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Different types of Symmetric key Cryptography


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Different types of Symmetric key Cryptography

  2. 2. 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
  3. 3. Symmetric encryption scheme has five components (see Figure 1): i) Plaintext ii) Encryption algorithm iii)Secret Key iv)Ciphertext v) Decryption Algorithm
  4. 4. 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
  6. 6.  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
  7. 7. 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
  8. 8.  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)
  9. 9. 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
  10. 10. FUNCTIONS: 3LFSR use 3 polynomial functions: i) X19+X5+X2+X +1 ii) 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)
  12. 12.  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)
  13. 13. 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)
  14. 14.  ENCRYPTION AND DECRYPTION : Li Ri Li+1 Ri+1 + F(Ri, Ki ) 32 bits XOR 32 bits 32 bits 32 bits32 bits 32 bits Ki 48 bits Li Ri Li+1 Ri+1 + F(Ri, Ki ) 32 bits XOR 32 bits Ki 48 bits 32 bits32 bits 32 bits32 bits ENCRYPTION ROUND DECRYPTION ROUND DATA ENCRYPTION STANDARD(DES) (Contd…)
  15. 15.  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 Ri 32 Bits Ki 48 Bits S-boxes  DISADVANTAGE: The key size is too small it’s only 64 bits long DATA ENCRYPTION STANDARD(DES) (Contd…)
  16. 16. 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)
  17. 17.  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 TripleDES TripleDES ENCRYPTION 3DES DECRYPTION 3DES N.B: Key 1 and Key3 are same in this version TRIPLE DES(3DES) (Contd…)
  18. 18. 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)
  19. 19. 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…)
  20. 20. 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
  21. 21. 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
  22. 22. 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