Cryptography

CONTENTS
o Introduction
o Need of Cryptography
o Types of Attacks
o Techniques of Cryptography
o Encryption Algorithm
• Symmetric
• Asymmetric
o Digital Signature
o Conclusion

INTRODUCTION
 What is Cryptography?
 “Hidden Writing”

 Mainly used to protect Information.

 Goal of Cryptography

 Ensure security of communication over insecure
medium

 Communicate even with possibility of adversary

NEED OF ENCRYPTION
 Confidentiality

 Integrity

 Authentication

 Nonrepudiation

 Access Control
 Availability

TYPES OF ATTACKS
 A General View

 A Technical View

 A Practical Side of Attacks

 Programs that Attack

PLAIN PLAIN
TEXT TEXT

ENCRYPTION DECRYPTION

CIPHER
TEXT
SENDER RECEIVER

BASIC BLOCK DIAGRAM

BASIC TERMINOLOGIES
 Encryption
 Encryption is the process of encoding a message so that its
meaning is not obvious
 Decryption
 Decryption is the reverse process, transforming an encrypted
message back into its normal, original form
 Cryptosystem
 A system for encryption and decryption is called a
cryptosystem.

BASIC TERMINOLOGIES
 Plaintext
 Cipher text
 Key –
 key refers to a sequence of symbols or a numerical value used
by an algorithm to alter information & making that information
secure
 Encryption algorithm
 The cryptosystem involves a set of rules for how to encrypt the
plaintext and how to decrypt the cipher text.
 Cryptanalysis
 Cryptanalysis is an attempt to break the cipher text.

TECHNIQUES OF CRYPTOGRAPHY
 Substitution Technique

 Caesar Cipher
 Monoalphabetic Cipher
 Homophonic Cipher
 Polyalphabetic Cipher
 Transposition Technique

 Rail Fence Technique
 Vernam Cipher(One -time Pads)
 Simple Columnar Cipher

ENCRYPTION ALGORITHM
 Symmetric
 Same key for encryption and decryption

 Key distribution problem

 Asymmetric
 Key pairs for encryption and decryption

 Public and private keys

SYMMETRIC ALGORITHM
 It is also called as Secret Key Cryptography
 Single key used for both encrypt & decrypt
 Key must be known to both the parties

Key

Original
Plaintext Ciphertext Plaintext
Encryption Decryption

Symmetric Cryptosystem

ASYMMETRIC ALGORITHM
 Private keys are used for decrypting.
 Public keys are used for encrypting

encryption
plaintext ciphertext
public key

decryption
ciphertext plaintext
private key

COMPARISON
Secret Key (Symmetric) Public Key (Asymmetric)
Number of Key 1 2
Protection of Key Must be kept secret One key must be kept secret
& other can be freely
exposed
Best Uses secrecy and integrity of Key exchange, authentication
data

Key Distribution Problematic Safer

Speed Fast Slow; typically, 10,000 times
slower than secret key

SYMMETRIC ALGORITHM

 Data Encryption Standard (DES):
56 bits key

 Advance Encryption Standard (AES):
128, 192 or 256 bits key

 International
Data Encryption
Algorithm(IDEA):
128 bits key

DATA ENCRYPTION STANDARD
 Developed by IBM and it is known as the Data Encryption
Standard
 It is also known as Data Encryption Algorithm

 The DES algorithm is a careful and complex combination of
two fundamental building blocks of encryption:
 Substitution and
 Transposition
 DES uses only standard arithmetic and logical operations on
numbers up to 64 bits long

Plain text (64 bits)

Initial Permutation (IP)

LPT RPT

key 16 rounds 16 rounds key

Final Permutation

Cipher text (64 bits)

BROAD LEVEL STEPS IN DES

DATA ENCRYPTION STANDARD
 1st 64 bit plain text is handed over to initial
permutation function.
 IP is performed over the plain text.
 IP produces two halves of the permuted blocks
left plain text (LPT) & right plain text (RPT).
 Now LPT & RPT goes 16 rounds of encryption
process, each with its own key.
 Now LPT & RPT are rejoined and FINAL
PERMUTATION (FP) is performed on the
combined block.
 The result is 64 bit cipher text.

DETAILS OF ONE ROUND IN DES

Key Transformation

Expansion Permutation

S- box Substitution

P- box Permutation

XOR and Swap

ADVANTAGES OF DES:

o DES is also an ANSI and ISO standard - anybody
can learn the details and implement it.
o Since DES was designed to run on hardware, it is
fast in hardware.
o Hard to crack.

DISADVANTAGES OF DES:

o Hardware implementations of DES are very fast;
DES was not designed for software and hence runs
relatively slowly.

ASYMMETRIC ALGORITHM
 Rivest Shamir Adleman (RSA) Encryption:

Based on factoring the product of large prime numbers.

 Knapsack Algorithm:

If M1,M2…., Mn are given values & S is the sum,
S=b1M1+b2M2….+bnMn
where, bi can be 0 or 1

RSA

 It is named after its three inventors Rivest
Shamir and Adleman
 This algorithm was introduced in 1978 and to
date remains secure.
 RSA has been the subject of extensive
cryptanalysis, and no serious flaws have yet been
found.
 The encryption algorithm is based on the
underlying problem of factoring large numbers.

RSA

 p and q are two large prime numbers

 n=p.q

 m = (p-1)(q-1)

 a is such that 1 < a < m and gcd (m , a) = 1.

 b is such that ( a. b) mod m = 1

RSA ENCRYPTION & DECRYPTION

 Message M < n.
 Encryption key = (a , n).

 Decryption key = (b , n).

 Encrypt => E = Ma mod n.

 Decrypt => M = Eb mod n.

 Advantage

 Individuals can post their public key on their Web
site.
 The number of the keys is only twice of the
number of user.

 Disadvantage

 It is slower than symmetric algorithms.
 The cipher text may be larger than plain text.

DIGITAL SIGNATURE
 When an author signs a document, it cannot
be changed.

 When you send a document
electronically, you can also sign it.

 Digital signature can be done in two ways:

 You can sign the whole document

 You can sign a digest of the document

Signing the whole document

 You can not provide these aspects of security using the
secret key.
 The method provides authentication & non-repudiation.

Signing the digest

 The two most common hash functions are:
 Message digest 5 (MD5)
 Secure hash algorithm (SHA-1)
 The properties of hash function
 One-way: the digest can only be created from the
message, but not vice versa
 One-to-one: be very difficult to find two messages that
create the same digest.

APPLICATIONS OF ENCRYPTION
 Cryptographic Hash Function

 Digital Signature

 Certificate

 Secure electronic transactions

 Office equipment

CONCLUSION
 RSA finds its strongest application when parties
who have no prior relationship want to share
sensitive data with each other.
 Thus , even though slower as compared to
symmetric algorithms it is & will be widely used as it
can be used in digital signature for long haul
transmission.

Cryptography

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