DIFFERENT TYPES OF SYMMETRIC KEY CRYPTOGRAPHY PRESENTED BY: Ankita Dutta & Subhradeep Mitra
May 22, 2015
DIFFERENT TYPES OF SYMMETRIC KEY CRYPTOGRAPHY
PRESENTED BY: Ankita Dutta & Subhradeep Mitra
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
Symmetric encryption scheme has five components (see Figure 1):
i) Plaintextii) Encryption algorithmiii) Secret Keyiv) Ciphertextv) Decryption Algorithm
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
SEMMETRIC KEY CRYPTOGRAPHY
STREAM CIPHER
BLOCK CIPHER
CLASSIFICATION
STREAM CIPHER
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
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
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)
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
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)
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
BLOCK CIPHER
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)
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)
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…)
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…)
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)
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…)
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)
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…)
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
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
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/
Question?
THANK YOU