Network Security Introduction: Confidentiality, Integrity, Availability, Authenticity

1. In the name of god By: Nasoor Bagheri (na_bagheri@yahoo.com) Network Security

2. Introduction • In most systems security is an important concern • Communications should be secure against eavesdropping and tampering • Servers/clients should be able to verify the identity of their clients/servers • The originator of a message should be verifiable after the message has been delivered

3. Security Requirements • Confidentiality • Integrity • Availability • Authenticity

4. Security Levels • unconditional security • no matter how much computer power is available, the cipher cannot be broken since the ciphertext provides insufficient information to uniquely determine the corresponding plaintext • computational security • given limited computing resources (eg time needed for calculations is greater than age of universe), the cipher cannot be broken

5. 1965-75 1975-89 1990-99 Current Platforms Multi-user Distributed systems The Internet, wide- The Internet + mobile timesharing based on local area services devices computers networks Shared Memory, files Local services (e.g. Email, web sites, Distributed objects, resources NFS), local networks Internet commerce mobile code Security User identification and Protection of services Strong security for Access control for requirements authentication commercial individual objects, transactions secure mobile code Security Single authority, Single authority, Many authorities, Per-activity management single authorization delegation, repli- no network-wide authorities, groups environment database (e.g. /etc/ cated authorization authorities with shared passwd) databases (e.g. NIS) responsibilities Evolution of System Security

6. Techniques • Security mechanisms are based on three techniques • Cryptography • Used to conceal information • Used in support of authentication • Used to implement digital signatures • Authentication • Validate the identity of the sender • Access Control • Allow resources to be accessed only by authorized individuals

7. Goals • Authentication: Are you who you claim to be? • Authorization: Are you authorized to do this? • Integrity: Is the message what the user has sent? • Confidentiality: Can one read this? • Availability (Denial-of-service attacks) • Accountability (Non-repudiation): How can we prove that the message was indeed sent by the sender?

8. Tools • Cryptography: Symmetric and asymmetric • Symmetric: A single secret key for encryption and decryption • Asymmetric: A key pair: Public key and private key; sender encrypts using key owner’s public key; key owner decrypts using corresponding private key

9. Tools (contd.) • Encryption/Decryption algorithms • Digital signatures: Sender uses private key to generate a special tag and appends to the message. Receiver cross checks the integrity by rechecking with the sender’s public key. (Third party can resolve a dispute) • Message authentication code (MAC): Secret key, when applied on a message, gives MAC. The receiver can regenerate the MAC and ensure the integrity of the message • Cryptographic hash functions

10. Security • Persons managing the security of a valued resource consider • Prevention: taking measures that prevent damage. • E.g., one-time passwords (s/key) • Detection: measures that allow detection of when an asset has been damaged, altered, or copied. • E.g., intrusion detection, network forensics • Risk assessment: the value of a resource should determine how much effort (or money) is spent protecting it. • E.g., If you have nothing in your house of value do you need to lock your doors other than to protect the house itself? • If you have an $16,000,000 artwork, you might consider a security guard.

11. Threats and Form of Attacks • Security threats common to computer systems fall into four broad classes • Leakage • Acquisition of information by unauthorized parties • Tampering • The unauthorized alteration of information • Resource Stealing • use of facilities without authorization • Vandalism • interference with proper operation • 2 main categories of attack: passive and active.

12. Passive Attacks • Eavesdropping on transmissions 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

13. Active Attacks • Masquerade • Pretending to be a different entity • Replay • Modification of messages • Denial of service • Easy to detect • Detection may lead to deterrent • Hard to prevent

14. Methods of attack • Eavesdropping • Replaying DoOperation . . . (continue) GetRequest . execute . SendReply replay eaves- drop

15. Methods of attack cont. • Masquerading GetRequest . execute . SendReply client imposter DoOperation . . . (continue) server imposter

16. Original Plaintext Plaintext Ciphertext Encryption Decryption Cryptography • The most widely used tool for securing information and services is cryptography. • Cryptography relies on ciphers: mathematical function used for encryption and decryption of a message. • Encryption: the process of disguising a message in such a way as to hide its substance. • Ciphertext: an encrypted message • Decryption: the process of returning an encrypted message back into plaintext.

17. Example Ciphers • Caesar cipher: each plaintext characters is replaced by a character k to the right. • “Watch out for Brutus!” => “Jngpu bhg sbe Oehghf!” • Only 25 choices! Not hard to break by brute force. • Substitution Cipher: each character in plaintext is replaced by a corresponding character of ciphertext. • E.g., cryptograms in newspapers. plaintext code: 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 ciphertext code: 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 • 26! Possible pairs.

18. Ciphers • For some message M, let’s denote the encryption of that message into cipher text as Ek(M) = C Similarly, the decryption into plain text as Dk(C) = M Notice, Dk(Ek(M)) = M symmetric key algorithms. Some algorithms use different keys for each operation: Dk1(Ek2(M))= M public-key algorithms.

19. Figure 16.1 Simplified Model of Symmetric Encryption

20. Ingredients • Plain text • Encryption algorithm • Secret key • Cipher text • Decryption algorithm

21. Requirements for Security • Strong encryption algorithm • Even if known, should not be able to decrypt or work out key • Even if a number of cipher texts are available together with plain texts of them • Sender and receiver must obtain secret key securely • Once key is known, all communication using this key is readable

22. Attacking Encryption • Cryptanalysis • Relay on nature of algorithm plus some knowledge of general characteristics of plain text • Attempt to deduce plain text or key • Brute force • Try every possible key until plain text is achieved

23. Timing Attacks • developed in mid-1990’s • exploit timing variations in operations • eg. multiplying by small vs large number • or IF's varying which instructions executed • infer operand size based on time taken • RSA exploits time taken in exponentiation • countermeasures • use constant exponentiation time • add random delays • blind values used in calculations

24. Cryptanalysis Ke Kd C = EKe(plaintext) Encrypt Decrypt plaintext plaintext Invader Side information plaintext Cryptanalysis

25. Cryptanalysis • Cryptanalysis is the science of recovering the plaintext of a message without access to the key. • Doesn’t have to discover the key necessarily. • The loss of a key without cryptanalysis is called a compromise. • Ciphertext-only attack • The attacker has to recover the plaintext from only the ciphertext. • Known-plaintext attack • Portions of the cipher are known as plaintext. The rest may be easier to recover • Chosen-plaintext attack • The attacker can choose what plaintext to encrypt, again making it easier to recover other ciphertext.

26. Encryption Algorithms • Block cipher • Process plain text in fixed block sizes producing block of cipher text of equal size • Data encryption standard (DES) • Triple DES (TDES) • Advanced Encryption Standard

27. Simple Block Cipher Plaintext message B3 B2 B1 B0 encrypt B0 B1 B2 B3

28. Problem • If the same block is encrypted twice with the same key, the resulting ciphertext blocks are the same • It is desirable to make identical plaintext blocks encrypt to different ciphertext blocks. • Two methods are commonly used for this: • CBC mode: a ciphertext block is obtained by first xoring the plaintext block with the previous ciphertext block, and encrypting the resulting value. • CFB mode: a ciphertext block is obtained by encrypting the previous ciphertext block, and xoring the resulting value with the plaintext.

29. CBC XOR Bn+4 Bn+3 Bn+2 Bn+1 encrypt Bn-3 Bn-2 Bn-1 Bn

30. Stream Ciphers • For some applications encryption in blocks will not work • Telephone conversation • Radio Broadcast • … • White noise…

31. Stream Cipher number generator encrypt K3 K2 K1 K0 buffer keystream XOR Plaintext stream Encrypted stream

32. Data Encryption Standard • US standard • 64 bit plain text blocks • 56 bit key • Broken in 1998 by Electronic Frontier Foundation • Special purpose machine • Less than three days • DES now worthless

33. Triple DEA • ANSI X9.17 (1985) • Incorporated in DEA standard 1999 • Uses 3 keys and 3 executions of DEA algorithm • Effective key length 112 or 168 bit • Slow • Block size (64 bit) too small

34. Advanced Encryption Standard • National Institute of Standards and Technology (NIST) in 1997 issued call for Advanced Encryption Standard (AES) • Security strength equal to or better than 3DES • Improved efficiency • Symmetric block cipher • Block length 128 bits • Key lengths 128, 192, and 256 bits • Evaluation include security, computational efficiency, memory requirements, hardware and software suitability, and flexibility • 2001, AESissued as federal information processing standard (FIPS 197)

35. AES Description • Assume key length 128 bits • Input is single 128-bit block • Depicted as square matrix of bytes • Block copied into State array • Modified at each stage • After final stage, State copied to output matrix • 128-bit key depicted as square matrix of bytes • Expanded into array of key schedule words • Each four bytes • Total key schedule 44 words for 128-bit key • Byte ordering by column • First four bytes of128-bit plaintext input occupy first column of in matrix • First four bytes of expanded keyoccupy first column of w matrix

36. Location of Encryption DevicesFigure: Encryption Across a Packet Switching Network

37. Link Encryption • Each communication link equipped at both ends • All traffic secure • High level of security • Requires lots of encryption devices • Message must be decrypted at each switch to read address (virtual circuit number) • Security vulnerable at switches • Particularly on public switched network

38. End to End Encryption • Encryption done at ends of system • Data in encrypted form crosses network unaltered • Destination shares key with source to decrypt • Host can only encrypt user data • Otherwise switching nodes could not read header or route packet • Traffic pattern not secure • Use both link and end to end

39. Key Distribution Question: How to deliver a shared key to 2 parties that wish to exchange data without others to see the key? • Key selected by A and delivered to B • Third party selects key and delivers to A and B • Use old key to encrypt and transmit new key from A to B • Use old key to transmit new key from third party to A and B

40. Message Authentication • Protection against active attacks • Falsification of data • Eavesdropping • Message is authentic if it is genuine and comes from the alleged source • Authentication allows receiver to verify that message is authentic • Message has not altered • Message is from authentic source • Message timeline

41. Authentication Using Encryption • Assumes sender and receiver are only entities that know key • Message includes: • error detection code • sequence number • time stamp

42. Authentication Without Encryption • Authentication tag generated and appended to each message • Message not encrypted • Useful for: • Messages broadcast to multiple destinations • Have one destination responsible for authentication • One side heavily loaded • Encryption adds to workload • Can authenticate random messages • Programs authenticated without encryption can be executed without decoding

43. Message Authentication Code • Generate authentication code based on shared key and message • Common key shared between A and B • If only sender and receiver know key and code matches: • Receiver assured message has not altered • Receiver assured message is from alleged sender • If message has sequence number, receiver assured of proper sequence

44. Figure : Message Authentication Using a Message Authentication Code

45. One Way Hash Function • Accepts variable size message and produces fixed size tag (message digest) • Advantages of authentication without encryption • Encryption is slow • Encryption hardware expensive • Encryption hardware optimized to large data • Algorithms covered by patents • Algorithms subject to export controls (from USA)

46. Secure Hash Functions • Hash function must have following properties: • Can be applied to any size data block • Produce fixed length output • Easy to compute • Not feasible to reverse • Not feasible to find two message that give the same hash

47. SHA-1 • Secure Hash Algorithm 1 • Input message less than 264 bits • Processed in 512 bit blocks • Output 160 bit digest

48. Public Key Encryption • Based on mathematical algorithms • Asymmetric • Use two separate keys • Ingredients • Plain text • Encryption algorithm • Public and private key • Cipher text • Decryption algorithm

49. Figure 16.9 Public-Key Cryptography

50. Public Key Encryption - Operation • One key made public • Used for encryption • Other kept private • Used for decryption • Infeasible to determine decryption key given encryption key and algorithm • Either key can be used for encryption, the other for decryption