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### SE571Security in Computing

Chap 2: Elementary Cryptography

Chap 2 Examines…

Concepts of encryption

Cryptanalysis: how encryption systems are “broken”

Symmetric (secret key) encryption and the DES and AES algorithms

Asymmetric (public key) encryption and the RSA algorithm

Key exchange protocols and certificates

Digital signatures

Cryptographic hash functions

SE571 Security in Computing Dr. Ogara

Common Terminologies

Cryptography - practice and study of hiding information/using encryption to conceal text

Cryptoanalysis - to find some weakness or insecurity in a cryptographic scheme

Cryptology - research into and study of encryption and decryption; it includes both cryptography and cryptanalysis

SE571 Security in Computing Dr. Ogara

Common Terminologies

Decryption – the method of turning cipher text back into plaintext

Encryption algorithm – set of rules or procedures that dictates how to encrypt and decrypt data, also called an encryption cipher

Encryption – method of transforming data (plaintext) into an unreadable format

Plaintext – the format(usually readable) of data before encrypted

SE571 Security in Computing Dr. Ogara

Common Terminologies

Ciphertext – the scrambled format of data after being encrypted

Key – a value used in the encryption process to encrypt and decrypt/ also called cryptovariable

SE571 Security in Computing Dr. Ogara

Encryption

SE571 Security in Computing Dr. Ogara

Symmetric Encryption

SE571 Security in Computing Dr. Ogara

Uses one key for both encryption and decryption

Receiver and sender share same key (private key) to lock and unlock

Also called private key encryption

Must securely distribute keys to other parties

Symmetric Encryption

SE571 Security in Computing Dr. Ogara

- Anyone with key can either encrypt or decrypt (similar to password)
- Very fast to encrypt or decrypt
- Provides authentication as long as key remains secret
- Problem
- How do A and B obtain their shared secret key?
- Key distribution is e.g. n users communicating in pairs need n*(n-1)/2 keys

Asymmetric Encryption

SE571 Security in Computing Dr. Ogara

Receiver and sender have two keys – public and private

Public key can be sent in an e-mail message or posted in a public directory

Public key used to encrypt and private key to decrypt or vise-versa

Requires a lot of resources

Asymmetric and Symmetric Encryption

SE571 Security in Computing Dr. Ogara

Encryption

SE571 Security in Computing Dr. Ogara

Cryptanalysis attempts to do six things…

break a single message

recognize patterns in encrypted messages, to be able to break subsequent ones by applying a straightforward decryption algorithm

infer some meaning without even breaking the encryption, such as noticing an unusual frequency of communication or determining something by whether the communication was short or long

SE571 Security in Computing Dr. Ogara

Cryptanalysis attempts to do six things…

deduce the key, to break subsequent messages easily

find weaknesses in the implementation or environment of use of encryption

find general weaknesses in an encryption algorithm, without necessarily having intercepted any messages

SE571 Security in Computing Dr. Ogara

Forms of Ciphers

- Confusion (substitution)
- One letter is exchanged for another
- Basis of many cryptographic algorithms used for diplomatic communication through the first half of the twentieth century
- Basis for some widely used commercial-grade encryption algorithms
- Examples:
- Ceasar cipher
- One-Time Pad
- The Vernam cipher

SE571 Security in Computing Dr. Ogara

Forms of Ciphers

- Diffusion (Transposition)
- Order of the letters is rearranged
- Basis for some widely used commercial-grade encryption algorithms
- Goal - widely spread the information from the message or the key across the ciphertext (diffusion)
- Also known as permutation (rearrangement of symbols of a message)

SE571 Security in Computing Dr. Ogara

Ceasar cipher

- Romans used a shift cipher called Ceasar cipher
- Shift ciphers simply shift characters in an alphabet
- Advantages
- Easy to memorize and implement
- Disadvantage
- Pattern is obvious

SE571 Security in Computing Dr. Ogara

One-Time Pads

- Large, non-repeating set of keys is written on sheets of paper, glued together into a pad
- Requires a prearranged chart called Vigenere table (contains 26 letters in each column in some scrambled order)
- Receiver needs a pad similar to the sender

SE571 Security in Computing Dr. Ogara

One-Time Pads

- Example:
- Message has 300 characters in length
- Keys are 20 characters long
- Sender needs 15 pages of keys
- Sender writes keys one at a time above the letters of plain text
- Sender encipher plain text with Vigenere chart
- Receiver uses appropriate number of keys to decipher message

SE571 Security in Computing Dr. Ogara

One-Time Pads

- Problems
- Requires absolute synchronization between sender and receiver
- Difficult to store and account for the keys

SE571 Security in Computing Dr. Ogara

The Vernam Cipher

Developed by Gilbert Vernam for AT&T

Is immune to most cryptanalytic attacks

Uses long non-repeating sequence of numbers that are combined with the plaintext

Used long punched paper tape that fed into a teletype machine

Tape contained random numbers that were combined with characters typed into the teletype

sequence of random numbers had no repeats, and each tape was used only once

SE571 Security in Computing Dr. Ogara

The Vernam Cipher

SE571 Security in Computing Dr. Ogara

The Vernam Cipher - Example

SE571 Security in Computing Dr. Ogara

Plain text - VERNAM CIPHER

Ciphertext - tahrspitxmab

Columnar Transposition

- Plaintext characters are rearranged into columns
- Example:
- Plain text - THIS IS A MESSAGE TO SHOW HOW A COLUMNAR TRANSPOSITION WORKS
- Ciphertext - tssohoaniwhaasolrstoimghwutpirseeoamrookistwcnasns

SE571 Security in Computing Dr. Ogara

Columnar Transposition

SE571 Security in Computing Dr. Ogara

Characteristics of Good Ciphers

The amount of secrecy needed should determine the amount of labor appropriate for the encryption and decryption

The set of keys and the enciphering algorithm should be free from complexity

SE571 Security in Computing Dr. Ogara

Characteristics of Good Ciphers

The implementation of the process should be as simple as possible

Errors in ciphering should not propagate and cause corruption of further information in the message

The size of the enciphered text should be no larger than the text of the original message

SE571 Security in Computing Dr. Ogara

Properties of Trustworthy Encryption Systems

It is based on sound mathematics

It has been analyzed by competent experts and found to be sound

It has stood the test of time

SE571 Security in Computing Dr. Ogara

Stream and Block Ciphers

Stream ciphers - encrypt one bit or character or symbol of plaintext into bit or symbol of Ciphertext at a time e.g. diffusion

Block ciphers encrypt a group of plaintext symbols as one block e.g. columnar transposition

Block ciphers can effectively act as a stream cipher

SE571 Security in Computing Dr. Ogara

Advantages of Stream Ciphers

SE571 Security in Computing Dr. Ogara

- Speed of transformation - the time to encrypt a symbol depends only on the encryption algorithm itself, not on the time it takes to receive more plaintext
- Low error propagation - error in the encryption process affects only a character

Disdvantages of Stream Ciphers

SE571 Security in Computing Dr. Ogara

- Low diffusion - Each symbol is separately enciphered. Therefore, all the information of that symbol is contained in one symbol of the ciphertext.
- Susceptibility to malicious insertions and modifications - Because each symbol is separately enciphered, an active interceptor who has broken the code can splice together pieces of previous messages and transmit a spurious new message that may look authentic.

Advantages of Block Ciphers

SE571 Security in Computing Dr. Ogara

- High diffusion - Information from the plaintext is diffused into several ciphertext symbols. One ciphertext block may depend on several plaintext letters
- Immunity to insertion of symbols - Because blocks of symbols are enciphered, it is impossible to insert a single symbol into one block. The length of the block would then be incorrect, and the decipherment would quickly reveal the insertion

Disdvantages of Block Ciphers

SE571 Security in Computing Dr. Ogara

- Slowness of encryption - The person or machine using a block cipher must wait until an entire block of plaintext symbols has been received before starting the encryption process
- Error propagation - An error will affect the transformation of all other characters in the same block

Three commonly used encryption schemes

DES – Data Encryption Standards

AES – Advanced Encryption Standards

RSA – Rives-Shamir-Adelman Encryption

SE571 Security in Computing Dr. Ogara

DES

Developed by U.S government for general public (adopted in 1976)

Based on data encryption algorithm developed by IBM

Combines two fundamental building blocks of encryption – substitution and transposition

Uses only standard arithmetic and logical operations on numbers up to 64 bits long

SE571 Security in Computing Dr. Ogara

Double and Tripple DES

Lack of trust with DES 56-bit key length

Development of double encryption for greater secrecy

Two keys perform two encryptions thus making it hard to unlock [C=E(k2, E(k1,m))]

Unfortunately the assumption is false

Three keys adds significant strength [C = E(k3, E(k2, E(k1,m)))]

SE571 Security in Computing Dr. Ogara

Double and Tripple DES

1997 researchers using over 3,500 machines in parallel were able to infer a DES key in four months’ work

1998 for approximately $100,000, researchers built a special “DES cracker” machine that could find a DES key in approximately four days

Hence need for better and stronger algorithm

SE571 Security in Computing Dr. Ogara

AES

Algorithm is called Rijndael – named after the two creators (Vincent Rijmen and Joan Daemen)

Adopted in 2001

Uses substitution; transposition; and the shift, exclusive OR, and addition operations

Keys based on 128, 192 and 256 bits

SE571 Security in Computing Dr. Ogara

AES

Does it have flaws?

How long will it remain sound?

Cryptanalysts have not found any flaws yet

SE571 Security in Computing Dr. Ogara

Rives-Shamir-Adelman Encryption (RAS)

Public key system introduced in 1978

Named after three inventors

Uses two keys for encryption and dceryption

SE571 Security in Computing Dr. Ogara

Four applications of encryption

Hash functions

Key exchange

Digital signatures

Certificates

SE571 Security in Computing Dr. Ogara

Hash Functions

- Important for integrity
- Put a shield or seal around a file by computing a cryptographic function called hash or checksum or message digest of a file
- Examples:
- MD4, MD5 (Message Digest) – produce 128 bit
- SHA/SHS (Secure Algorithm or Standards) – produce 160-bit digest

SE571 Security in Computing Dr. Ogara

Key exchange

Example: Web browser connecting to shopping website

Encrypted session must be established

S = sender of protected information

R = receiver of protected information

Establish assurance that information came from S

Public key cryptography can help here

SE571 Security in Computing Dr. Ogara

Key exchange

Use lockboxes and keys

S puts protected information into lockbox that can be opened by S public key

S puts lockbox into another one that can be opened by ONLY by R’s private key

R uses private key to open outer box and S public key to open inner box (proof it came from S)

SE571 Security in Computing Dr. Ogara

Diffie–Hellman key exchange protocol

Does not require preshared public keys

S and R uses simple arithmetic to exchange a secret

They agree on field number n and starting number g

Each thinks of a secret number, say, s and r.

S sends to R gs and R sends to S gr.

Then S computes (gr)s and R computes (gs)r, which are the same, so grs = gsr becomes their shared secret.

SE571 Security in Computing Dr. Ogara

Digital Signatures

Provide reliable means to ensure the origin of data

Cryptographic hash codes are used to support digital signatures

Cryptographic hash codes offer a fast, fairly reliable way of determining whether a piece of data has been modified between sender and receiver

SE571 Security in Computing Dr. Ogara

Digital Signatures

It must be unforgeable

It must be authentic

It is not alterable

It is not reusable

SE571 Security in Computing Dr. Ogara

Public Key Encryption

Ideally suited to digital signatures

If S wishes to send M to R, S uses the authenticity transformation to produce D(M, KS). S then sends D(M, KS) to R. R decodes the message with the public key transformation of S

SE571 Security in Computing Dr. Ogara

Cetificates

Binds a public key and users’ identity

Signed by Certificate of Authority (CA)

Example – Two people

Edward posts his public key in public but retains private key

Diana creates public key and includes it into message with her identity

Edward signs (affirms Diana’s public key and identity) by creating has value and then encrypting message and hash value with private key

SE571 Security in Computing Dr. Ogara

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