Cryptography Computer Security - Lecture Slides | CS 1060, Study notes of Computer Science

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CS 1060

Explorations in Computer Science

November 26, 2008 — LECTURE 30

Cryptography

Computer security

Information Security

● The techniques and polices used to ensure proper access to data. ● (^) Examples of data that must be protected and how? ● (^) Any good solution for information security must address ● (^) Confidentiality —ensuring that data is protected from unauthorized access. ● (^) Integrity —ensuring that data can only be modified by appropriate mechanisms. ● (^) Availability —the degree to which authorized users can access information for legitimate purposes.

Substitution Ciphers

● (^) Substitute one character in the plaintext message with another character. ● (^) To decode, perform the opposite substitution. ● Example : The Caesar cipher shifts the characters of a plaintext message by X positions in the alphabet. ● (^) The key consists of the value of X and the direction to shift. ● (^) Plaintext “BRUTUS WILL BETRAY YOU” ● (^) Ciphertext “ZPSRSQ UGEE ZCRYW WMS” ● (^) What is the key to the cipher used in this encryption?

Transposition Ciphers

● Rearrange the order of existing characters in the plaintext message in a certain way. ● Example : The route cipher lays out the message as a grid of characters and specifies a route through the grid. ● (^) The key consists of the dimensions of the grid and the route. ● (^) Plaintext “BRUTUS WILL BETRAY YOU” ● (^) Ciphertext “BSBYRWEYUITOTLRUULA” ● (^) What is the key to the cipher used in this encryption?

Book Cipher

● Traditionally, replace words in the plaintext of a message with the location of words from a book. ● (^) What if a word does not appear in the book? ● (^) Another approach substitutes the first letter of a word in the book with that word's position. ● This Beale ciphertext used a particular edition of the US Declaration of Independence. ● (^) The 115th word in the Declaration of Independence is "instituted”. Use the first letter of that word, "I".

More on Cryptography

● (^) Cryptanalysis is the attempt to figure out the plaintext message without the cipher or its key. ● (^) A challenge for substitution or transposition ciphers? ● (^) How can a computer be used for code breaking? ● Notice that the sender and receiver of an encrypted message must share the cipher key. ● (^) The key must be kept secret otherwise. ● (^) Isn't the key at as much risk of interception as the messages? ● (^) Should the key be encrypted?

Computer Security

● Controlling access to computers involves forcing users to identify themselves using authentication credentials. ● (^) based on something the user knows (e.g., password) ● (^) based on something the user has (e.g., smart card) ● (^) based on biometrics (e.g., fingerprint) ● Unfortunately, keeping unauthorized users out of a computer system is not enough. ● (^) In what other ways might security be compromised?

Malicious Code

● (^) Virus —a malicious program that embeds a copy of itself in another program. ● (^) Typically does damage by corrupting or deleting files on a particular computer. ● Worm —a stand-alone, self-replicating malicious program. ● (^) Typically does damage on networks by consuming bandwidth. ● (^) Trojan horse —a program that appears to be helpful but actually causes some kind of problem when executed. ● (^) Stand-alone, but does damage to computer on which it executes.