Distributed Information Systems Lecture 9 - Security, Lecture notes of Computers and Information technologies

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Security

Dr Simon Blake

Topics for this lecture

•^

Security^ – Significance / Importance of security– Threats and Attacks– Security techniques:

• Cryptography• Digital signatures• Certificates• Access control• Credentials

•^

Kerberos^ – The architecture and operation of Kerberos

Security Requirements

• Secrecy (Confidentiality)• Data Integrity• Authentication• Non-repudiation• Availability• …

The Enemy^ Communication channel

Process

p^

Process

q

The enemy

m’

Copy of

m m

“Know thy enemy and know thyself, find naught in fear for 100 battles”

Sun Tzu, 600 BC

Methods of Attack

• Eavesdropping

– Release of message contents and trafficanalysis

• Masquerading• Message Tampering (Modification)

– Man-in-the-middle attack

• Replaying• Denial of Service• Mobile Code

Cryptography

• Caesar Cypher or Caesar Shift
  • Substitutes a letter in a message by anotherletter– Substituted according to the length of a key– Example

Key = Caesar Message = THIS IS EASY TO CRACKMessage

= ZNOY OY KGYE ZU IXGIQkey

Terminology

Alice and Bob share a secret key K

.AB

1.^

Alice uses K

and an agreed encryption function E(KAB

, M)AB

to encrypt and send any number of messages {M

}KABi

to Bob.

2.^

Bob reads the encrypted messages using the correspondingdecryption function D(K

, M).AB

Alice and Bob can go on using K

as long as it is safe to assume that KAB

has notAB

been

compromised

.

Key challenges:

-^

Key distribution

and

Freshness of

Shared secret key communication

Authenticated communication

with a server

• Key challenges:

– Timing and replay issues

• These are addressed in Needham and Schroederand Kerberos

– Not suitable for e-commerce becauseauthentication service do not scale well

Bob has a public/private key pair <K

Bpub

, K

Bpriv

1.^

Alice obtains a certificate that was signed by a trustedauthority 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

Authenticated communication

with public keys

Summary of cryptography techniques • Symmetric (secret key)^ –

E(K, M) = {M}K – D(K, E(K, M)) = M – The same key is used for Encryption as Decryption– 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 aknown 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

Digital signatures with a secure

digest function

• Digest function – secure hash function

• takes a long string (or message) of any length asinput and produces a fixed length string as output

• THIS IS THE ORIGINAL TEXT = GRE3245AS144G

• notation H(M)– require that H(M)

≠^

H(M’) for all messages M..M’

• THIS IS THE ORIGINAL TEXTT = GD2T535RD7HG342• THIS WAS THE ORIGINAL TEXT = 45ER32657845TFG

Certificates

1.^ Certificate type

:^

Account number

2.^ Name

:^

Alice

3.^ Account

:^

4.^ Certifying authority

:^

Bob’s Bank

5.^ Signature

:^

{ Digest

( field 2 + field 3

)}^ K

Bpriv

•A statement signed by an appropriate authority (a CA)•Information is provided and certified by a CA•CA signs information using {Digest(F,F)}K

privB

•Others can check signature and validate the data using K

pubB

•But.. How is K

pubB

certified

•Requires another CA to provide K

pubB

•But.. How are they certified?

Cipher Block Chaining

• Eencryption algorithms use a set block size

(typically 64bits)

• Similar blocks result in similar encrypted

patterns

• This is a weakness that needs to be

overcome

• Solution – cipher block chaining with time

stamp initialization vector