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Week 2 - Friday

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What did we talk about last time? Substitution ciphers Vigenère ciphers One-time pad

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Michael Franzese

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Claude Shannon is the guy that invented Shannon secrecy and is considered the father of information theory He proposed 5 characteristics for a good cipher: 1. The amount of secrecy needed should determine the amount of labor appropriate for encryption and decryption 2. The set of keys and the enciphering algorithm should be free from complexity 3. The implementation of the process should be as simple as possible 4. Errors in ciphering should not propagate and cause corruption of further information in the message 5. The size of the enciphered text should be no larger than the text of the original message

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Shannon was focused on hand encryption Modern commercial users of cryptography want the following characteristics for their cryptosystems: Based on sound mathematics Analyzed by competent experts and found to be sound Stood the test of time

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A common way of dividing ciphers is into stream ciphers and block ciphers Block ciphers divide messages into fixed length parts (or blocks) and encipher each part with the same key Stream ciphers encipher each message character by character Some other authors define a stream cipher to be like a block cipher except that the key changes with each block based on the message

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Self-synchronous ciphers are stream ciphers that get the key from the message itself The simplest such cipher is an autokey cipher that uses the message itself for the key Essentially, this is similar to the Vigenère cipher with the key coming from the message Example: Message: THISISTHEREMIX Key: QTHISISTHEREMI Alternatively, the key can be drawn from the ciphertext Message: THEBOYHASTHECAT Key: XQXBCQOVVNGNRTT Ciphertext: QXBCQOVVNGNRTTM

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Confusion is the property of a cryptosystem that changing a single character in the plaintext should not have a predictable effect Diffusion is the property of a cryptosystem that each character in the plaintext should impact many characters in the ciphertext Examples: Caesar cipher has poor confusion and no diffusion One time pad has good confusion but no diffusion Auto-key ciphers may have poor confusion but good diffusion AES and DES have good confusion and diffusion within a block

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We measure a cryptosystem based on its resistance to an adversary or attacker Kinds of attacks: Ciphertext only Full or partial plaintext Chosen plaintext Chosen ciphertext Ciphertext and plaintext pairs

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Attacker only has access to an encrypted message, with a goal of decrypting it This is the assumption we have made so far when cryptanalyzing the classical ciphers The world is filled with ciphertext data This model gives the attacker very little to work with

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Attacker has access to a plaintext and its matching ciphertext, with a goal of discovering the key It is possible that the full or partial plaintext is available because it is an encrypted broadcast of public (or soon to be public) information Perhaps a secret transmission informed everyone of a new policy Then, the policy is made public Some messages are very common “Nothing to report.” If these messages are predictable, the ciphertext could be intercepted and the plaintext guessed

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Attacker may ask to encrypt any plaintext, with a goal of discovering the key This model seems unusual, but it comes up in practice Military forces seize a transmission room and start transmitting messages Perhaps they don’t have enough knowledge to learn the encryption settings, but the known messages could be analyzed later All public key cryptosystems allow this kind of attack, since anyone can generate encrypted messages

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It is unusual that an attacker can pick a ciphertext and ask for it to be decrypted Why not just ask for any particular ciphertext that you’re interested in? If you have access to code that can encrypt huge amounts of plaintext quickly, it is possible to attempt a brute force encryption that will approximate choosing the ciphertext

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As an extension of known plaintext, it may be the case that you have many ciphertext/plaintext pairs that are encrypted with the same key

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Humans allow some of the scenarios described above through error Operators transmit the same message with two different keys Operators transmit some information in the clear Operators transmit a repeat of a message but make small mistakes the second time As usual, humans are a problem

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A block cipher is a symmetric key cipher that works on a block of data of a given size For compatibility with hardware, block sizes are often powers of two: 64 bits, 128 bits, 256 bits, etc. Block ciphers are a fundamental part of many modern cryptosystems To encrypt a message longer than a single block: First break the message into blocks Then, each block could be encrypted individually Or data from the first block can be used in the encryption of the second, and so on

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Data Encryption Standard DES is a typical block cipher It was chosen as the government's standard for encryption in 1976 (but has since been deprecated) DES works on blocks 64 bits in size DES uses a 56 bit key NSA helped design it… amidst some controversy

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In the 1970’s, the National Bureau of Standards (NBS) saw the need for a publicly available encryption standard They called for proposals that met the following criteria: High level of security Easy to understand Publishable (no security through obscurity) Available to everyone Adaptable for many applications Economical to implement in hardware Efficient to use Able to be validated Exportable A cryptosystem called Lucifer developed by IBM was adapted into the resulting DES NBS was reorganized into the National Institute of Standards and Technology in 1988

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After WWII (the birth of modern cryptography), many governments saw the immense value of crypto Countries like the US with good crypto didn’t want their enemies to have it Strong encryption was listed as an Auxiliary Weapons Technology on the US Munitions List 40 bit or weaker encryption could be exported 2 40 possibilities can be brute forced in days (or hours) In 1996, Bill Clinton signed an executive order that moved commercial encryption from the Munitions List to the Commerce Control List It is still technically possible to be arrested for exporting software that can perform strong encryption and decryption But it is no longer illegal arms trafficking Although DES is longer than 40 bits, its 56 bits seem to be in the range that never really posed a problem for the feds

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DES has 16 rounds The book calls them cycles In each round, the input is broken into 2 halves, manipulated, and combined with part of the key Input Permutation Left 0 Right 0 f f Key 1 Left 1 Right 1 + + Left 0 Right 0

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More on DES Maybe start on AES Yuki Gage presents

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Read Section 2.5 Start working on Assignment 1 Due next Friday by midnight

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