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DIFFERENT TYPES OF SYMMETRIC KEY CRYPTOGRAPHY PRESENTED BY: Ankita Dutta & Subhradeep Mitra

Different types of Symmetric key Cryptography

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Page 1: Different types of Symmetric key Cryptography

DIFFERENT TYPES OF SYMMETRIC KEY CRYPTOGRAPHY

PRESENTED BY: Ankita Dutta & Subhradeep Mitra

Page 2: Different types of Symmetric key Cryptography

INTRODUCTION

Symmetric key cryptography has following properties:

1) The encryption key is trivially related to the decryption key, in that they may be identical or there is a simple transformation to go between the two keys.

2) The keys, in practice, represent a shared secret between two or more parties that can be used to maintain a private information link

3) A secret key, which can be a number, a word, or just a string of random letters, is applied to the text of a message to change the content in a particular way

N.B: Other terms for symmetric-key encryption are secret-key, single-key, shared- key, one-key, and private-key encryption

Page 3: Different types of Symmetric key Cryptography

Symmetric encryption scheme has five components (see Figure 1):

i) Plaintextii) Encryption algorithmiii) Secret Keyiv) Ciphertextv) Decryption Algorithm

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REQUIRMENTS:

1) Do not need to keep the algorithm secret; we need to keep only the key secret.

2) Sender and the receiver must have obtained copies of the secret key in a secure fashion and must keep the key secure. If someone can discover the key and knows the algorithm, all communications using this key is readable

FIG: 1

Page 5: Different types of Symmetric key Cryptography

SEMMETRIC KEY CRYPTOGRAPHY

STREAM CIPHER

BLOCK CIPHER

CLASSIFICATION

Page 6: Different types of Symmetric key Cryptography

STREAM CIPHER

Page 7: Different types of Symmetric key Cryptography

DEFINATION: A stream cipher is a symmetric key cipher where plaintext digits are combined with a pseudorandom cipher digit stream (keystream).

[N.B: An alternative name of stream cipher is state cipher and a shift register is used to generate pseudorandom stream]

TYPES OF STREAM CIPHER:

1) Synchronous stream ciphers(For e.g., Binary additive stream cipher.)

2) Self-synchronous stream ciphers

SOME IMPORTANT STREAM CIPHER:

A) RC4 (RIVEST CIPHER)

B) A5/1

Page 8: Different types of Symmetric key Cryptography

RIVEST CIPHER(RC4)

DESIGNED BY: RC4 was designed by Ron Rivest of RSA Security in 1987

COMPONANTS:

i) A pseudorandom keystream (using key-scheduling algorithm a) A permutation of all 256 possible byte(denoted S as figure)

b) Two 8bit index pointer(denoted i and j)

ii) Plaintext

iii) Exclusive or

Page 9: Different types of Symmetric key Cryptography

PSEUDO-RANDOM GENERATION ALOGRITHM(PRGA): This key generation algorithm is used here

i := 0 j := 0 while GeneratingOutput: i := (i + 1) mod 256 j := (j + S[i]) mod 256 swap values of S[i] and S[j] K := S[(S[i] + S[j]) mod 256] output K endwhile

RIVEST CIPHER(RC4) (Contd..)

(Lecture Notes on Stream Cipher and RC4 by Rick Wash)

Page 10: Different types of Symmetric key Cryptography

A5/1

DEVELOPED BY: A5/1 was developed by 1987 .It is mainly used by European countries and A5/2 is used other countries.

COMPONANTS: A5/1 is the symmetric cipher used for encrypting over the air transmissions in the GSM standard's he Main components are: i) Linear Feedback Shift Register(LFSR) ii)X-OR iii)Plaintext

Page 11: Different types of Symmetric key Cryptography

FUNCTIONS: 3LFSR use 3 polynomial functions:

i) X19+X5+X2+X +1ii) X22+X +1

iii) X23+X15+X2+X +1

A5/1 (Contd..)

(Cryptanalysis of the A5/1 GSM Stream Cipher NES/DOC/TEC/WP3/005/a by Eli Biham and Orr Dunkelman)

Page 12: Different types of Symmetric key Cryptography

STREAM CIPHER CREATION YEAR EFFECTIVE KEY LENGHTH

FISH 1993 VARIABLE

PIKE 1994 VARIABLE

PANAMA 1998 256

MUGI 1998-2002 128

RABBIT 2003 128

SALSA20 2004 256

VEST 2005VARIABLE(80-256)

OTHER STREAM CIPHER

Page 13: Different types of Symmetric key Cryptography

BLOCK CIPHER

Page 14: Different types of Symmetric key Cryptography

DEFINATION: Block ciphers take a number of bits and encrypt them as a single unit, padding the plaintext so that it is a multiple of the block size. Blocks of 64 bits have been commonly used.

ENCRYPTION & DECRYPTION FUNCTION:

EK(P) =E(K,P) : {0,1}K {0,1}n {0,1}n

Dk (C) = EK-1 (C)=D(K,C): {0,1}K {0,1}n

Where input and output block size are n bits and a key of size k

EXAMPLES: A) DATA ENCRYPTION STANDARD(DES) B) TRIPLE DES(3DES) C) ADVANCE ENCRYPTON STANDRAD(AES)

(Block Cipher Principle by Yuan Xue)

Page 15: Different types of Symmetric key Cryptography

DATA ENCRYPTION STANDARD(DES)

DESIGNED BY: IBM and adopted by the U.S governments as the standard encryption method

COMPONANTS:

i) 2 transposition blocks(P-Boxes)

ii) 16 complex round cipher

iii) Round key generator

INITIAL PERMUTATION

FINAL PERMUTATION

ROUND 1

ROUND 2

ROUND 16

64-Bit Plaintext

• •

ROUND KEY

GENERATOR

K1

K2

K16

64 BIT KEY

64-Bit Ciphertext

(Fourth Edition – Data Communications and Networking by Behrouz A Forouzan)

Page 16: Different types of Symmetric key Cryptography

ENCRYPTION AND DECRYPTION :

Li Ri

Li+1 Ri+1

+

F(Ri, Ki )

32 bits

XOR

32 bits 32 bits

32 bits32 bits

32 bits

Ki48 bits

Li Ri

Li+1 Ri+1

+

F(Ri, Ki )

32 bits

XOR

32 bits

Ki48 bits

32 bits32 bits

32 bits32 bits

ENCRYPTION ROUND DECRYPTION ROUND

DATA ENCRYPTION STANDARD(DES) (Contd…)

Page 17: Different types of Symmetric key Cryptography

DES FUNCTION : DES function is the heart of DES. This function applies 48 bit keys

and 32 Bits Ri to produce a 32 Bit output

This function is made up to four operations :

i) an XOR ii) an expansion permutation iii) a group of S-boxes iv) a straight permutation

S

EXPANSION PERMUTATION

STRAIGHT PERMUTATION

+

S S S S S S S

XOR

48 Bits

48 Bits

32 Bits

32 Bits

Ri32 Bits

Ki48 BitsS-boxes DISADVANTAGE: The key size is

too small it’s only 64 bits long

DATA ENCRYPTION STANDARD(DES) (Contd…)

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TRIPLE DES(3DES)

COMPONANTS: i) Use 3 DES blocks ii) Encrypt block uses a encryption-decryption-encryption iii) Decrypt block uses a decryption-encryption-decryption

DEFINITION: It’s the improved version of DES

CLASSIFICATION: i) 3DES with 2Keys ii) 3DES with 3Keys

ADVANTAGE: Key size of 3DES is larger than DES

DISADVANTAGE: The process of 3DES is very slow

(Fourth Edition – Data Communications and Networking by Behrouz A Forouzan)

Page 19: Different types of Symmetric key Cryptography

ENCRYPTION AND DECRYPTION :

ENCRYPT DES

DECRYPT DES

ENCRYPT DES

DECRYPT DES

ENCRYPT DES

DECRYPT DES

64-Bit Plaintext

64-Bit Ciphertext

64-Bit Plaintext

64-Bit Ciphertext

Key1

Key2

Key3

Key1

Key2

Key3

Trip

le D

ES

Trip

le D

ES

ENCRYPTION 3DES DECRYPTION 3DES

N.B: Key 1 and Key3 are same in this version

TRIPLE DES(3DES) (Contd…)

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ADVANCE ENCRYPTION STANDARD(AES)

COMPONANTS: i) Initital X-OR function ii) Ten or Twelve or fourteen complex round cipher iii) Round key generator

CONFIGURATION:

Size of Data Block

128 Bits

Number of Round Key Size

10 128 bits

12 192 bits

14 256 bits

DESIGNED BY: The National Institute of Standards and Technology(NIST) chose Rijndael Algorithm invented by Vincent Rijmen & Joan Daemen

(Fourth Edition – Data Communications and Networking by Behrouz A Forouzan)

Page 21: Different types of Symmetric key Cryptography

Round 1

+

Round Key

GeneratorRound 2

Round 10

128-Bit plaintext

128-Bit ciphertext

128-Bit Key

AES

Byte subsititution

Byte permutation

Complex opration (missing in round 10)

128-Bit data

+

SubByte

ShiftRow

MixColoum

AddRoundKey

Ki

128-Bit data

ROUND i

ADVANCE ENCRYPTION STANDARD(AES) (Contd…)

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All other block cipher have same principles but difference is usually in the size of block or key, the no of rounds and the function used. They are following :

Block Cipher Name Block Size Key Size

IDEA 64 128

Blowfish 64 32-448

CAST-12864 64 128

RC5 32-128 0-2040

OTHER BLOCK CIPHER

Page 23: Different types of Symmetric key Cryptography

STREAM CIPHER VS BLOCK CIPHER

STREAM CIPHER BLOCK CIPHER

Stream ciphers combine plain-text bits with a pseudorandom cipher bits stream using XOR operation.

Block ciphers encrypt fixed length blocks of bits

Stream ciphers use varying transformations

Block ciphers use the same transformation

Stream ciphers usually execute faster

Block ciphers usually execute slower

Stream ciphers are relatively less complex

Block ciphers are relatively more complex

ENCRYPTION TECHNIQUE

TYPE OF TRANSFORMATION

EXCUTABLE TIME

HARDWARE COMPLEXITY

Page 24: Different types of Symmetric key Cryptography

REFERENCES

(Lecture Notes on Stream Cipher and RC4 by Rick Wash)

(Cryptanalysis of the A5/1 GSM Stream Cipher NES/DOC/TEC/WP3/005/a by Eli Biham and Orr Dunkelman)

(Block Cipher Principle by Yuan Xue)

(Fourth Edition – Data Communications and Networking by Behrouz A Forouzan)

Difference between Stream Cipher and Block Cipher from http://www.differencebetween.com/difference-between-stream-cipher-and-vs-block-cipher/

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Question?

Page 26: Different types of Symmetric key Cryptography

THANK YOU