CS 843 - Distributed Computing Systems Chapter 7: Security Chin-Chih Chang, From Coulouris, Dollimore and Kindberg Distributed Systems:

CS 843 - Distributed Computing Systems Chapter 7: Security Chin-Chih Chang, chang@cs.twsu.edu From Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edition 3, © Addison-Wesley 2001

Learning Objectives Security model  Types of threat – eavesdropping, masquerading, tampering, and denial of service Security techniques  Cryptographic techniques oSecrecy and integrity oAuthentication  Certificates and credentials  Access control Symmetric and asymmetric encryption algorithms Digital signatures Approaches to secure system design Pragmatics and case studies

Introduction Why do we need Security?  The need to protect the integrity and privacy of information and other resources belonging to individuals and organizations is pervasive in both the physical and the digital world.  Figure 7.1 summarize the evolution of security needs. Some Common Terms  Security Policies provide for the sharing of resources within specified limits. For example, access policy.  Security Mechanisms enforce security policies. For example, security guard. Security Policy and Security Mechanism are used to determine the security of a system. For example, lock - mechanism and rules of using lock - policy.

Figure 7.1 Historical context: the evolution of security needs

Security Model (Ch 2) Protecting objects Securing processes and their interactions The enemy Defeating security threats Uses of this model

Principal (user)Principal (server) Chapter 2: Objects and Principals Access rights Network invocation result Client Server Object 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 Figure 2.13

Protecting Objects Server: Collection of objects on behalf of some users. Clients send invocations and Server replies Access rights to users Associate each invocation and each result the authority on which it is issued. Authority: principal Server verifies identity of each principal.

Securing Processes & Interactions, Enemy Processes interact by sending messages Messages are exposed to attack because of open networks Integrity of messages threatened by security violations and communication failures Model to analyze these threats An enemy is capable of Sending any message to any process and reading or copying any message between a pair of processes. Example: Computers on a network running a program to read network messages or make false requests to processes.

Chapter 2: The enemy Communication channel Process p q The enemy m’ Copy of m m 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 Figure 2.14

Chapter 2: Secure channels 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 Figure 2.15 Principal A Secure channel Process p q Principal B The enemy Cryptography

The Emergence of Cryptography Cryptography provides the basis for most computer security mechanisms. The names shown in Figure 7.2 are used extensively in the security literature.

Figure 7.2 Familiar names for the protagonists in security protocols AliceFirst participant BobSecond participant CarolParticipant in three- and four-party protocols DaveParticipant in four-party protocols EveEavesdropper MalloryMalicious attacker SaraA server

Defeating Security Threats  Cryptography  Authentication  Secure Channels Uses Of Security Model:  Provides basis for analysis and design of secure systems keeping costs at a minimum.

Threats & Attacks Types of Security Threats generally fall into three board classes:  Leakage: Acquisition of information by unauthorized recipients.  Tampering: Unauthorized alteration of information.  Vandalism: Interference with proper operation of system without gain to the perpetrator

Threats and Forms of Attack on a Communication Channel Eavesdropping  obtaining private or secret information Masquerading  assuming the identity of another user/principal Message tampering altering the content of messages in transit oman 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

Threats not defeated by secure channels or other cryptographic techniques Denial of service attacks  Deliberately excessive use of resources to the extent that they are not available to legitimate users oE.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: oNew software installation oMobile code downloaded dynamically by existing software (e.g. Java applets) oAccidental execution of programs transmitted surreptitiously  Defences: code authentication (signed code), code validation (type checking, proof), sandboxing.

The February 2000 IP Spoofing DoS attack Echo request | source = x.x.x.x | destination = n.n.n.i Echo reply | source = n.n.n.i | destination = x.x.x.x Untrue! Compromised host on each local network sends repeatedly (for all i): resulting in:

Threats From Mobile Code Programs can be loaded into a process from a remote server and then executed locally. What is the danger? Internal interfaces and objects within an executing process exposed to attacks. E.g. Java Virtual Machine employs security manager.

Java Virtual Machine (JVM) Measures in JVM to protect local environment  Downloaded classes are stored separately from local classes  Code-validation: Bytecodes are checked for validity.  Type-checking: Valid java bytecode is composed of JVM instructions from a specified set. These instructions are checked to ensure they will not produce certain errors when the program runs. Information leakage  If the transmission of a message between two processes can be observed, some information can be gathered just by observation.

Securing Electronic Transactions Email: authenticate an auction bid by email Purchase of Goods Banking Transactions Micro-transactions  Businesses are looking for ways to charge for access to Web pages, which may for example hold stock market quotes or give access to interactive games. The cost of processing such small sums, called micro-transactions (or sometimes micro-payments)  One slightly-cynical provider has commented that the true meaning of micro-transaction is any transaction whose value is currently too small to be worth bothering with - about one US dollar at the moment.

Securing Web Purchase Authenticate the vendor to the buyer. Keep the buyer’s credit card number and other payment details from falling into the hands of the third party and ensure that they are sent unaltered. If the goods are in a form suitable for downloading, ensure that their contents is delivered without alteration. Authenticate the identity of the account holder to bank before giving them access to their account.

Designing Secure Systems Designing Secure Systems is difficult. The design goal is to exclude all possible attacks and loopholes. This is analogous to that of programmer whose aim is to exclude all bugs from his program. Worst Case Assumptions (The box on page 260):  Interfaces are exposed – an attacker can send a message to any interface.  Networks are insecure  Limit the lifetime and scope of each secret - password  Algorithms and code are available to attackers  Attackers may have access to large resources  Minimize the trusted base – The system for security implementation should be kept small.

Security Techniques Familiar names for the main players in security protocols are introduced in Figure 7.2 and the notations for encrypted and signed items in Figure 7.3. Cryptography:  Encryption: Process of encoding a message to hide its contents  Cryptography includes several encryption/decryption algorithms based on use of secrets called Keys.  Cryptographic key: Parameter used in an encryption algorithm in such a way that the encryption cannot be reversed without the knowledge of the key

Figure 7.3 Cryptography notations KAKA Alice’s secret key KBKB Bob’s secret key K AB Secret key shared between Alice and Bob K Apriv Alice’s private key (known only to Alice) K Apub Alice’s public key (published by Alice for all to read) {M} K MessageM encrypted with keyK [M]K]K MessageM signed with key K

Security notations KAKA Alice’s secret key KBKB Bob’s secret key K AB Secret key shared between Alice and Bob K Apriv Alice’s private key (known only to Alice) K Apub Alice’s public key (published by Alice for all to read) {M} K MessageM encrypted with keyK [M ]K]K MessageM signed with key K AliceFirst participant BobSecond participant CarolParticipant in three- and four-party protocols DaveParticipant in four-party protocols EveEavesdropper MalloryMalicious attacker SaraA server

Two main classes:  Shared Secret keys: Both use secret keys  Public/Private keys: Sender uses public key Comparison  Public key encryption requires 100/1000 times more processing power Cryptography is used in:  Secrecy and integrity  Authentication  Digital signatures Security Techniques

Alice and Bob share a secret key K AB. 1.Alice uses K AB and an agreed encryption function E(K AB, M) to encrypt and send any number of messages {M i } K AB to Bob. 2.Bob reads the encrypted messages using the corresponding decryption function D(K AB, M). Alice and Bob can go on using K AB as long as it is safe to assume that K AB has not been compromised. Scenario 1: Secret communication with a shared secret key Issues: Key distribution: How can Alice send a shared key K AB to Bob securely? Freshness of communication: How does Bob know that any {M i } isn’t a copy of an earlier encrypted message from Alice that was captured by Mallory and replayed later?

Authentication: Authenticated Communication With Server Scenario: Alice wishes to access files held by Bob, a file server on the LAN she works. Sara is an authentication server that is securely managed. Sara issues users with passwords and holds secret keys for all principals in the system (K A & K B ). Ticket: Encrypted item issued by authentication server, containing the identity of the principal to whom it is issued and a shared key that has been generated for the current communication session. Pros and Cons:  Pros: Useful in small organizations  Cons: Inappropriate for Electronic Commerce

Bob is a file server; Sara is an authentication service. Sara shares secret key K A with Alice and secret key K B with Bob. 1.Alice sends an (unencrypted) message to Sara stating her identity and requesting a ticket for access to Bob.  2.Sara sends a response to Alice. {{Ticket} K B, K AB } K A. It is encrypted in K A and consists of a ticket (to be sent to Bob with each request for file access) encrypted in K B and a new secret key K AB. 3.Alice uses K A to decrypt the response. 4.Alice sends Bob a request R to access a file: {Ticket} K B, Alice, R. 5.The ticket is actually {K AB, Alice} K B. Bob uses K B to decrypt it, checks that Alice's name matches and then uses K AB to encrypt responses to Alice. 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 has a public/private key pair 1.Alice obtains a certificate that was signed by a trusted authority stating Bob's public key K Bpub 2.Alice creates a new shared key K AB, encrypts it using K Bpub using a public-key algorithm and sends the result to Bob. 3.Bob uses the corresponding private key K Bpriv 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.) 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.

Digital Signatures Can be achieved by encrypting a compressed form of message (digest) using a key known only to signer. This encrypted digest acts as a signature that accompanies the message.

Alice wants to publish a document M in such a way that anyone can verify that it is from her. 1.Alice computes a fixed-length digest of the document Digest(M). 2.Alice encrypts the digest in her private key, appends it to M and makes the resulting signed document (M, {Digest(M)} K Apriv ) available to the intended users. 3.Bob obtains the signed document, extracts M and computes Digest(M). 4.Bob uses Alice's public key to decrypt {Digest(M)} K Apriv and compares it with his computed digest. If they match, Alice's signature is verified. Scenario 4: Digital signatures with a secure digest function The digest function must be secure against the birthday attack

Certificates 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.

Certificates 1.Certificate type:Account number 2.Name:Alice 3.Account:6262626 4.Certifying authority:Bob’s Bank 5.Signature:{Digest(field 2 + field 3)} K Bpriv Figure 7.4 Alice’s bank account certificate Figure 7.5 Public-key certificate for Bob's Bank 1.Certificate type:Public key 2.Name:Bob’s Bank 3.Public key:K Bpub 4.Certifying authority:Fred – The Bankers Federation 5.Signature: {Digest(field 2 + field 3)} K Fpriv

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 Figure 7.5 Public-key certificate for Bob's Bank 1.Certificate type: Public key 2.Name:Bob’s Bank 3. Public key:K Bpub 4.Certifying authority:Fred – The Bankers Federation 5. Signature : {Digest(field 2 + field 3)} K Fpriv 1.Certificate type:Account number 2.Name:Alice 3.Account:6262626 4.Certifying authority:Bob’s Bank 5.Signature:{Digest(field 2 + field 3)} K Bpriv Figure 7.4 Alice’s bank account certificate

Access control Protection domain  A set of pairs Two main approaches to implementation:  Access control list (ACL) associated with each object oE.g. Unix file access permissions  oFor more complex object types and user communities, ACLs can become very complex  Capabilities associated with principals oLike a key oFormat: oMust be unforgeable oProblems: 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

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.

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.

Firewalls Protect intranets, performing filtering operations on incoming, outgoing communications. All external communication is intercepted No protection from attacks from inside Not affective against denial of service attacks

Summary Security Model Threats and Attacks Designing Secure Systems Security Techniques: Cryptography Secret/ Public Keys Digital Signatures Certificates Firewalls

Content Cryptographic algorithms :  Symmetric algorithms  Asymmetric algorithms  Block ciphers  Stream ciphers Digital signatures :  Digital signatures with public keys  Digital signatures with private keys  Secure digest functions

Cryptographic algorithms A message is encrypted by the sender applying some rule to transform the plaintext message (any sequence of bits) to a ciphertext (a different sequence of bits). The recipient must know the inverse rule in order to transform the ciphertext into the original plaintext.

The encryption transformation is defined with two parts, a function E and a key K. The resulting message is written {M} . Decryption is carried out using an inverse function D, which also takes a key as a parameter. Symmetric vs. Asymmetric:  For secret-key cryptography, the key used for decryption is the same as that used for encryption.  For public-key cryptography, the key used for encryption and decryption are different. Cryptographic algorithms

Cryptographic Algorithms Symmetric (secret key) E(K, M) = {M} K D(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: K e, K d D(K d. E(K e, 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. Message M, key K, published encryption functions E, D

Most algorithms work on 64-bit blocks. For some applications, such as the encryption of telephone conversations, encryption in blocks is inappropriate because the data streams are produced in real time in small chunks. A keystream is an arbitrary-length sequence of bits that can be used to obscure the contents of a data stream by XOR-ing the keystream with the data stream. Cryptographic algorithms

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

Shannon’s principles of confusion and diffusion is to conceal the content of a ciphertext block M, combing it with a key to protect against brute-force attacks. Non-destructive operations such as XOR and circular shifting are used to combine each block of plaintext with the key, producing a new bit pattern that obscures the relationship between the blocks in M and {M} . There is usually repetition and redundancy in the plaintext. Diffusion dissipates the regular patterns that result by transposing portions of each plaintext block. Design of Cryptographic Algorithms

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

Symmetric encryption algorithms  AES: A proposed US Advanced Encryption Standard (1997). 128/256-bit key. The above speeds are for a Pentium II processor at 330 MHZ. Today's PC's (May 2002) should achieve a 5 x speedup. There are many other effective algorithms. Schneier [1996] describes more than 25 symmetric algorithms, many of which are identified as secure against known attacks. (Schneier, B. (1996). Applied Cryptography, 2 nd Ed. New York: John Wiley.)

TEA (Tiny Encryption Algorithm) The TEA algorithm uses rounds of integer addition, XOR (the ^ operator) and bitwise logical shifts ( >) to achieve diffusion and confusion of the bit patterns in the plaintext. The plaintext is a 64-bit block represented as two 32-bit integers in the vector text[]. The key is 128bits long, represented as four 32-bit integer.

TEA encryption function 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 > 5) + k[1]);5 z += ((y > 5) + k[3]);6 } text[0] = y; text[1] = z; } Lines 5 & 6 perform confusion (XOR of shifted text) and diffusion (shifting and swapping) Figure 7.8 key 4 x 32 bits plaintext and result 2 x 32 Exclusive OR logical shift

TEA: TEA decryption function 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 > 5) + k[3]); y -= ((z > 5) + k[1]); sum -= delta; } text[0] = y; text[1] = z; }

TEA: TEA in use 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 to 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 */ }

DES (Data Encryption Standard) Data Encryption Standard (DES) was developed by IBM and adopted as a US national standard. The encryption function maps a 64-bit plaintext input into a 64-bit encrypted output using a 56-bit key. The algorithm has 16 key-dependent stages known as rounds and was time-consuming. In June 1997, it was successfully cracked. Only used for the protection of low-value information. Triple-DES: apply DES three times with two keys k1, k2. Give strength against brute-force attacks.

IDEA (International Data Encryption Algorithm) IDEA was developed in the early 1990s as a successor to DES. Use a 128-bit key to encrypt 64-bit blocks. For both DES and IDEA, the same function is used for encryption and decryption. No significant weakness have been found. It performs encryption and decryption at about 3 times the speed of DES.

AES (Advanced Encryption Standard) 1997, the US NIST issued an invitation. NIST is pleased to announce the approval of the Federal Information Processing Standard (FIPS) for the Advanced Encryption Standard, FIPS-197. This standard specifies Rijndael as a FIPS-approved symmetric encryption algorithm that may be used by U.S. Government organizations (and others) to protect sensitive information.

AES (Advanced Encryption Standard) The two researchers who developed and submitted Rijndael for the AES are both cryptographers from Belgium: Dr. Joan Daemen (Yo'-ahn Dah'-mun) of Proton World International and Dr. Vincent Rijmen (Rye'-mun), a postdoctoral researcher in the Electrical Engineering Department (ESAT) of Katholieke Universiteit Leuven. http://csrc.nist.gov/encryption/aes/ http://csrc.nist.gov/encryption/aes/round2/aesfact.ht mlhttp://csrc.nist.gov/encryption/aes/round2/aesfact.ht ml

Asymmetric (public-key) algorithm Only a few practical public-key schemes have been developed to date. 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. 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)

Asymmetric (public-key) algorithm RSA – The Rivest, Shamir, Adelman (RSA) design for a public-key cipher is based on the use of the product of two very large prime numbers, relying on the fact that the determination of the prime factors of such large numbers is so computationally difficult as to be effectively impossible to compute. 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.

RSA Encryption To find a key pair e, d: 1. Choose two large prime numbers, P and Q (each greater than 10100), and form: N = P x Q Z = (P–1) x (Q–1) 2. For d choose any number that is relatively prime with Z (that is, such that d has no common factors with Z). We illustrate the computations involved using small integer values for P and Q: P = 13, Q = 17 –> N = 221, Z = 192 d = 5 3.To find e solve the equation: e x d = 1 mod Z That is, e x d is the smallest element divisible by d in the series Z+1, 2Z+1, 3Z+1,.... e x d = 1 mod 192 = 1, 193, 385,... 385 is divisible by d e = 385/5 = 77

RSA Encryption To encrypt text using the RSA method, the plaintext is divided into equal blocks of length k bits where 2 k < N (that is, such that the numerical value of a block is always less than N; in practical applications, k is usually in the range 512 to 1024). k = 7, since 27 = 128 The function for encrypting a single block of plaintext M is: E'(e,N,M) = M e mod N for a message M, the ciphertext is M 77 mod 221 The function for decrypting a block of encrypted text c to produce the original plaintext block is: D'(d,N,c) = c d mod N Rivest, Shamir and Adelman proved that E' and D' are mutual inverses (that is, E'(D'(x)) = D'(E'(x)) = x) for all values of P in the range 0 ≤ P ≤ N. The two parameters e,N can be regarded as a key for the encryption function, and similarly d,N represent a key for the decryption function. So we can write K e = and K d =, and we get the encryption function: E(K e, M) ={M} K (the notation here indicating that the encrypted message can be decrypted only by the holder of the private key K d ) and D(K d, ={M} K ) = M.

RSA 1.choose 2 large primes, p and q > 10^100. 2.compute n=pq and phi(n)=(p-1)(q-1). 3.choose a number relatively prime to phi(n) and call it d. 4.find e s.t. e.d mod phi(n) = 1. Group P into blocks s.t. C=P^e (mod n) and P=C^d(mod n), 0 <= P < n E.g. (pedagogical): P=3 q=11 => n=33 phi(n) = 20. | let d=7 <--| 7.e = 1 (mod 20) => e=3 ---- C = P^3 (mod 33), P = C^7 (mod 33)

Digital Signatures Strong digital signatures are an essential requirement for secure systems. Handwritten signatures are used to verify that document is:  Authentic  Unforgettable  Non-repudiable What is needed is a means to irrevocably bind a signer’s identity to the entire sequence of bits representing a document. Two techniques:  Digital signing  Digest functions

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)

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: ofast to compute ohard to invert - hard to compute M given H(M) ohard 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)

Secure digest functions h=H(M) should have the following properties: 1. Given M, it is hard to compute h. 2. Given h, it is hard to compute M. 3. Given M, it is hard to find another message M’, such that H(M) = H(M’). Two widely used digest functions: MD5, SHA

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

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

Certificate standards X.509 is the most widely standard format as shown in Figure 7.13.

X509 Certificate format Figure 7.13

Cryptography pragmatics Performance of cryptographic algorithms. comparison of various algorithms. 330 Mhz PII systems used

Performance of encryption and secure digest algorithms Key size/hash size (bits) Extrapolated speed (kbytes/sec.) PRB optimized speed (kbytes/s) TEA128700- DES563507746 Triple-DES 1121202842 IDEA1287004469 RSA 512 7- RSA2048 1- MD5128174062425 SHA 16075025162 PRB = Preneel, Rijmen and Bosselaers [Preneel 1998] Algorithm Public key Secret key Digest speeds are for a Pentium II processor at 330 MHZ Figure 7.14

Applications & obstacles Political obstacles: oThe resistance had two sources: NSA and FBI oIn January 2000, the US government introduced a new policy to allow US software vendors to export software that incorporates strong encryption. Applications: PGP (Pretty Good Privacy) www.pgp.com www.pgp.com oDeveloped by Philip Zimmermann oIt uses RSA public-key encryption for authentication and to transmit secret keys to the intended party oIt uses the IDEA or 3 DES secret-key encryption algorithms to encrypt mail messages

Case studies Needham-Schroeder Kerberos SSL Millicent

Case study: Needham and Schroeder 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 ocomputers too slow for trap-door calculations oRSA algorithm not available until 1978 An authentication protocol in response to the need for a secure means of managing keys and passwords in a network.

Case study: Needham - Schroeder protocol 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. The solution to authentication and key distribution is based on an authentication server to supply keys to the clients.

Figure 7.15 The Needham–Schroeder secret-key authentication protocol HeaderMessageNotes 1. A->S: A, B, N A A requests S to supply a key for communication with B. 2. S->A:{N A, B, K AB, {K AB, A} K B } K A S returns a message encrypted in A’s secret key, containing a newly generated key K AB and a ‘ticket’ encrypted in B’s secret key. The nonce N A 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. 3. A->B: A sends the ‘ticket’ to B. 4. B->A: B decrypts the ticket and uses the new key K AB to encrypt another nonce N B. 5. A->B: A demonstrates to B that it was the sender of the previous message by returning an agreed transformation of N B. {K AB, A} K B {N B } K AB {N B - 1} K AB

Case study: Needham - Schroeder protocol N A 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. Weakness: Message 3 might not be fresh - and K AB could 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.

Kerberos – 1980s Secures communication with servers on a local network  Developed at MIT in the 1980s to provide security across a large campus network > 5000 users  based on Needham - Schroeder protocol. Time stamp is used as nonces. Introduced for authentication and security facilities in campus and other intranets.

Case study: Kerberos authentication and key distribution service Standardized and now included in many operating systems  Internet RFC 1510, OSF DCE  BSD UNIX, Linux, Windows 2000, NT, XP, etc.  Available from MIT Three kinds of security objects:  ticket  authentication  session key 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

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

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

E-transactions with Secure Sockets (SSL) Developed by Netscape and adopted as standard with name Transport Layer Security. Features:  Negotiable encryption and authentication algorithms.  Bootstrapped secure communication

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

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

Client A Server B ClientHello ServerHello Figure 7.18 SSL handshake protocol Establish protocol version, session ID, cipher suite, compression method, exchange random start values Certificate Certificate Request ServerHelloDone Optionally send server certificate and request client certificate Certificate Certificate Verify Send client certificate response if requested Change Cipher Spec Finished Change Cipher Spec Finished Change cipher suite and finish handshake Includes key master exchange. Key master is used by both A and B to generate: 2 session keys2 MAC keys K AB M AB K BA M BA

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

Figure 7.20 SSL record protocol Application data abcdefghi abcdefghi Record protocol units Fragment/combine Compressed units Compress MAC Hash Encrypted Encrypt TCP packet Transmit

Milicent protocol Used for low level(low priced) electronic transactions. Drawbacks of Existing systems:  Credit cards – high cost  Online accounts – the initial overhead  Digital cash – double spending because of undetectable copies

The Millicent Scheme Developed scrip - specialized digital cash for low level transactions. Scrip is valid for a particular vendor only. Format of scrip: vendorValueScrip IDCustomer IDExpiry DatePropertiesCertificate

Figure 7.21 Millicent architecture

Summary Introduction - Threats, e-transactions, designing secure systems Overview of security techniques - cryptography, access control, firewalls Cryptographic algorithms - asymmetric algorithms, secret-key/symmetric algorithms, hybrid algorithms, digital signatures Case-studies - Needham-Schroeder, kerberos, SSL, Millicent

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.

CS 843 - Distributed Computing Systems Chapter 7: Security Chin-Chih Chang, From Coulouris, Dollimore and Kindberg Distributed Systems:

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CS 843 - Distributed Computing Systems Chapter 7: Security Chin-Chih Chang, From Coulouris, Dollimore and Kindberg Distributed Systems:

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