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Symmetric Encryption and MessageConfidentiality

Comp Sci 3600 Security

Part 1:ModernSymmetricEncryption

SymmetricEncryptionPrinciples

Cryptography

Crpytanalysis

Feistel CipherStructure

DataEncryptionStandard

Triple DES

AdvancedEncryptionStandard

Overview

AlgorithmDetails

Outline

1 Symmetric Encryption PrinciplesCryptographyCrpytanalysisFeistel Cipher Structure

2 Data Encryption StandardTriple DES

3 Advanced Encryption StandardOverviewAlgorithm Details



Cryptography

Crpytanalysis



Triple DES


Overview

AlgorithmDetails

Symmetric Encryption

• Also referred to as: Conventional, Secret-key, or single-keyencryption

• Only alternative before public-key encryption in 1970’s

• Still most widely used alternative

• Has five ingredients:

1 Plaintext2 Encryption algorithm3 Secret key4 Ciphertext5 Decryption algorithm



Cryptography

Crpytanalysis



Triple DES


Overview

AlgorithmDetails

Outline






Cryptography

Crpytanalysis



Triple DES


Overview

AlgorithmDetails

Classified along Three Independent Dimensions

1 The type of operations used for transforming plaintext tociphertext

• Substitution – each element in the plaintext is mappedinto another element

• Transposition – elements in plaintext are rearranged

2 The number of keys used• Sender and receiver use same key – symmetric• Sender and receiver each use a different key - asymmetric

3 The way in which the plaintext is processed• Block cipher – processes input one block of elements at a

time• Stream cipher – processes the input elements continuously



Cryptography

Crpytanalysis



Triple DES


Overview

AlgorithmDetails

Outline






Cryptography

Crpytanalysis



Triple DES


Overview

AlgorithmDetails

Cryptanalyis



Cryptography

Crpytanalysis



Triple DES


Overview

AlgorithmDetails

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 effortrequired to break

• Can estimate time/cost of a brute-force attack



Cryptography

Crpytanalysis



Triple DES


Overview

AlgorithmDetails

Outline






Cryptography

Crpytanalysis



Triple DES


Overview

AlgorithmDetails

Classic Fiestel Network

Ciphertext (2w bits)

Figure 20.1 Classical Feistel Network

Ki

Round i

Li Ri

F

w bits w bits

K1

Round 1

Kn

Round n

Ln Rn

Ln+1 Rn+1

F

L0 R0

L1 R1

F

Plaintext (2w bits)



Cryptography

Crpytanalysis



Triple DES


Overview

AlgorithmDetails

Block Cipher Structure

• Symmetric block cipher consists of:• A sequence of rounds• With substitutions and permutations controlled by key

• Parameters and design features:• Block size• Key size• Number of rounds• Round key generation algorithm• Round function• Fast software encryption/decryption



Cryptography

Crpytanalysis



Triple DES


Overview

AlgorithmDetails

Outline






Cryptography

Crpytanalysis



Triple DES


Overview

AlgorithmDetails

Data Encryption Standard (DES)

• Very widely used

• Minor variaton of the Fiestel Network



Cryptography

Crpytanalysis



Triple DES


Overview

AlgorithmDetails

Outline






Cryptography

Crpytanalysis



Triple DES


Overview

AlgorithmDetails

Triple DES

E D E

K1 K2 K3

K3 K2 K1

P CA B

(a) Encryption

D E DC P

(b) Decryption

Figure 20.2 Triple DES

B A



Cryptography

Crpytanalysis



Triple DES


Overview

AlgorithmDetails

Outline






Cryptography

Crpytanalysis



Triple DES


Overview

AlgorithmDetails

Advanced Encryption Standard (AES)

• The Advanced Encryption Standard (AES) was issued as afederal information processing standard (FIPS 197).

• It is intended to replace DES and triple DES with analgorithm that is more secure and efficient.



Cryptography

Crpytanalysis



Triple DES


Overview

AlgorithmDetails

Outline






Cryptography

Crpytanalysis



Triple DES


Overview

AlgorithmDetails

AES Enrcyption an Decryption

Add round key

w[4, 7]

w[0, 3]

Plaintext

Substitute bytes Expand key

Shift rows

Mix columnsRou

nd 1

Rou

nd 9

Rou

nd 1

0

Add round key

Substitute bytes

Shift rows

Mix columns

Add round key

Substitute bytes

Shift rows

Add round key

Ciphertext

(a) Encryption

Figure 20.3 AES Encryption and Decryption

Key

Add round key

Plaintext

Inverse sub bytes

Inverse shift rows

Inverse mix cols

Rou

nd 1

0R

ound

9R

ound

1

Add round key

Inverse sub bytes

Inverse shift rows

Inverse mix cols

Add round key

Inverse sub bytes

Inverse shift rows

Add round key

Ciphertext

(b) Decryption

w[36, 39]

w[40, 43]



Cryptography

Crpytanalysis



Triple DES


Overview

AlgorithmDetails

AES Encryption Round

SSubBytes

State

State

State

State

State

ShiftRows

MixColumns

AddRoundKey

Figure 20.4 AES Encryption Round

S S S S S S S S S S S S S S S

M M M M

r0 r1 r2 r3 r4 r5 r6 r7 r8 r9 r10 r11 r12 r13 r14 r15



Cryptography

Crpytanalysis



Triple DES


Overview

AlgorithmDetails

Outline






Cryptography

Crpytanalysis



Triple DES


Overview

AlgorithmDetails

Substitute Bytes Transformation



Cryptography

Crpytanalysis



Triple DES


Overview

AlgorithmDetails

S-box



Cryptography

Crpytanalysis



Triple DES


Overview

AlgorithmDetails

Inverse S-box



Cryptography

Crpytanalysis



Triple DES


Overview

AlgorithmDetails

ShiftRows

• To move individual bytes from one column to another andspread bytes over columns

• On encryption left rotate each row of State by 0,1,2,3bytes respectively

• Decryption does reverse



Cryptography

Crpytanalysis



Triple DES


Overview

AlgorithmDetails

Mix Column Transformation: MixColumns

• Operates on each column individually

• Mapping each byte to a new value that is a function of allfour bytes in the column

• Use of equations over finite fields

• To provide good mixing of bytes in column



Cryptography

Crpytanalysis



Triple DES


Overview

AlgorithmDetails

Add Round Key Transformation

• Simply XOR State with bits of expanded key

• Security from complexity of round key expansion and otherstages of AES


StreamCiphers andRC4

Steam CipherStructure

RC4 Algorithm

Block CipherModes ofOperation

ElectronicCodebook Mode

Cipher BlockChaining Mode

Cipher FeedbackMode

Counter Mode

Location ofSymmetricEncryption

KeyDistribution

Outline

4 Stream Ciphers and RC4Steam Cipher StructureRC4 Algorithm

5 Block Cipher Modes of OperationElectronic Codebook ModeCipher Block Chaining ModeCipher Feedback ModeCounter Mode

6 Location of Symmetric Encryption

7 Key Distribution




RC4 Algorithm




Cipher FeedbackMode

Counter Mode


KeyDistribution

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