Distributed Systems Security - Distributed Operating Systems - Lecture Slides, Slides of Computer Science

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Case Study

On

Distributed Systems Security

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Historical context: the evolution of

security needs

1965-75 1975-89 1990-99 Current

Platforms Multi-user timesharing computers

Distributed systems based on local networks

The Internet, wide- area services

The Internet + mobile devices

Shared resources

Memory, files Local services (e.g. NFS), local networks

Email, web sites, Internet commerce

Distributed objects, mobile code

Security requirements

User identification and authentication

Protection of services Strong security for commercial transactions

Access control for individual objects, secure mobile code

Security management environment

Single authority, single authorization database (e.g. /etc/ passwd)

Single authority, delegation, repli- cated authorization databases (e.g. NIS)

Many authorities, no network-wide authorities

Per-activity authorities, groups with shared responsibilities

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Secure channels

 Properties

 Each process is sure of the identity of the other

 Protection against repetition and reordering of data

 Employs cryptography

 Secrecy based on cryptographic concealment

 Authentication based on proof of ownership of secrets

 Cryptographic concealment is based on:

 Confusion and diffusion

 Conventional shared crypto keys

 Public/private key pair

Principal A

Process p (^) Secure channel Process q

The enemy^ Principal^ B

Cryptography

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Definitions

• Asymmetric algorithm – public/private key cryptography that utilizes a trap-door function, such as RSA
• Availability - system services are available when they are needed
• Certificate - an electronic document containing a statement signed by a principal
• Confidentiality - secrecy, information is only disclosed according to security policy
• Integrity - information is not destroyed or corrupted and the system performs data operations correctly
• DES - Data Encryption Standard adopted by National Bureau of Standards in 1977; famous cryptosystem that uses a 56-bit secret key
• Digital Signature - emulates a conventional signature by using cryptography to verify to a third party that a message or document is produced by the signer
• RSA - Rivest, Shamir, Adleman encryption algorithm; first public-key cryptosystem; based on the use of trap door function of two very large prime numbers
• Symmetric algorithm – secret-key cryptography that uses a common key and a one-way encryption function, such as DES

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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} KB, KAB} KA. 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} KB, Alice, R.

5. The ticket is actually {K AB, Alice} KB. Bob uses K B to decrypt it, checks

that Alice's name matches and then uses K AB to encrypt responses to Alice.

Features

Scenario 2: Authenticated communication

with a server

• 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.
• This is a simplified version of the Needham and Schroeder (and Kerberos) protocol.

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Bob has a public/private key pair <K Bpub , K Bpriv>

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.)

Features

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.

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Features

X509 Certificate format

Subject (^) Distinguished Name, Publ ic Key

Issuer Distinguished Name, Signature Period of validity Not Before Date, Not Aft er Date Admi nistrat ive information Version, Serial Number Extended Informati on

Certificate: a statement signed by an appropriate authority.

Certificates require:

• An agreed standard format
• Agreement on the construction of chains of trust
• Expiry dates, so that certificates can be revoked.

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Certificates as credentials

• Certificates can act as credentials

– Evidence for a principal's right to access a resource

• The two certificates shown could act as credentials

for Alice to operate on her bank account

– She would need to add her public key certificate

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Structure

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 (one-way function) Usual form of attack is brute-force: try all possible key values

• Asymmetric (public key)

Separate encryption and decryption keys: Ke , Kd

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

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Structure

Cipher block chaining and stream ciphers

Most algorithms work on 64-bit blocks. Weakness of simple block cipher:- repeated patterns can be detected.

n

n+3 n+2 n+1 XOR E(K, M)

n-3 n-2 n-

plaintext blocks

ciphertext blocks

Cipher block chaining (CBC)

XOR

generatornumber n+3^ n+2^ n+1^ E(K, M)

plaintext stream

ciphertext stream

buffer

keystream

Stream cipher

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Asymmetric encryption algorithms

RSA: The first practical algorithm (Rivest, Shamir and Adelman 1978) and still the

most frequently used. Key length is variable, 512-2048 bits.

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.

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 to factor

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Implementation

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)

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Implementation

Digital signatures with public keys

Signing

H(doc)^ h

D(Kpub,{h}) (^) h'

Verifying^ h = h'?authentic:forged

M

H(M)

128 bits

h (^) E(Kpri, h) {h}Kpri

M

signed doc

M

{h}Kpri

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Implementation

Low-cost signatures with a shared secret

key

Signing

Verifying

M

K

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

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