The document discusses encryption and decryption techniques. It describes substitution and transposition as two types of encryption techniques. It then discusses symmetric key cryptography which uses a single key for encryption and decryption, providing examples of algorithms like DES, Triple DES, and Blowfish. It proceeds to describe the DES algorithm in more detail, outlining its key generation process and the steps involved, including initial permutation, expansion, substitution using S-boxes, and final permutation. Finally, it briefly introduces asymmetric key algorithms like RSA which use public and private key pairs.

1. Submitted by:Smruti Ranjan Sahoo
Underguidance by:Prof. Mukesh Tiwari

3. A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
D E F G H I J K L M N O P Q R S T U V W X Y Z A B C

4. Data encryption and Decryption
Encryption – The process of converting plain text message
into unintelligible format in known as encryption .

5. Decryption – The process of converting
unreadable message i.e cipher text into
plain text is called data decryption.

6. There are two types of technique used for
encryption i.e
1. Substitution
2. Transposition

7. Symmetric key Cryptography
In this we use only one key for both
encryption and decryption of data.
There are various algorithms such as DES,
Triple
DES,RC4,RC5,Blowfish,Twofish….etc.

8. DES Algorithm

9. How DES works ??
56-bit 56-bit
key key
Block 1 Block 2
64 bit plain
text
DES
64 bit cipher
text
64-bit plain
text
DES
64-bit cipher
text

10. Key generation of DES
 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48
49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64

11. Steps involved in DES algorithm

12.  Initial Permutation(IP)
Division into 32-bit blocks:
58 50 42 34 28 18 10 2 60 52 44 36 28 20 12 4
62 54 46 38 30 22 14 6 64 56 48 40 32 24 16 8
57 49 41 33 25 17 9 1 59 51 43 35 27 19 11 3
61 53 45 37 29 21 13 5 63 55 47 39 31 23 15 1

13. Expansion Function:
The 32 bit block are expand into 48 bit block . The table is called “Expansion Table”.

14. S-box Substitution
……
48-bit input block
6-bit sub block 6-bit sub block
6-bit sub block
S-box 1 S-box 2 S-box 8
4-bit output 4-bit output 4-bit output
32-bit output block

16. P-box permutation:
Final Permutation:
16 7 20 21 29 12 28 17 1 15 23 26 5 18 31 10
2 8 24 14 32 27 3 9 19 13 30 6 22 11 4 25

17. Asymmetric Key Algorithm
Public-key cryptography, also known as asymmetric cryptography, is a class of
cryptographic algorithms which requires two separate keys, one of which is secret (or
private) and one of which is public. Although different, the two parts of this key pair are
mathematically linked.

19. 1.Choose two large prime numbers P & Q.
2.Calculate N= P*Q
3.Select public key (i.e encryption key) E such that it is not a factor of (P-1) and (Q-1)
4.Select the private key D such that following equation is true
(D * E) mod (P-1)(Q-1) = 1
5.For encryption ,calculate the cipher text “CT” from the plain text “PT” as follows
CT = 𝑃𝑇 𝐸
𝑚𝑜𝑑 𝑁
6.Send CT as cipher text to the receiver
7.For decryption ,calculate the plain text “PT” from cipher text “CT” as follows
PT = 𝐶𝑇 𝐷
𝑚𝑜𝑑 𝑁

20.  Five Basic steps
 Large Prime Number Generation: Two large prime numbers p and
q need to be generated. These numbers are very large: At least 512
digits, but 1024 digits is considered safe.
 Modulus: From the two large numbers, a modulus n is generated by
multiplying p and q.
 Totient: The totient of n,ϕ(n) is calculated.
 Public Key: A prime number is calculated from the range [3,ϕ(n))
that has a greatest common divisor of 1 with ϕ(n).
 Private Key: Because the prime in step 4 has a gcd of 1 with ϕ(n),
we are able to determine it's inverse with respect to modϕ(n).