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Distributed Systems Course Security

Distributed Systems Course Security. Chapter 2 Revision: Security model Chapter 7: 7.2 Overview of security techniques 7.3 Cryptographic algorithms 7.4 Digital signatures 7.5 Cryptography pragmatics 7.6 Case studies. Learning objectives. Security model Types of threat Basic techniques

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Distributed Systems Course Security

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  1. Distributed Systems CourseSecurity Chapter 2 Revision: Security model Chapter 7: 7.2 Overview of security techniques 7.3 Cryptographic algorithms 7.4 Digital signatures 7.5 Cryptography pragmatics 7.6 Case studies

  2. Learning objectives • Security model • Types of threat • Basic techniques • Cryptographic techniques • Secrecy • Authentication • Certificates and credentials • Access control • Audit trails • Symmetric and asymmetric encryption algorithms • Digital signatures • Approaches to secure system design • Pragmatics and case studies 2 *

  3. Access rights Principal (user) Principal (server) Chapter 2 Revision: Objects and principals Figure 2.13 • Object (or resource) • Mailbox, system file, part of a commercial web site • Principal • User or process that has authority (rights) to perform actions • Identity of principal is important Object invocation Client Server result Network 3 *

  4. m Copy of The enemy m’ p m Process q Process Communication channel Chapter 2 Revision: The enemy Figure 2.14 • Attacks • On applications that handle financial transactions or other information whose secrecy or integrity is crucial • Enemy (or adversary) • Threats • To processes, to communication channels, denial of service 4 *

  5. Cryptography B Principal A Principal The enemy p Process Process q Secure channel Chapter 2 Revision: Secure channels Figure 2.15 • Properties • Each process is sure of the identity of the other • Data is private and protected against tampering • Protection against repetition and reordering of data • Employs cryptography • Secrecy based on cryptographic concealment • Authentication based on proof of ownership of secrets • Ownership of secrets: • Conventional shared crypto keys • Public/private key pair • Cryptographic concealment is based on: • Confusion and diffusion 5 *

  6. Threats and forms of attack • Eavesdropping • obtaining private or secret information • Masquerading • assuming the identity of another user/principal • Message tampering • altering the content of messages in transit • man in the middle attack (tampers with the secure channel mechanism) • Replaying • storing secure messages and sending them at a later date • Denial of service • flooding a channel or other resource, denying access to others 6 *

  7. Threats not defeated by secure channelsor other cryptographic techniques • Denial of service attacks • Deliberately excessive use of resources to the extent that they are not available to legitimate users • E.g. the Internet 'IP spoofing' attack, February 2000 • Trojan horses and other viruses • Viruses can only enter computers when program code is imported. • But users often require new programs, for example: • New software installation • Mobile code downloaded dynamically by existing software (e.g. Java applets) • Accidental execution of programs transmitted surreptitiously • Defences: code authentication (signed code), code validation (type checking, proof), sandboxing. 7 *

  8. Scenario 1: Secret communication with a shared secret key Issues: Key distribution: How can Alice send a shared key KAB to Bob securely? Freshness of communication: How does Bob know that any {Mi} isn’t a copy of an earlier encrypted message from Alice that was captured by Mallory and replayed later? Alice and Bob share a secret key KAB. Alice uses KAB and an agreed encryption function E(KAB, M) to encrypt and send any number of messages {Mi}KAB to Bob. Bob reads the encrypted messages using the corresponding decryption function D(KAB, M). Alice and Bob can go on using KAB as long as it is safe to assume that KAB has not been compromised. 9 *

  9. Scenario 2: Authenticated communication with a server • This is a simplified version of the Needham and Schroeder (and Kerberos) protocol. • Timing and replay issues – addressed in N-S and Kerberos. • Not suitable for e-commerce because authentication service doesn't scale… Bob is a file server; Sara is an authentication service. Sara shares secret key KA with Alice and secret key KB with Bob. Alice sends an (unencrypted) message to Sara stating her identity and requesting a ticket for access to Bob. Sara sends a response to Alice. {{Ticket}KB, KAB}KA. It is encrypted in KA and consists of a ticket (to be sent to Bob with each request for file access) encrypted in KB and a new secret key KAB. Alice uses KA to decrypt the response. Alice sends Bob a request R to access a file: {Ticket}KB, Alice, R. The ticket is actually {KAB, Alice}KB. Bob uses KB to decrypt it, checks that Alice's name matches and then uses KAB to encrypt responses to Alice. A ticket is an encrypted item containing the identity of the principal to whom it is issued and a shared key for a communication session. 10 *

  10. Scenario 3: Authenticated communication with public keys • Mallory might intercept Alice’s initial request to a key distribution service for Bob’s public-key certificate and send a response containing his own public key. He can then intercept all the subsequent messages. Bob has a public/private key pair <KBpub,KBpriv> Alice obtains a certificate that was signed by a trusted authority stating Bob's public key KBpub Alice creates a new shared key KAB , encrypts it using KBpub using a public-key algorithm and sends the result to Bob. 3. Bob uses the corresponding private key KBpriv to decrypt it. (If they want to be sure that the message hasn't been tampered with, Alice can add an agreed value to it and Bob can check it.) 11 *

  11. Scenario 4: Digital signatures with a secure digest function The digest function must be secure against the birthday attack Alice wants to publish a document M in such a way that anyone can verify that it is from her. Alice computes a fixed-length digest of the document Digest(M). Alice encrypts the digest in her private key, appends it to M and makes the resulting signed document (M, {Digest(M)}KApriv) available to the intended users. Bob obtains the signed document, extracts M and computes Digest(M). Bob uses Alice's public key to decrypt {Digest(M)}KApriv and compares it with his computed digest. If they match, Alice's signature is verified. 12 *

  12. Birthday attack Birthday paradox Statistical result: if there are 23 people in a room, the chances are even that 2 of them will have the same birthday. 1. Alice prepares two versions M and M' of a contract for Bob. M is favourable to Bob and M' is not. Alice makes several subtly different versions of both M and M' that are visually indistinguishable from each other by methods such as adding spaces at the ends of lines. She compares the hashes of all the versions of M with all the versions of M'. (She is likely to find a match because of the Birthday Paradox). When she has a pair of documents M and M' that hash to the same value, she gives the favourable document M to Bob for him to sign with a digital signature using his private key. When he returns it, she substitutes the matching unfavourable version M', retaining the signature from M. • If our hash values are 64 bits long, we require only 232 versions of M and M’ on average. • This is too small for comfort. We need to make our hash values at least 128 bits long to guard against this attack. 13 *

  13. 1. Certificate type : Account number 2. Name : Alice 3. Account : 6262626 Figure 7.5 Public-key certificate for Bob's Bank 4. Certifying authority : Bob’s Bank 5. Signature : {Digest(field 2 + field 3)} {Digest(field 2 + field 3)} KFpriv KBpriv 1. Certificate type : Public key 2. Name : Bob’s Bank 3. Public key : KBpub 4. Certifying authority : Fred – The Bankers Federation 5. Signature : Certificates Figure 7.4 Alice’s bank account certificate • Certificate: a statement signed by an appropriate authority. • Certificates require: • An agreed standard format • Agreement on the construction of chains of trust (see Section 7.4.4). • Expiry dates, so that certificates can be revoked. 14 *

  14. S u b jec t D i s t i n g u is he d N a m e, Pu b l ic K e y Iss ue r D i s t i n g u is he d N a m e, Si g n at u r e Pe ri o d o f v a li d i t y N o t Be f o r e Da t e, No t A f t e r D ate A d m i ni str a t ive i n fo rma ti o n V er si o n , S e r i a l N u mb e r Ex t en d e d I n f or m a t i o n X509 Certificate format Figure 7.13 15

  15. Figure 7.4 Alice’s bank account certificate 1. Certificate type : Account number 2. Name : Alice 3. Account : 6262626 Figure 7.5 Public-key certificate for Bob's Bank 4. Certifying authority : Bob’s Bank {Digest(field 2 + field 3)} 5. Signature : {Digest(field 2 + field 3)} KFpriv KBpriv Public key 1. Certificate type : 2. Name : Bob’s Bank 3. Public key : KBpub 4. Certifying authority : Fred – The Bankers Federation 5. Signature : Certificates as credentials • Certificates can act as credentials • Evidence for a principal's right to access a resource • The two certificates shown in the last slide could act as credentials for Alice to operate on her bank account • She would need to add her public key certificate * 16

  16. Access control • Protection domain • A set of <resource, rights> pairs • Two main approaches to implementation: • Access control list (ACL) associated with each object • E.g. Unix file access permissions  • For more complex object types and user communities, ACLs can become very complex • Capabilities associated with principals • Like a key • Format: <resource id, permitted operations, authentication code> • Must be unforgeable • Problems: eavesdropping, difficulty of cancellation drwxr-xr-x gfc22 staff 264 Oct 30 16:57 Acrobat User Data -rw-r--r-- gfc22 unknown 0 Nov 1 09:34 Eudora Folder -rw-r--r-- gfc22 staff 163945 Oct 24 00:16 Preview of xx.pdf drwxr-xr-x gfc22 staff 264 Oct 31 13:09 iTunes -rw-r--r-- gfc22 staff 325 Oct 22 22:59 list of broken apps.rtf 17 *

  17. Credentials • Requests to access resources must be accompanied by credentials: • Evidence for the requesting principal's right to access the resource • Simplest case: an identity certificate for the principal, signed by the principal. • Credentials can be used in combination. E.g. to send an authenticated email as a member of Cambridge University, I would need to present a certificate of membership of CU and a certificate of my email address. • The speaks for idea • We don't want users to have to give their password every time their PC accesses a server holding protected resources. • Instead, the notion that a credential speaks for a principal is introduced. E.g. a user's PK certificate speaks for that user. 18 *

  18. Delegation • Consider a server that prints files: • wasteful to copy the files, should access users' files in situ • server must be given restricted and temporary rights to access protected files • Can use a delegation certificate or a capability • a delegation certificate is a signed request authorizing another principal to access a named resource in a restricted manner. • CORBA Security Service supports delegation certificates. • a capability is a key allowing the holder to access one or more of the operations supported by a resource. • The temporal restriction can be achieved by adding expiry times. 19 *

  19. Cryptographic Algorithms Message M, key K, published encryption functions E, D • Symmetric (secret key) E(K, M) = {M}KD(K, E(K, M)) = M Same key for E and D M must be hard (infeasible) to compute if K is not known. Usual form of attack is brute-force: try all possible key values for a known pair M, {M}K. Resisted by making K sufficiently large ~ 128 bits • Asymmetric (public key) Separate encryption and decryption keys: Ke, Kd D(Kd. E(Ke, M)) = M depends on the use of a trap-door function to make the keys. E has high computational cost. Very large keys > 512 bits • Hybrid protocols - used in SSL (now called TLS) Uses asymmetric crypto to transmit the symmetric key that is then used to encrypt a session. 20 *

  20. Figure 7.6 Cipher block chaining (CBC) XOR n+3 n+2 n+1 plaintext blocks E(K, M) n-3 n-2 n-1 n ciphertext blocks Figure 7.7 Stream cipher keystream number E(K, M) buffer n+3 n+2 n+1 generator XOR ciphertext plaintext stream stream Cipher blocks, chaining and stream ciphers Most algorithms work on 64-bit blocks. Weakness of simple block cipher:- repeated patterns can be detected. 21 *

  21. Symmetric encryption algorithms These are all programs that perform confusion and diffusion operations on blocks of binary data TEA: a simple but effective algorithm developed at Cambridge U (1994) for teaching and explanation. 128-bit key, 700 kbytes/sec DES: The US Data Encryption Standard (1977). No longer strong in its original form. 56-bit key, 350 kbytes/sec. Triple-DES: applies DES three times with two different keys. 112-bit key, 120 Kbytes/sec IDEA: International Data Encryption Algorithm (1990). Resembles TEA. 128-bit key, 700 kbytes/sec AES: A proposed US Advanced Encryption Standard (1997). 128/256-bit key. There are many other effective algorithms. See Schneier [1996]. The above speeds are for a Pentium II processor at 330 MHZ. Today's PC's (January 2002) should achieve a 5 x speedup. 22 *

  22. key 4 x 32 bits plaintext and result 2 x 32 Exclusive OR logical shift TEA encryption function Figure 7.8 • Lines 5 & 6 perform confusion (XOR of shifted text)and diffusion (shifting and swapping) void encrypt(unsigned long k[], unsigned long text[]) { unsigned long y = text[0], z = text[1]; unsigned long delta = 0x9e3779b9, sum = 0; int n; for (n= 0; n < 32; n++) { sum += delta; y += ((z << 4) + k[0]) ^ (z+sum) ^ ((z >> 5) + k[1]); 5 z += ((y << 4) + k[2]) ^ (y+sum) ^ ((y >> 5) + k[3]); 6 } text[0] = y; text[1] = z; } 23 *

  23. TEA decryption function Figure 7.9 void decrypt(unsigned long k[], unsigned long text[]) { unsigned long y = text[0], z = text[1]; unsigned long delta = 0x9e3779b9, sum = delta << 5; int n; for (n= 0; n < 32; n++) { z -= ((y << 4) + k[2]) ^ (y + sum) ^ ((y >> 5) + k[3]); y -= ((z << 4) + k[0]) ^ (z + sum) ^ ((z >> 5) + k[1]); sum -= delta; } text[0] = y; text[1] = z; } 24 *

  24. TEA in use Figure 7.10 void tea(char mode, FILE *infile, FILE *outfile, unsigned long k[]) { /* mode is ’e’ for encrypt, ’d’ for decrypt, k[] is the key.*/ char ch, Text[8]; int i; while(!feof(infile)) { i = fread(Text, 1, 8, infile); /* read 8 bytes from infile into Text */ if (i <= 0) break; while (i < 8) { Text[i++] = ' ';} /* pad last block with spaces */ switch (mode) { case 'e': encrypt(k, (unsigned long*) Text); break; case 'd': decrypt(k, (unsigned long*) Text); break; } fwrite(Text, 1, 8, outfile); /* write 8 bytes from Text to outfile */ } } 25 *

  25. A trapdoor provides a secret way into a room. If you're inside, the way out is obvious, if you're outside, you need to know a secret to get in. Asymmetric encryption algorithms They all depend on the use of trap-door functions • A trap-door function is a one-way function with a secret exit - e.g. product of two large numbers; easy to multiply, very hard (infeasible) to factorize. RSA: The first practical algorithm (Rivest, Shamir and Adelman 1978) and still the most frequently used. Key length is variable, 512-2048 bits. Speed 1-7 kbytes/sec. (350 MHz PII processor) Elliptic curve: A recently-developed method, shorter keys and faster. Asymmetric algorithms are ~1000 x slower and are therefore not practical for bulk encryption, but their other properties make them ideal for key distribution and for authentication uses. See Section 7.3.2 for a description and example of the RSA algorithm. 26 *

  26. Digital signatures Requirement: • To authenticate stored document files as well as messages • To protect against forgery • To prevent the signer from repudiating a signed document (denying their responsibility) Encryption of a document in a secret key constitutes a signature • impossible for others to perform without knowledge of the key • strong authentication of document • strong protection against forgery • weak against repudiation (signer could claim key was compromised) 29 *

  27. Secure digest functions • Encrypted text of document makes an impractically long signature • so we encrypt a secure digest instead • A secure digest function computes a fixed-length hash H(M) that characterizes the document M • H(M) should be: • fast to compute • hard to invert - hard to compute M given H(M) • hard to defeat in any variant of the Birthday Attack • MD5: Developed by Rivest (1992). Computes a 128-bit digest. Speed 1740 kbytes/sec. • SHA: (1995) based on Rivest's MD4 but made more secure by producing a 160-bit digest, speed 750 kbytes/second • Any symmetric encryption algorithm can be used in CBC (cipher block chaining) mode. The last block in the chain is H(M) 30 *

  28. signed doc M {h} h Kpri E(K , h) H(M) pri M 128 bits {h} D(K ,{h}) h' Kpri pub M h = h'?authentic:forged h H(doc) Digital signatures with public keys Figure 7.11 Signing Verifying 31 *

  29. M signed doc h H(M+K) M K M h H(M+K) h = h'?authentic:forged h' K MACs: Low-cost signatures with a shared secret key MAC: Message Authentication Code Figure 7.12 Signing Signer and verifier share a secret key K Verifying 32 *

  30. Key size/hash size Extrapolated PRB optimized speed (bits) speed (kbytes/sec.) (kbytes/s) TEA 128 700 - DES 56 350 7746 112 120 2842 Triple-DES IDEA 128 700 4469 512 7 - RSA RSA 2048 1 - MD5 128 1740 62425 160 750 25162 SHA Performance of encryption and secure digest algorithms Figure 7.14 speeds are for a Pentium II processor at 330 MHZ Algorithm Secret key Publickey Digest PRB = Preneel, Rijmen and Bosselaers [Preneel 1998] 33 *

  31. Case study: Needham - Schroeder protocol In early distributed systems (1974-84) it was difficult to protect the servers • E.g. against masquerading attacks on a file server • because there was no mechanism for authenticating the origins of requests • public-key cryptography was not yet available or practical • computers too slow for trap-door calculations • RSA algorithm not available until 1978 Needham and Schroeder therefore developed an authentication and key-distribution protocol for use in a local network • An early example of the care required to design a safe security protocol • Introduced several design ideas including the use of nonces. * 34

  32. Header Message Notes A requests S to supply a key for communication 1. A->S: A, B, NA with B. S returns a message encrypted in A’s secret key, 2. S->A: {NA , B, KAB, containing a newly generated key KAB and a {KAB, A}KB}KA Ticket ‘ticket’ encrypted in B’s secret key. The nonce NA demonstrates that the message was sent in response to the preceding one. A believes that S sent the message because only S knows A’s secret key. A sends the ‘ticket’ to B. {KAB, A}KB 3. A->B: B decrypts the ticket and uses the new key KAB to {NB}KAB 4. B->A: encrypt another nonce NB. A demonstrates to B that it was the sender of the {NB - 1}KAB 5. A->B: previous message by returning an agreed transformation of NB. The Needham–Schroeder secret-key authentication protocol Weakness: Message 3 might not be fresh - and KABcould have been compromised in the store of A's computer. Kerberos (next case study) addresses this by adding a timestamp or a nonce to message 3. Figure 7.15 NA is a nonce. Nonces are integers that are added to messages to demonstrate the freshness of the transaction. They are generated by the sending process when required, for example by incrementing a counter or by reading the (microsecond resolution) system clock. 35 *

  33. Case study: Kerberos authentication and key distribution service • Secures communication with servers on a local network • Developed at MIT in the 1980sto provide security across a large campus network > 5000 users • based on Needham - Schroeder protocol • Standardized and now included in many operating systems • Internet RFC 1510, OSF DCE • BSD UNIX, Linux, Windows 2000, NT, XP, etc. • Available from MIT • Kerberos server creates a shared secret key for any required server and sends it (encrypted) to the user's computer • User's password is the initial secret shared with Kerberos 36 *

  34. Needham - Schroeder protocol Kerberos Key Distribution Centre TGS: Ticket-granting service 1. A->S: A, B, NA Authentication database Step A 2. S->A: {NA , B, KAB, Ticket- Authen- granting tication 1. Request for {KAB, A}KB}KA service T service A TGS ticket {KAB, A}KB 3. A->B: 2. TGS ticket {NB}KAB 4. B->A: Step B 3. Request for {NB - 1}KAB 5. A->B: server ticket Login Step C 4. Server ticket session setup 5. Service Server request Service session setup function Request encrypted with session key DoOperation Reply encrypted with session key Server Client System architecture of Kerberos Figure 7.16 Step A once per login session Step B once per server session Step C once per server transaction 37 *

  35. Kerberized NFS • Kerberos protocol is too costly to apply on each NFS operation • Kerberos is used in the mount service: • to authenticate the user's identity • User's UserID and GroupID are stored at the server with the client's IP address • For each file request: • UserID and GroupID are sent encrypted in the shared session key • The UserID and GroupID must match those stored at the server • IP addresses must also match • This approach has some problems • can't accommodate multiple users sharing the same client computer • all remote filestores must be mounted each time a user logs in 38 *

  36. Case study: The Secure Socket Layer (SSL) • Key distribution and secure channels for internet commerce • Hybrid protocol; depends on public-key cryptography • Originally developed by Netscape Corporation (1994) • Extended and adopted as an Internet standard with the name Transport Level Security (TLS) • Provides the security in all web servers and browsers and in secure versions of Telnet, FTP and other network applications • Design requirements • Secure communication without prior negotation or help from 3rd parties • Free choice of crypto algorithms by client and server • communication in each direction can be authenticated, encrypted or both 39

  37. SSL SSL Change SSL Alert Handshake HTTP Telnet Cipher Spec Protocol protocol SSL Record Protocol Transport layer (usually TCP) Network layer (usually IP) SSL protocols: Other protocols: SSL protocol stack Figure 7.17 changes thesecure channel to a new spec negotiates cipher suite, exchanges certificates and key masters implements the secure channel 40 *

  38. ClientHello ServerHello Component Description Example Certificate Cipher suite Optionally send server certificate and Key exchange the method to be used for RSA with public-key Certificate Request request client certificate method exchange of a session key certificates ServerHelloDone Cipher for data the block or stream cipher to be IDEA Certificate ClientA ServerB Send client certificate response if transfer used for data Certificate Verify requested Message digest for creating message SHA function authentication codes (MACs) Change Cipher Spec Change cipher suite and finish Finished handshake Change Cipher Spec Finished SSL handshake protocol Figure 7.18 Establish protocol version, session ID, cipher suite, compression method, exchange random start values Includes key master exchange. Key master is used by both A and B to generate: 2 session keys 2 MAC keys KAB MAB KBA MBA 41 *

  39. Component Description Example Key exchange the method to be used for RSA with public-key method exchange of a session key certificates Cipher for data the block or stream cipher to be IDEA transfer used for data Message digest for creating message SHA function authentication codes (MACs) SSL handshake configuration options Figure 7.19 42 *

  40. abcdefghi Application data Fragment/combine abc def ghi Record protocol units Compress Compressed units Hash MAC Encrypt Encrypted Transmit TCP packet SSL record protocol Figure 7.20 43 *

  41. Summary • It is essential to protect the resources, communication channels and interfaces of distributed systems and applications against attacks. • This is achieved by the use of access control mechanisms and secure channels. • Public-key and secret-key cryptography provide the basis for authentication and for secure communication. • Kerberos and SSL are widely-used system components that support secure and authenticated communication. 44 *

  42. Worst case assumptions and design guidelines [p. 260] • Interfaces are exposed • Networks are insecure • Limit the lifetime and scope of each secret • Algorithms and program code are available to attackers • Attackers may have access to large resources • Minimize the trusted base 45

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