# Feistel Encryption and Decryption

Document Sample

```					Feistel
Encryption
and
Decryption
Example - Data Encryption Standard
(DES)
• Introduced in 1977 by NIST, based on IBM Lucifer
System –key of 128 bits
• Extensive controversy over influence of NSA on key
size
• 64 bit data blocks/56 bit keys
• Only intended to have limited lifetime
• 1994 -NIST reaffirmed DES for another 5 years for
federal (nonclassified) use
• 1999 – NIST announced that DES is just for legacy
system, and Triple-DES will serve instead
DES          DES
Algorithm   Encryption
Algorithm
DES Single Iteration
Calculation of F(R, K)
An Example of 6-by-4 S-Box unit
Middle 4 bits of input
S5
0000   0001   0010   0011   0100   0101   0110   0111   1000    1001   1010   1011   1100   1101   1110   1111

00   0010   1100   0100   0001   0111   1010   1011   0110   1000    0101   0011   1111   1101   0000   1110   1001

01   1110   1011   0010   1100   0100   0111   1101   0001   0101    0000   1111   1010   0011   1001   1000   0110
Outer
bits
10   0100   0010   0001   1011   1010   1101   0111   1000   1111    1001   1100   0101   0110   0011   0000   1110

11   1011   1000   1100   0111   0001   1110   0010   1101   0110    1111   0000   1001   1010   0100   0101   0011
Why not Double DES?
Can we find a key K3 such that

In this case, the Double DES degraded into
single key DES?

Ans: We can find many combinations of (K1, K2)
For Double DES which yield the same result by
using K1 in the single DES case.
• Total Time = 257
64-bit plaintext
Other Conventional Systems
• DES still under export control
• Single DES near end of useful lifetime
• Triple DES suffers speed penalty
• International Data Encryption Algorithm (IDEA)
proposed by Massey and Lai
• 128 bit keys/64 bit data blocks
• “Easy” to implement
Multiplication   Addition & Modulo by 216
modulo 216+1
Exclusive Or
Bigger is Better?
• There is a tradeoff between size, speed and power
consumption of a conventional cryptosystem
• Ideally, designers of cryptographic systems will use
an algorithm that will be secure for a time window
into the future with the intention that the system will
be replaced as techniques and technology advances
• NIST created a program for the development of
Advanced Encryption Standard (AES) (first call Sept.
97)
- Cut off for participation June 1998
- Cut off for public comments (1st round) April 1999.
- “Winner” – Rijndael, announced May 2001
- Standardized by NIST May 2002.
• Rijndael (Daemen and Rijmen) supports keys of 128,
192 or 256 bits and messages of 128, 192 or 256 bits
• Designed to be resistant to linear or differential
cryptanalysis
• Fast and efficient in hardware and software
implementations
Rijndael
• Each round is composed of three invertible (and
uniform) transforms –(layers)
– Linear mixing layer –high diffusion over multiple
rounds
– Non-linear layer –parallel S-boxes –each has worst-
case non-linear properties
– Key Addition layer –XOR of round keys
ByteSub Transform
• Non-linear, invertible byte substitution (S-box)
Byte Sub
• Along with Shift Row, MixColumn Transform
provides diffusion to AES
32 bits

One MD5 operation-MD5 consists of 64 of these operations, grouped
in four rounds of 16 operations. F is a nonlinear function; one function
is used in each round. Mi denotes a 32-bit block of the message input,
and Ki denotes a 32-bit constant, different for each operation.
hash value
A = 67452301
B = EFCDAB89
D = 10325476
E = C3D2E1F0
H
<< n

<< n

One iteration within the SHA-1 compression function. A, B,
C, D and E are 32-bit words of the state; H is a nonlinear
function that varies; n denotes a left bit rotation by n places; n
varies for each operation. denotes addition modulo 232. Kt is a
constant.
• Some other useful approaches
- ANSI X9.17
- Used to generate the IV and keys of DES
• FIPS 168 Generator
- Used in SHA and DES as a one-way function

```
DOCUMENT INFO
Shared By:
Categories:
Stats:
 views: 536 posted: 9/4/2010 language: English pages: 72
How are you planning on using Docstoc?