1. SOFTWARE IMPLEMENTATION OF OCB MODE
Under the guidance of
Dr. Jens-Peter Kaps by
Paradesi Ravi kiran
Baba Prashanth Namatheertham
Bharath Govindool
Ritesh Suppala
12/19/2006
Software Implementation of OCB

2. Software Implementation of OCB
Need for Encryption
Data Confidentiality
Data Integrity
Sender Authentication
12/19/2006
Software Implementation of OCB

3. Classification of Ciphers
Classical
Modern
Rotor Machines
Substitution
Transposition
Private Key
Public Key
Stream
Block
12/19/2006
Software Implementation of OCB

4. Private Key Encryption
Symmetric key encryption can be done at high speeds
Key distribution problem
Block ciphers provide high security
DES and AES are the most commonly used block ciphers
12/19/2006
Software Implementation of OCB

5. Software Implementation of OCB
AES or DES?
DES encryption uses 64 bit key which is proved to be vulnerable
AES encryption uses 128 bit key which is still considered to be secure
In the present work AES is used
12/19/2006
Software Implementation of OCB

6. Software Implementation of OCB
Modes of Operation
ECB mode
CBC mode
Counter mode
OFB mode
OCB mode
12/19/2006
Software Implementation of OCB

7. OCB (Offset Code Book) Mode
New mode of operation
Invented by Phil Rogway
Authenticated Encryption Scheme
Cost efficient algorithm
Platform independent
12/19/2006
Software Implementation of OCB

8. Simple representation of OCB Algorithm
1L+R
2L+R
3L+R
4L+R
Nonce E E E E E k k k k k
4L+R
1L+R
2L+R
3L+R R
12/19/2006
Software Implementation of OCB

9. Software Implementation of OCB
Steps Involved
Message partitioning
Random number generation
L value generation
Hash function generation
Encryption
Tag generation
12/19/2006
Software Implementation of OCB

10. Software Implementation of OCB
Encryption Scheme
Any block cipher can be used
Advanced Encryption Standard
Rijndael AES
Complex and secure
128 bit key
AES is faster than 3DES
12/19/2006
Software Implementation of OCB

11. Software Implementation of OCB
AES
128 bit message and 128 bit key
Key expansion
Ten rounds
Substitution
Shift rows
Mix columns
Add round key
12/19/2006
Software Implementation of OCB

12. Software Implementation of OCB
Block diagram of AES
Key
Plain Text
Add Round Key
Key expansion
Substitute Bytes
Round Key 1
Shift rows
Round Key 2
Mix Columns
Add round Key
Round Key 10
Substitute Bytes
Shift rows
Add round Key
Cipher Text
12/19/2006
Software Implementation of OCB

13. Software Implementation of OCB
Message partitioning
Input message is a text file
AES 128 bits
Input message divided into blocks of 128 bits
Last block ( 1 to 128 bits)
Each block arranged in 4x4 matrix
12/19/2006
Software Implementation of OCB

14. Software Implementation of OCB
Random Number
128 bit random number
Generated using special functions offered by C compiler
Srand ( ) under the header <math.h>
Arranged in a 4x4 matrix
Encrypted to generate nonce(R)
Used in offset
12/19/2006
Software Implementation of OCB

15. Software Implementation of OCB
L values
Significant feature of OCB
128 bits of zeroes are encrypted (L[0])
Number of L values generated is equal to number of message blocks
L[-1]= L[0]>>1 if MSB is 0 otherwise L[-1]=L[0]>>1 xored with
L[i] for i>0, L[i]=L[i-1]<<1 if MSB is 0 otherwise L[i]=L[i-1]<<1 xor with
12/19/2006
Software Implementation of OCB

16. Software Implementation of OCB
Last Block
Contains less than or equal to 128 bits
Length is found out
Xored with L[-1]
Xored with offset (L[m]+R)
Encrypted
Xored with M[m] (message)
12/19/2006
Software Implementation of OCB

17. Offset value and Encryption
Offset values are generated for each message block
Offset value =L[i]+R
Before encryption, message xored with offset
M[i]=M[i] xor offset (for i=1 to m-1)
Last block
All messages encrypted with AES
After encryption cipher is again xored with offset
12/19/2006
Software Implementation of OCB

18. Software Implementation of OCB
Tag Generation
Simple hash function is generated
Hash function xored with final offset( L[m])
Encrypted
Tag value generated
12/19/2006
Software Implementation of OCB

19. Software Implementation of OCB
Block diagram of OCB
Hash
M[1]
M[2]
M [m]
Zeroes
Random
length
L[1]+R
L[2]+R
L[m]+R+C[m]
L[-1]+Len[m]
L[M]+R
AES(K)
AES(K)
AES(K)
AES(K)
AES(K)
AES(K)
L[1]+R
L[2]+R
L[0]
NONCE
L[-1]
C[1]
C[2]
C[ m]
Tag
12/19/2006
Software Implementation of OCB
L[m]

20. Software Implementation of OCB
Comparison
Compared with CBC mode
More clock cycles than CBC
Authentication provided
Parallelism can be implemented
More efficient than CBC
12/19/2006
Software Implementation of OCB

21. Software Implementation of OCB
12/19/2006
Software Implementation of OCB

22. Software Implementation of OCB
12/19/2006
Software Implementation of OCB