ECE454 /CS594 Computer and Network Security

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ECE454/CS594 Computer and Network Security

Dr. Jinyuan (Stella) SunDept. of Electrical Engineering and Computer ScienceUniversity of Tennessee Fall 2011

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Introduction to Cryptography• What is cryptography?• Types of cryptography• Attacks on cryptosystem• SKC, PKC, Hash• Security notions• Early ciphers

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What Is Cryptography?• Cryptography is the art of secret writing• Traditional use: encryption• Cryptographic systems: algorithm+secret• Cryptology: cryptography+cryptanalysis

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Types of Cryptography• Operations used to transform plaintext to ciphertext: - Substitution: each element in plaintext is mapped into another element - Transposition: elements in plaintext are rearranged• Number of keys used: - Single-key: secret-key cryptography - Two-key: public-key cryptography - Zero-key: hash functions • The way plaintext is processed: - Block cipher: input & output in blocks - Stream cipher: input & output in bits

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Conventional Cryptography: Symmetric Encryption

• Plaintext: original message• Encryption algorithm:

substitutions/transpositions• Secret key: independent of plaintext and

algorithm• Ciphertext: depends on plaintext and key,

appears random• Decryption algorithm: reverse of encryption

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Attacking Cryptosystems• Cryptanalysis: attempts to deduce the plaintext and/or the key being used, with knowledge of the nature of the algorithm + general characteristics of plaintext + some sample plaintext-ciphertext pairs

• Brute-force attack: attacker tries every possible key on a piece of ciphertext until an intelligible translation into plaintext is obtained. On average, half of all possible keys must be tried to achieve success.

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Types of Cryptanalytic Attacks

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Brute-force Attacks

Average Time Required for Exhaustive Key Search

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Secret Key Cryptography (SKC)Aka: conventional cryptography, symmetric

cryptographyUse a single keyCiphertext about the same length as plaintextExamples: Captain Midnight code,

monoalphabetic cipher, DES, AES, RC4

Alice

plaintext

ciphertext

encryption

ciphertext

plaintextdecryptio

n

shared secret key Bob

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SKC ApplicationsTransmitting over an insecure channelSecure storage on insecure mediaAuthentication:

Integrity check: message authentication code (MAC), aka, message integrity check (MIC)

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Public Key Cryptography (PKC)Aka: asymmetric cryptography, invented in

1970sUse two keys: a public key known to

everyone, a private key kept secret to the owner

Encryption/decryption: encryption can be done by everyone using the recipient’s public key, decryption can be done only by the recipient with his/her private key

Alice

plaintext

ciphertext

encryption

ciphertext

plaintextdecryption

( , )A Apub pri ( , )B Bpub priBpub

Bpri

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PKC ApplicationsEverything that SKC does can be done

by PKCTransmitting over an insecure channelSecure storage over insecure mediaAuthentication

Key exchange: establish a shared session key with PKC

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PKC Applications (Cont’d)Digital signature: non-repudiation

Alice

plaintext

Signed message

sign

Signed message

True or falseverify

( , )A Apub pri ( , )B Bpub priApubApri

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Hash FunctionsAka: message digests, one-way transformationsTake a message m of arbitrary length

(transformed into a string of bits) and computes from it a fixed-length (short) number h(m)

Properties:- easy-to-compute: for any message m, it is relatively easy to compute h(m)- non-reversible: given h(m), there is no way to find an m that hashes to h(m) except trying all possibilities of m- computationally infeasible to find m and m’ such that h(m)=h(m’) and m!=m’

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Applications of Hash FunctionsPassword hashingMessage integrity: keyed hash

File fingerprintDownline load securityDigital signature efficiency

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Security NotionsUnconditionally secure: ciphertext generated by the

scheme does not contain enough information to determine uniquely the corresponding plaintext, no matter how much ciphertext is available. Perfectly secure, unlimited power of adversary

Provably secure: under the assumption of well-known hard mathematical problem, e.g., factoring large numbers, discrete logarithm problem

Computationally secure: if cost of breaking the cipher exceeds the value of the encrypted information, or time required to break the cipher exceeds the useful lifetime of the information, practical security

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Early Ciphers: Substitution• Substitution: letters of plaintext are replaced by other letters or by numbers or symbols, involves replacing plaintext bit patterns with ciphertext bit patterns • Caesar cipher• Captain Midnight Secret Decoder Rings• Monoalphabetic cipher• Hill cipher• Polyalphabetic cipher (Vigenere)• One-time pad

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Caesar Cipher• Replacing each letter of the alphabet with the letter three places further down the alphabet, e.g.,

• Captain Midnight Secret Decoder Ring (slightly enhanced): If a numerical value is assigned to each letter, for each plaintext letter p, the ciphertext letter C=E(k, p)=(p+k) mod 26 where k takes on the value in [1,25]:

• Subject to brute-force attack: simply try all 25 possible k

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Monoalphabetic Cipher• The "cipher" line can be any permutation of the 26 alphabetic characters, 26! or around 4x1026 possible keys:

• Is it secure enough to resist cryptanalysis? Consider:

• By exploiting the regularities of the language and counting letter frequencies:

• The following can be recovered:

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Hill Cipher• Takes m successive plaintext letters and substitutes for them m ciphertext letters• The substitution is determined by m linear equations in which each character is assigned a numerical value (a = 0, b = 1 ... z = 25)• For example: m = 3

for plaintext “paymoremoney” and encryption key

we have ciphertext: LNSHDLEWMTRW

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Polyalphabetic Cipher• Use different monoalphabetic substitutions as one proceeds through the plaintext message• Vigenere cipher: best known polyalphabetic cipher. The set of related monoalphabetic substitution rules consists of the 26 Caesar ciphers, with shifts of 0 through 25.• For keyword “deceptive” and plaintext “we are discovered save yourself”, the encryption works as:

• Vulnerable to cryptanalysis: the key and plaintext share the same language regularities and frequency distribution of letters. Solutions?

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One-time Pad• Use a random key that is as long as the message so that the key need not be repeated• The key is used to encrypt and decrypt a single message, and then is discarded• Perfectly secure: unbreakable because it producesrandom output (from the random key) that bears no statistical relationship to the plaintext

• Drawbacks: large quantities of random keys needed, key distribution and protection (both sender and receiver)

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Early Ciphers: Transposition

• Different from substituting a ciphertext symbol for a plaintext symbol• Transposition cipher: performs some sort of permutation/rearrangement on plaintext letters• Cryptanalysis is straightforward: a transposition cipher has the same letter frequency as plaintext• Can be made more secure by performing more than one stage of transposition (result is not easily reconstructed)

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Rail Fence Cipher• Simplest transposition cipher• Plaintext is written down as a sequence of diagonals and then read off as a sequence of rows

• Trivial to attack

Plaintext:

Ciphertext:

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Double Transposition• A more complex scheme: permute the order of the columns with a key

• Double transposition: more than one permutation, number the above plaintext letters 1-28 and after the first permutationwe have

• After the 2nd permutation?

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Cryptography vs. Steganography• Cryptography conceals the context of message• Steganography conceals the existence of message, useful when the fact of secret communication should be concealed - an arrangement of words/letters of the overall message spells out the hidden message - character marking: selected letters overwritten in pencil, not visible unless the paper is held at an angle to bright light - invisible ink: substances used for writing but leave no visible trace until heat or some chemical is applied to the paper - pin punctures: small pin punctures on selected letters are not visible unless the paper is held up in front of a light

• Drawbacks of steganography: high overhead to hide a relatively few bits of information, becomes worthless once the system is discovered (can make insertion depend on key)

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Steganography

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Reading Assignments

[Kaufman] Chapter 2