Cryptography and Network Security

CryptographyCryptography
& Network& Network
SecuritySecurity
By
M.Ramki
S.Vigneshwaran

Introduction
The art of war teaches us to rely not on the
likelihood of the enemy's not coming, but on
our own readiness to receive him; not on
the chance of his not attacking, but rather
on the fact that we have made our position
unassailable.
—The Art of War, Sun Tzu

Security Services (X.800)
• Authentication - assurance that the communicating entity
is the one claimed
• Access Control - prevention of the unauthorized use of a
resource
• Data Confidentiality –protection of data from
unauthorized disclosure
• Data Integrity - assurance that data received is as sent
by an authorized entity
• Non-Repudiation - protection against denial by one of the
parties in a communication

Security Mechanisms (X.800)
• specific security mechanisms:
– encipherment, digital signatures, access controls, data
integrity, authentication exchange, traffic padding,
routing control, notarization
• pervasive security mechanisms:
– trusted functionality, security labels, event detection,
security audit trails, security recovery

Classify Security Attacks
• passive attacks - eavesdropping on, or
monitoring of, transmissions to:
– obtain message contents, or
– monitor traffic flows
• active attacks – modification of data stream to:
– masquerade of one entity as some other
– replay previous messages
– modify messages in transit
– denial of service

Classical Encryption
Techniques
Many savages at the present day regard their
names as vital parts of themselves, and
therefore take great pains to conceal their
real names, lest these should give to evil-
disposed persons a handle by which to
injure their owners. —The Golden Bough,
Sir James George Frazer

Symmetric Encryption
• or conventional / private-key / single-key
• sender and recipient share a common key
• all classical encryption algorithms are
private-key
• was only type prior to invention of public-
key in 1970’s

Cryptography
• can be characterized by:
– type of encryption operations used
• substitution / transposition / product
– number of keys used
• single-key or private / two-key or public
– way in which plaintext is processed
• block / stream

Types of Cryptanalytic Attacks
• ciphertext only
– only know algorithm / ciphertext, statistical, can identify plaintext
• known plaintext
– know/suspect plaintext & ciphertext to attack cipher
• chosen plaintext
– select plaintext and obtain ciphertext to attack cipher
• chosen ciphertext
– select ciphertext and obtain plaintext to attack cipher
• chosen text
– select either plaintext or ciphertext to en/decrypt to attack cipher

Caesar Cipher
• earliest known substitution cipher
• by Julius Caesar
• first attested use in military affairs
• replaces each letter by 3rd letter on
• example:
meet me after the toga party
PHHW PH DIWHU WKH WRJD SDUWB

Cryptanalysis of Caesar Cipher
• only have 26 possible ciphers
– A maps to A,B,..Z
• could simply try each in turn
• a brute force search
• given ciphertext, just try all shifts of letters
• do need to recognize when have plaintext
• eg. break ciphertext "GCUA VQ DTGCM"

Language Redundancy and Cryptanalysis
• human languages are redundant
• eg "th lrd s m shphrd shll nt wnt"
• letters are not equally commonly used
• in English e is by far the most common letter
• then T,R,N,I,O,A,S
• other letters are fairly rare
• cf. Z,J,K,Q,X
• have tables of single, double & triple letter frequencies

Encrypting and Decrypting
• plaintext encrypted two letters at a time:
1. if a pair is a repeated letter, insert a filler like 'X',
eg. "balloon" encrypts as "ba lx lo on"
2. if both letters fall in the same row, replace each with letter
to right (wrapping back to start from end), eg. “ar"
encrypts as "RM"
3. if both letters fall in the same column, replace each with the
letter below it (again wrapping to top from bottom), eg. “mu"
encrypts to "CM"
4. otherwise each letter is replaced by the one in its row in the
column of the other letter of the pair, eg. “hs" encrypts to
"BP", and “ea" to "IM" or "JM" (as desired)

Polyalphabetic Ciphers
• another approach to improving security is to use multiple
cipher alphabets
• called polyalphabetic substitution ciphers
• makes cryptanalysis harder with more alphabets to guess
and flatter frequency distribution
• use a key to select which alphabet is used for each letter
of the message
• use each alphabet in turn
• repeat from start after end of key is reached

One-Time Pad
• if a truly random key as long as the message is used, the
cipher will be secure
• called a One-Time pad
• is unbreakable since ciphertext bears no statistical
relationship to the plaintext
• since for any plaintext & any ciphertext there exists a
key mapping one to other
• can only use the key once though
• have problem of safe distribution of key

Transposition Ciphers
• now consider classical transposition or
permutation ciphers
• these hide the message by rearranging the
letter order
• without altering the actual letters used
• can recognise these since have the same
frequency distribution as the original text

Steganography
• an alternative to encryption
• hides existence of message
– using only a subset of letters/words in a
longer message marked in some way
– using invisible ink
– hiding in LSB in graphic image or sound file
• has drawbacks
– high overhead to hide relatively few info bits

Block vs Stream Ciphers
• block ciphers process messages in into blocks,
each of which is then en/decrypted
• like a substitution on very big characters
– 64-bits or more
• stream ciphers process messages a bit or byte
at a time when en/decrypting
• many current ciphers are block ciphers
• hence are focus of course

Confusion and Diffusion
• cipher needs to completely obscure statistical
properties of original message
• a one-time pad does this
• more practically Shannon suggested combining
elements to obtain:
• diffusion – dissipates statistical structure of
plaintext over bulk of ciphertext
• confusion – makes relationship between
ciphertext and key as complex as possible

Differential Cryptanalysis
• one of the most significant recent (public)
advances in cryptanalysis
• known by NSA in 70's cf DES design
• Murphy, Biham & Shamir published 1990
• powerful method to analyse block ciphers
• used to analyse most current block ciphers with
varying degrees of success
• DES reasonably resistant to it, cf Lucifer

Linear Cryptanalysis
• another recent development
• also a statistical method
• must be iterated over rounds, with decreasing
probabilities
• developed by Matsui et al in early 90's
• based on finding linear approximations
• can attack DES with 247
known plaintexts, still in
practise infeasible

AES Evaluation Criteria
• initial criteria:
– security – effort to practically cryptanalyse
– cost – computational
– algorithm & implementation characteristics
• final criteria
– general security
– software & hardware implementation ease
– implementation attacks
– flexibility (in en/decrypt, keying, other factors)

The AES Cipher - Rijndael
• designed by Rijmen-Daemen in Belgium
• has 128/192/256 bit keys, 128 bit data
• an iterative rather than feistel cipher
– treats data in 4 groups of 4 bytes
– operates an entire block in every round
• designed to be:
– resistant against known attacks
– speed and code compactness on many CPUs
– design simplicity

AES Decryption
• AES decryption is not identical to encryption
since steps done in reverse
• but can define an equivalent inverse cipher with
steps as for encryption
– but using inverses of each step
– with a different key schedule
• works since result is unchanged when
– swap byte substitution & shift rows
– swap mix columns & add (tweaked) round key

Triple-DES with Two-Keys
• hence must use 3 encryptions
– would seem to need 3 distinct keys
• but can use 2 keys with E-D-E sequence
– C = EK1[DK2[EK1[P]]]
– nb encrypt & decrypt equivalent in security
– if K1=K2 then can work with single DES
• standardized in ANSI X9.17 & ISO8732
• no current known practical attacks

Triple-DES with Three-Keys
• although are no practical attacks on two-
key Triple-DES have some indications
• can use Triple-DES with Three-Keys to
avoid even these
– C = EK3[DK2[EK1[P]]]
• has been adopted by some Internet
applications, eg PGP, S/MIME

Confidentiality using Symmetric
Encryption
• have two major placement alternatives
• link encryption
– encryption occurs independently on every link
– implies must decrypt traffic between links
– requires many devices, but paired keys
• end-to-end encryption
– encryption occurs between original source and final
destination
– need devices at each end with shared keys

Key Distribution
• symmetric schemes require both parties
to share a common secret key
• issue is how to securely distribute this
key
• often secure system failure due to a
break in the key distribution scheme

Key Distribution
• given parties A and B have various key
distribution alternatives:
1. A can select key and physically deliver to B
2. third party can select & deliver key to A & B
3. if A & B have communicated previously can use
previous key to encrypt a new key
4. if A & B have secure communications with a
third party C, C can relay key between A & B

Key Distribution Issues
• hierarchies of KDC’s required for large
networks, but must trust each other
• session key lifetimes should be limited for
greater security
• use of automatic key distribution on behalf of
users, but must trust system
• use of decentralized key distribution
• controlling purposes keys are used for

Random Numbers
• many uses of random numbers in cryptography
– nonces in authentication protocols to prevent replay
– session keys
– public key generation
– keystream for a one-time pad
• in all cases its critical that these values be
– statistically random
• with uniform distribution, independent
– unpredictable cannot infer future sequence on previous values

Private-Key Cryptography
• traditional private/secret/single key
cryptography uses one key
• shared by both sender and receiver
• if this key is disclosed communications are
compromised
• also is symmetric, parties are equal
• hence does not protect sender from receiver
forging a message & claiming is sent by sender

Public-Key Cryptography
• probably most significant advance in the 3000
year history of cryptography
• uses two keys – a public & a private key
• asymmetric since parties are not equal
• uses clever application of number theoretic
concepts to function
• complements rather than replaces private key
crypto

Public-Key Cryptography
• public-key/two-key/asymmetric cryptography
involves the use of two keys:
– a public-key, which may be known by anybody, and can
be used to encrypt messages, and verify signatures
– a private-key, known only to the recipient, used to
decrypt messages, and sign (create) signatures
• is asymmetric because
– those who encrypt messages or verify signatures cannot
decrypt messages or create signatures

Public-Key Certificates
• certificates allow key exchange without real-
time access to public-key authority
• a certificate binds identity to public key
– usually with other info such as period of validity, rights
of use etc
• with all contents signed by a trusted Public-Key
or Certificate Authority (CA)
• can be verified by anyone who knows the public-
key authorities public-key

Message Authentication Code (MAC)
• generated by an algorithm that creates a small
fixed-sized block
– depending on both message and some key
– like encryption though need not be reversible
• appended to message as a signature
• receiver performs same computation on message
and checks it matches the MAC
• provides assurance that message is unaltered
and comes from sender

Hash Functions
• condenses arbitrary message to fixed size
• usually assume that the hash function is public
and not keyed
– cf. MAC which is keyed
• hash used to detect changes to message
• can use in various ways with message
• most often to create a digital signature

Keyed Hash Functions as MACs
• have desire to create a MAC using a hash function
rather than a block cipher
– because hash functions are generally faster
– not limited by export controls unlike block ciphers
• hash includes a key along with the message
• original proposal:
KeyedHash = Hash(Key|Message)
– some weaknesses were found with this
• eventually led to development of HMAC

Digital Signature Properties
• must depend on the message signed
• must use information unique to sender
– to prevent both forgery and denial
• must be relatively easy to produce
• must be relatively easy to recognize & verify
• be computationally infeasible to forge
– with new message for existing digital signature
– with fraudulent digital signature for given message
• be practical save digital signature in storage

Arbitrated Digital Signatures
• involves use of arbiter A
– validates any signed message
– then dated and sent to recipient
• requires suitable level of trust in arbiter
• can be implemented with either private or
public-key algorithms
• arbiter may or may not see message

Authentication Protocols
• used to convince parties of each others
identity and to exchange session keys
• may be one-way or mutual
• key issues are
– confidentiality – to protect session keys
– timeliness – to prevent replay attacks

Digital Signature Standard (DSS)
• US Govt approved signature scheme FIPS 186
• uses the SHA hash algorithm
• designed by NIST & NSA in early 90's
• DSS is the standard, DSA is the algorithm
• a variant on ElGamal and Schnorr schemes
• creates a 320 bit signature, but with 512-1024 bit security
• security depends on difficulty of computing discrete
logarithms

Web Security
• Web now widely used by business, government,
individuals
• but Internet & Web are vulnerable
• have a variety of threats
– integrity
– confidentiality
– denial of service
– authentication
• need added security mechanisms

What is a Firewall?
• a choke point of control and monitoring
• interconnects networks with differing trust
• imposes restrictions on network services
– only authorized traffic is allowed
• auditing and controlling access
– can implement alarms for abnormal behavior
• is itself immune to penetration
• provides perimeter defence

Firewalls – Packet Filters
• simplest of components
• foundation of any firewall system
• examine each IP packet (no context) and permit or
deny according to rules
• hence restrict access to services (ports)
• possible default policies
– that not expressly permitted is prohibited
– that not expressly prohibited is permitted

Trusted Computer Systems
• have considered some application specific
security mechanisms
– eg. S/MIME, PGP, Kerberos, SSL/HTTPS
• however there are security concerns that
cut across protocol layers
• would like security implemented by the
network for all applications

Summary
• information security is increasingly important
• have varying degrees of sensitivity of information
– cf military info classifications: confidential, secret etc
• subjects (people or programs) have varying rights of access
to objects (information)
• want to consider ways of increasing confidence in systems to
enforce these rights
• known as multilevel security
– subjects have maximum & current security level
– objects have a fixed security level classification

Cryptography and Network Security

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