Lecture 4 - Chapter 20 Completed
Lecture 4 - Chapter 20 Completed CSCI 4531
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This 3 page Class Notes was uploaded by Leslie Ogu on Friday September 23, 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 5 views. For similar materials see Computer Security in Computer science at George Washington University.
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Date Created: 09/23/16
Leslie Ogu CSCI 4531 09/20/2016 C hapter 20 Continued Type of Attack Known to Cryptanalyst Ciphertext only ● Encryption algorithm ● Ciphertext to be decoded Known plaintext ● Encryption algorithm ● Ciphertext to be decoded ● One or more plaintextciphertext pairs formed with the secret key Chosen plaintext ● Encryption algorithm ● Ciphertext to be decoded ● Plaintext message chosen by cryptanalyst, together with its corresponding ciphertext generated with the secret key Chosen ciphertext ● Encryption algorithm ● Ciphertext to be decoded ● Purported ciphertext chosen by cryptanalyst, together with its corresponding decrypted plaintext generated with the secret key Chosen text ● Encryption algorithm ● Ciphertext to be decoded ● Plaintext message chosen by cryptanalyst, together with its corresponding decrypted plaintext generated with the secret key Computationally Secure Encryption Schemes + Encryption is computationally secure if: + Cost of breaking cipher exceeds value of information + Time required to break cipher exceeds the useful lifetime of the information + Usually very difficult to estimate the amount of effort required to break + Can estimate time/cost of a bruteforce attack Diagram on the slides Block Cipher Structure + Symmetric block cipher consists of: + A sequence of rounds + With substitutions and permutations by key + Parameters and design features: Block Size ⇒ Key Size ⇒ Number of Rounds ⇒ Subkey Generation Algorithm ⇒ Round Function ⇒ Fast Software Encryption / Decryption ⇒ Ease of Analysis Data Encryption Standard (DES) + Most widely used encryption scheme + Adopted in 1977 by National Bureau of Standards (now NIST) + FIPS PUB 46 + Algorithm is referred to as the Data Encryption Algorithm (DEA) + Minor variation of the Feistel network DES and AES Diagrams on Slides AES + U.S. FIPS PUB 197 + Block Cipher + Three different sizes: 128, 192, 256 + 10 rounds + SBoxes (Substitution Boxes) Mix Columns and Add Key + Mix columns + Operates on each columns individually + Mapping each byte to a new value that is a function of all four bytes in the column + Use of equations over finite fields + To provide good mixing of bytes + Add round key + Simply XOR state with bits of expanded keys + Security from complexity of round key expansion and other stages of AES Stream Ciphers Processes input elements continuously ⇒ Key input to a pseudorandom bit generator ● Produces stream of random like numbers ● Unpredictable without knowing input key ● XOR keystream output with plaintext bytes Electronic Codebook (ECB) ● Simplest mode ● Plaintext is handled b bits at a time and each block is encrypted using the same key ● “Codebook” because has unique ciphertext value for each plaintext block ○ Not secure for long messages since repeated plaintext is seen in repeated ciphertext ● To overcome security deficiencies, you need a technique where the same plaintext block, if repeated, produces different ciphertext blocks Diagrams available in slides Key Distribution ● The means of delivering a key to two parties that wish to exchange data without allowing others to see the key ● Two parties (A & B) can achieve this by: 1. A key could be selected by A and physically delivered to B 2. A third party could select the key and physically deliver it to A and B 3. If A and B have previously and recently used a key, one party could transmit the new key to the other, encrypted using the old key 4. If A and B each have an encrypted connection to a third party C, C could deliver a key on the encrypted links to A and B
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