New User Special Price Expires in

Let's log you in.

Sign in with Facebook


Don't have a StudySoup account? Create one here!


Create a StudySoup account

Be part of our community, it's free to join!

Sign up with Facebook


Create your account
By creating an account you agree to StudySoup's terms and conditions and privacy policy

Already have a StudySoup account? Login here

Lecture 3 - Symmetric Encryption and Message Confidentiality

by: Leslie Ogu

Lecture 3 - Symmetric Encryption and Message Confidentiality CSCI 4531

Marketplace > George Washington University > Computer science > CSCI 4531 > Lecture 3 Symmetric Encryption and Message Confidentiality
Leslie Ogu
GPA 3.01

Preview These Notes for FREE

Get a free preview of these Notes, just enter your email below.

Unlock Preview
Unlock Preview

Preview these materials now for free

Why put in your email? Get access to more of this material and other relevant free materials for your school

View Preview

About this Document

We finish chapter 2 about symmetric encryption and asymmetric encryption algorithms. There are diagrams available on lecture slides. Then we began an overview on random and pseudorandom numbers, cr...
Computer Security
Mohamed Tamer Abdelrahman Refaei
Class Notes
message, confidentiality, authentication, notes, Computer, Security, symmetric, asymmetric, algorithms, RSA, Digital, signatures, envelopes, random, Numbers, predictability, unpredicability, pseudorandom, Cryptography, transposition, Substitution, cipher
25 ?




Popular in Computer Security

Popular in Computer science

This 5 page Class Notes was uploaded by Leslie Ogu on Thursday September 15, 2016. The Class Notes belongs to CSCI 4531 at George Washington University taught by Mohamed Tamer Abdelrahman Refaei in Fall 2016. Since its upload, it has received 7 views. For similar materials see Computer Security in Computer science at George Washington University.

Similar to CSCI 4531 at GWU

Popular in Computer science


Reviews for Lecture 3 - Symmetric Encryption and Message Confidentiality


Report this Material


What is Karma?


Karma is the currency of StudySoup.

You can buy or earn more Karma at anytime and redeem it for class notes, study guides, flashcards, and more!

Date Created: 09/15/16
Leslie Ogu CSCI 4531  09/13/2016 ­ ​Chapter 20: Symmetric Encryption and Message Confidentiality    Finishing Chapter 2 (from last class)    Asymmetric Encryption Algorithms  + RSA (Rivest, Shamir, Adleman)  + Developed in 1977  + Most widely accepted and implemented approach to public­key encryption  + Block cipher in which the plaintext and ciphertext are integers between 0  and n­1 for some n  + Diffie­Hellman Key Exchange Algorithm  + Enables two users to secretly reach agreement about a shared secret that  can be used as a secret key for subsequent symmetric encryption of  messages  + Limited to the exchange of keys  + Digital Signature Standard (DSS)  + Provides only a digital signature function with SHA­1  + Cannot be used for encryption or key exchange  + Elliptic Curve Cryptography (ECC)  + Security like RSA, but with much smaller keys    Digital Signatures (diagram in slides)  ● Used for authenticating both source and data integrity  ● Created by encrypting hash code with private key  ● Does not provide confidentiality  ○ Even in the case of complete encryption  ○ Message is safe from alteration but not eavesdropping    Digital Envelopes (diagram in slides)  ● Protects a message without needing to first arrange for sender and receiver to  have the same secret key  ● Equates to the same thing as a sealed envelope containing an unsigned letter    Random Numbers  ● Uses include generation of:  ○ Keys for public key algorithms  ○ Stream key for symmetric stream cipher  ○ Symmetric key for use as a temporary session key or in creating a digital  envelope  ○ Handshaking to prevent replay attacks  ○ Session key  ● Requirements:  ○ Randomness  ■ Criteria:  ● Uniform distribution  ○ Frequency of occurrences of each of the numbers  should be approximately the same  ● Independence  ○ No one value in the sequences can be inferred from  the others  ○ Unpredictability  ■ Criteria:  ● Each number is statistically independent of other numbers in  sequence  ● Opponent should not be able to predict future elements of  the sequence on the basis of earlier elements    Random v. Pseudorandom  ● Cryptographic applications typically make use of algorithmic techniques for  random number generation  ○ Algorithms are deterministic, and therefore produce sequences of  numbers that are not statistically random  ● Pseudorandom numbers are:  ○ Sequences produced that satisfy statistical randomness tests  ○ Likely to be predictable  ● True random number generator (TRNG):  ○ Uses a nondeterministic source to produce randomness  ○ Most operate by measuring unpredictable, natural processes  ■ E.g., radiation, gas discharge, leaky capacitors  ○ Increasingly provided on modern processors    Practical Application: Encryption of Stored Data (visual on slides)    Chapter 20: Symmetric Encryption and Message Confidentiality    Symmetric Encryption  ● Also referred to as:  ○ Conventional encryption  ○ Secret­key or single­key encryption  ● Only alternative before public­key encryption in 1970’s  ○ Still most widely used alternative  ● Has 5 ingredients:  ○ Plaintext  ○ Encryption algorithm  ○ Secret key  ○ Ciphertext  ○ Decryption algorithm    Cryptography  ● Classified along three independent dimensions:  ○ The type of operations used for transforming plaintext to ciphertext  ■ Substitution:​ each element in the plaintext is mapped into another  element  ■ Transposition:​ elements in plaintext are rearranged  ○ The number of keys used  ■ Sender and receiver use some key ­ symmetric  ■ Sender and receiver each use a different key ­ asymmetric  ○ The way in which the plaintext is processed  ■ Block Cipher:​ processes one input block of elements at a time  ■ Stream Cipher:​ processes the input elements continuously    Transposition Cipher Example (on slide)    Substitution Ciphers  ● Change characters in plaintext to produce ciphertext  ● Example (Caesar Cipher)  ○ Use a left shift of k to protect messages  ○ Plaintext is HELLO WORLD  ○ k=3; Change each letter to the third letter following it (X goes to A), Y to B,  Z to C  ○ Ciphertext is KHOOR ZRUOG  ● How to break it? ​Brute Force  ○ Ciphertext: phhw ph diwhu wkh wrjd sduwb  ○ This is only possible because:  ■ The encryption / decryption algorithm is known  ■ The small key space  ■ The plaintext language is known  ● How to break it? ​Statistical Attack  ○ Susceptible to statistical attacks  ○ Statistical correlation function, OR  ■ Find letter that has the highest frequency  ■ Assume “e”  ■ Find the distance from “e”  ■ Decipher the rest of the message using distance as a key  ● Possible direction for improvement  ○ Make key longer  ■ Key space??  ■ Does that solve the statistically exposed language statistics  problem?  ○ Allow for arbitrary substitution  ■ Key space??  ■ Does that solve the statistically exposed language statistics  problem?  ○ Multiple letters in a key  ■ A cipher is polyalphabetic if the key has several different letters  ■ A cipher is monoalphabetic if the key has one letter    Vigenère Cipher  ● Pronounced “vedj­ih­nair”  ● Like Caesar cipher, but use a string for key  ● Example:  ○ Message THE BOY HAS A BALL  ○ Key: VIG (21,8,6)  ○ Encipher using Caesar cipher for each letter:  ■ Key: VIGVIGVIGVIGVIGV  ■ Plain: THEBOYHASTHEBALL  ■ Cipher: OPKWWECIYOPKWIRG  ○ Target Cipher (visual in slide)  ○ Attack the Cipher by Recognizing repetitions  ■ Notice cipher  ● T H E B O Y H A S T H E B A L L  ● O P K W  ​ W E C I Y ​O P K W​ I RG  ● V I G V​ I G V I G ​  I G V​ I G V ⇐corresponding key  ■ (Visual representations in slide)  ■ Since the distance from the beginning of the key to the beginning of  its repetition is 9, the key has to be a factor of that distance (1, 3, or  9)    One­Time Pad  ● A Vigenère cipher with:  ○ A random key at least as long as the message  ○ Encrypts/decrypts a single message  ● Ciphertext is random and bears no statistical relationship to plaintext  ● ** Provably unbreakable ** (Examples in slides)  ● In practice:  ○ Making large quantities of truly random keys  ○ Key distribution and protection  ■ For every message, an equally long key needs to be sent to the  receiver  ○ Hence, mechanism is of limited utility    Rotor Machines (diagram on slides) 


Buy Material

Are you sure you want to buy this material for

25 Karma

Buy Material

BOOM! Enjoy Your Free Notes!

We've added these Notes to your profile, click here to view them now.


You're already Subscribed!

Looks like you've already subscribed to StudySoup, you won't need to purchase another subscription to get this material. To access this material simply click 'View Full Document'

Why people love StudySoup

Bentley McCaw University of Florida

"I was shooting for a perfect 4.0 GPA this semester. Having StudySoup as a study aid was critical to helping me achieve my goal...and I nailed it!"

Jennifer McGill UCSF Med School

"Selling my MCAT study guides and notes has been a great source of side revenue while I'm in school. Some months I'm making over $500! Plus, it makes me happy knowing that I'm helping future med students with their MCAT."

Steve Martinelli UC Los Angeles

"There's no way I would have passed my Organic Chemistry class this semester without the notes and study guides I got from StudySoup."


"Their 'Elite Notetakers' are making over $1,200/month in sales by creating high quality content that helps their classmates in a time of need."

Become an Elite Notetaker and start selling your notes online!

Refund Policy


All subscriptions to StudySoup are paid in full at the time of subscribing. To change your credit card information or to cancel your subscription, go to "Edit Settings". All credit card information will be available there. If you should decide to cancel your subscription, it will continue to be valid until the next payment period, as all payments for the current period were made in advance. For special circumstances, please email


StudySoup has more than 1 million course-specific study resources to help students study smarter. If you’re having trouble finding what you’re looking for, our customer support team can help you find what you need! Feel free to contact them here:

Recurring Subscriptions: If you have canceled your recurring subscription on the day of renewal and have not downloaded any documents, you may request a refund by submitting an email to

Satisfaction Guarantee: If you’re not satisfied with your subscription, you can contact us for further help. Contact must be made within 3 business days of your subscription purchase and your refund request will be subject for review.

Please Note: Refunds can never be provided more than 30 days after the initial purchase date regardless of your activity on the site.