Wireless & Network Security 1Kemal Akkaya

Department of Computer ScienceSouthern Illinois University Carbondale

CS 591 – Wireless & Network Security

Lecture 5: Security Basics-1

Dr. Kemal AkkayaE-mail: kemal@cs.siu.edu

Slides are courtesy of Kenny Fong

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CryptographyCryptography is the study of mathematical techniques in the provision of information security services. It is the strongest and most widely used tool for defending against many kinds of security threats.Goals of cryptography

Confidentiality: keeping information secret from all but those who are authorized to see itIntegrity: ensuring information has not been altered by unauthorized or unknown meansAuthentication: corroborating the source of information or the identity of an entityNon-repudiation: preventing the denial of previous commitments or actions

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Symmetric-Key Encryption

A symmetric-key encryption scheme, or a cipher, consists of:

a secret key K shared by the sender and the receiver,an encryption algorithm EK that, with a message M(called the plaintext) and the secret key K as input, produces the encrypted message C = EK(M) as output (called the ciphertext), anda decryption algorithm DK that, with a ciphertext C and the secret key K as input, outputs the original message M = DK(C); i.e., given any key K,

DK(EK(M)) = Mfor all messages M.

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Symmetric-Key Encryption

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CryptanalysisCryptanalysis is the science of recovering the plaintext or deducing the key without access to the key.A brute-force attack, also called exhaustive key search, attempts to decrypt a given ciphertext message with every possible key until the resulting plaintext is meaningful (identified by means of frequency analysis).In general, a brute-force attack on a cipher with an n-bit key requires at most 2n trials. Therefore, the longer the key length, the more secure the cipher.

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Caesar CipherThe Caesar cipher (also called the shift cipher) replaces each letter of a message with another letter a fixed number of places after it in the alphabet (the alphabet is wrapped around).

Mathematical descriptionAssign a numerical equivalent to each letter: A – 0, B – 1, C – 2, …, Y – 24, Z – 25Encrypt: for each plaintext letter Mi, the ciphertext letter is

Ci = EK(Mi) = (Mi + K) mod 26where the secret key K is the shift amount (which is a number between 1 and 25)Decrypt: Mi = DK(Ci) = (Ci – K) mod 26

Example: M = KENNYFONG, C = NHQQBIRQJ, K = 3 Cryptanalysis: brute-force attack (there are only 25 possible keys)

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Vigenère CipherThe Vigenère cipher, invented by Giovan Batista Belaso in 1553 (but misattributed to Blaise de Vigenère in the 19th century), works as follows:

Let K = K0 K1 … Kd-1, where d is the number of letters in K. The key K is usually a word with 5 –8 letters.Encrypt: Ci = (Mi + Ki mod d) mod 26Decrypt: Mi = (Ci – Ki mod d) mod 26

Example: K = FALSEPlaintext KENNYISTHEGREATESTMANINTHEWORLDKey FALSEFALSEFALSEFALSEFALSEFALSEF

Ciphertext PEYFCNSEZILRPSXJSEEESIYLLJWZJPI

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Block CiphersA block cipher encrypts the plaintext message one block at a time (every block has the same fixed size).Most contemporary ciphers are block ciphers:

Data Encryption Standard (DES)Key length: 56 bits, block length: 64 bits

Advanced Encryption Standard (AES)Key length: 128/192/256 bits, block length: 128 bits

Triple DESInternational Data Encryption Algorithm (IDEA)BlowfishRon’s Code 5 (RC5)

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Block CiphersBlock ciphers generally apply the techniques of substitution and permutation to complicate the statistical relationship between the ciphertext and the plaintext, thereby thwarting cryptanalysis based on statistical analysis.A mode of operation specifies how a block cipher with a fixed block size (e.g., 128 bits for AES) can be extended to process messages of arbitrary length. Most of the modes of operation partition a message P in the most straightforward way: P = P1 P2 … PN, where Pi is an m-bit block for all 1 ≤ i ≤ N and m is the block size of the block cipher used, padding the last block PN if necessary.5 modes:

ECB, CBC, CFB, OFB, CTR

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Electronic Codebook Mode (ECB)Encrypt: The ciphertext to be sent is <C1, C2, …, CN>, where Ci = EK(Pi) for all 1 ≤ i ≤ N.

Decrypt: Pi = DK(Ci) for all 1 ≤ i ≤ N.

Though ECB is simple, it suffers the problem that if the same plaintext block appears more than once in the message, ECB always produces the same ciphertext block, allowing an adversary to gain some knowledge by observing repetitions in lengthy messages.

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Cipher Block Chaining Mode (CBC)Precomputation: an m-bit initialization vector IV is randomly selected, where m is the block size.Encrypt: The ciphertext to be sent is <IV, C1, C2, …, CN>, where C0= IV and Ci = EK(Ci-1 ⊕ Pi) for all 1 ≤ i ≤ N.

Decrypt: Pi = Ci-1 ⊕ DK(Ci) for all 1 ≤ i ≤ N, where C0 = IV.

The same plaintext block may not be encrypted to the same ciphertext block in CBC, because the ith ciphertext block depends on the ith plaintext block and all the previous plaintext blocks. However, unlike ECB, CBC encryption is not parallelizable (but CBC decryption is parallelizable).

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Cipher Feedback (CFB)

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Counter (CTR)

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Data Encryption Standard (DES)

Adopted in 1977 by NISTBlock cipher with 64 bits of blocks56 bits key length16 Rounds

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Triple DES (3DES)

first used in financial applicationsin DES FIPS PUB 46-3 standard of 1999uses three keys & three DES executions:

C = E(K3, D(K2, E(K1, P)))decryption same with keys reverseduse of decryption in second stage gives compatibility with original DES userseffective 168-bit key length, slow, secureAES will eventually replace 3DES

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Advanced Encryption Standard

(AES)

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AES Round Structure

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Stream CiphersProcesses input elements continuously

Bit by bit (or byte)

Key input to a pseudorandom bit generatorProduces stream of random like numbersUnpredictable without knowing input key

Are faster and use far less codeDesign considerations:

Encryption sequence should have a large periodKeystream approximates random number propertiesUses a sufficiently long key (i.e., 128 bits)

RC4 is the most commonOthers

A5/1, A5/2, FISH, Helix, ISAAC, MUGI, Panama, Phelix, Pike

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Diffie-Hellman Key Exchange

First practical scheme to exchange a secret key

Used in symmetric ciphersProposed by Diffie & Hellman in 1976

note: now know that Williamson (UK CESG) secretly proposed the concept in 1970

Practical method to exchange a secret keyUsed in a number of commercial productsSecurity relies on difficulty of computing discrete logarithms

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Diffie-Hellman

Algorithm

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Diffie-Hellman Example

Haveprime number q = 353 primitive root α = 3

A and B each compute their public keysA computes YA = 397 mod 353 = 40B computes YB = 3233 mod 353 = 248

Then exchange and compute secret key:for A: K = (YB)XA mod 353 = 24897 mod 353 = 160for B: K = (YA)XB mod 353 = 40233 mod 353 = 160

Attacker must solve:3a mod 353 = 40 which is hardDesired answer is 97, then compute key as B does

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Key Exchange Protocols