Lecture 3 - Symmetric Encryption and Message Confidentiality
Lecture 3 - Symmetric Encryption and Message Confidentiality CSCI 4531
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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.
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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)