafwkwmfanwjkalq, vabgxlxngboxklbmr, wbchymyohcpylmcns, xcdiznzpidqzmndot, ydejaoaqjeranoepu, zefkbpbrkfsbopfqv, afglcqcslgtcpqgrw, bghmdrdtmhudqrhsx, chineseuniversity, dijoftfvojwfstjuz, ejkpgugwpkxgtukva, fklqhvhxqlyhuvlwb, glmriwiyrmzivwmxc, hmnsjxjzsnajwxnyd, inotkykatobkxyoze, jopulzlbupclyzpaf, kpqvmamcvqdmzaqbg, lqrwnbndwrenabrch

18. 3.4.2 Monoalphabetic Cipher • Improvement on Caesar Cipher • Rather than substituting according to a regular pattern – any letter can be substituted for any other letter, as long as each letter has a unique substitute letter, and vice versa. • Example of a monoalphabetic cipher • Plain Text: a b c d e f g h i j k l m n o p q r s t u v w x y z • Cipher Text: m n b v c x z a s d f g h j k l p o i u y t r e w q • Complexity: • _______ Possible pairings of letters – so breaking code is not as easy as in the case of Caesar cipher. 26!

19. 3.4.2 Cryptanalysis: Simple Example • Mono-alphabetic substitution ciphers can be broken by simple frequency analysis • The relative frequency of letters in English text is distinct (shown in table) • Ciphertext frequencies can be used to guess plaintext letters

20. 3.4.2 Cryptanalysis • It is the art and science of breaking ciphers, ciphertexts, or keys. • Brute-force: A cryptanalysis where all possible keys are tested, and then the result possible plaintext are analyzed. • Statistical analysis: single-letter appearance frequency; “q” is mostly followed by “u”; multi-letter frequency. • Known plaintext-ciphertext combination.

21. 3.4.3 Polyalphabetic Encryption • Use multiple monoalphabetic/Caesar ciphers • Use a specific monoalphabetic/Caesar cipher to encode a letter in a specific position in the plain text message • This implies that same letter appearing in different positions in the plaintext might be encoded differently. Example: 2 Caesar ciphers; K = 5, K = 19 For every 5 bits in the plain text use the 2 Caesar ciphers in the following pattern: C1, C2, C2, C1, C2

22. 3.4.3 Example: Vigenere Cipher • In the Vigenere cipher, the character in the ciphertext is chosen from a two-dimensional table (26 x 26), in which each row is a permutation of 26 characters (A to Z).