Unit 4

ENCRYPTION
UNIT-IV
1/9/17 ABHISHEK SRIVASTAVA (CSE)

TABLE OF CONTENTS
• We will be talking about...
•
•
What is Encryption?
•
Why Do We Use Encryption?
•
How Does It Work? (x3)
•
Pros and Cons
•
Real Life Examples

WHAT IS ENCRYPTION?
• Encryption: The process of coding text
• Decryption: The process of decoding text
• Decryption is the opposite of encryption
•
• A security method used to protect data
• Files on computers
• Data being passed through the Internet
• ATM machines
• E-Commerce
• Facebook password.

WHY DO WE USE ENCRYPTION?
To secure important information e.g. :
• Health records
• Credit card information
• Student records
• Etc.
•
• Prevents information from getting stolen or read
•
• Without encryption, there is no reliable security

HOW DOES IT WORK?
●
Encryption allows the sender to transform data from plain
text into cipher text by using a key
• Cipher text: coded text
• Key: what is used to encrypt and decrypt text
•
Two different types of encryption:
• Asymmetric encryption (Public key encryption)
• Symmetric encryption
•
•
Adrianna wants to send Melissa a message...

REAL LIFE EXAMPLES
●
Used in the military and the government
●
●
Now used in everyday life:
• Online banking
• E-commerce
• Student records, health records, tax records etc.
• ATM machines
• Social networking (emails, texts, instant messengers)
• Businesses

DATA ENCRYPTION

CONTENT
• Data encryption definition.
• Types of data encryption.
• Difference between symmetric and asymmetric.
• Encryption Methods.
• Data Encryption Standard (DES).
• Public Key Cryptosystem
•
•
•
•
•

DATA ENCRYPTION
DEFINITION
Data encryption refers to mathematical calculations and
algorithmic schemes that transform plaintext into cypher text,
a form that is non-readable to unauthorized parties. The
recipient of an encrypted message uses a key which triggers
the algorithm mechanism to decrypt the data, transforming it
to the original plaintext version

A public key encryption scheme has six
ingredients
• Plaintext
•
• encryption algorithm
•
• public and private key
•
• cipher text
•
• decryption algorithm

TYPES OF DATA ENCRYPTION
• In symmetric (Single
key):
The encryption
and decryption keys
are the same.
•

A SYMMETRIC ENCRYPTION
SCHEME HAS FIVE INGREDIENTS
Plaintext
• Encryption Algorithm
• Secret key
• Cipher Text
• Decryption

.
• In asymmetric (two
keys):
• The
encryption
and
decryption
keys are
different.
•

.

DIFFERENCE B/W
ASYMMETRIC &
SYMMETRIC ENCRYPTION
• The symmetric one key is used for symmetric and a
symmetric but in asymmetric different keys are use for
encryption and description.
• In symmetric, sender and receiver must share the algorithm
and key but in a symmetric sender and receiver must have
one matched pair of keys.
• In symmetric key must be kept secret but in asymmetric
two keys must of kept secret.

PUBLIC KEY CRYPTOGRAPHY
• A form of cryptography in which the key used to encrypt a
message differs from the key used to decrypt it.
• In public key cryptography, a user has a pair of
cryptographic keys—a public key and a private key. The
private key is kept secret, while the public key may be
widely distributed.
• The two main branches of public key cryptography are:
1. Public key encryption
2. Digital signatures

PUBLIC KEY ENCRYPTION
A message encrypted with a recipient's public key cannot be decrypte
by anyone except the recipient possessing the corresponding private
key.
•
ContdContd..
Actual algorithms - two linked
keys:

KEY TERMS
Public and private keys:
This is a pair of keys that have been selected so that if one
is used for encryption, the other is used for decryption.
The exact transformations performed by the algorithm
depend on the public or private key that is provided as
input.
Cipher text:
This is the scrambled message produced as output. It
depends on the plaintext and the key. For a given message,
two different keys will produce two different cipher texts.

A GENERAL APPROACH
ContdContd..1/9/17 ABHISHEK SRIVASTAVA (CSE)

Step 1 : Each user generates a pair of keys to be used for the
encryption and decryption of messages.
Step 2 : Each user places public key in a public register or
other accessible file. As encryption figure suggests,
each user maintains a collection of public keys
obtained from others.
Step 3 : If user1 wishes to send a confidential message to
user2, user1 encrypts the message using user2's
public key.
Step 4 : When user1 receives the message, he decrypts it
using his private key. No other recipient can decrypt
the message because only user1 knows his private
key

Authentication & security:
• There is some source A that produces a message in plaintext,
X =[X1, X2,..., XM,]. The M elements of X are letters in
some finite alphabet. The message is intended for
destination B. B generates a related pair of keys: a public
key, PUb, and a private key, PRb. PRb is known only to B,
whereas PUb is publicly available.
•
• A generates another pair of keys: a public key, PUa, and a
private key, PRa. PRa is known only to A, whereas PUa is
publicly available.
Contd.Contd.1/9/17 ABHISHEK SRIVASTAVA (CSE)

SECRECY IN A PUBLIC KEY
ENCRYPTION :
Step 1:Step 1: A encrypt the massage using B’s public key PUb andA encrypt the massage using B’s public key PUb and
send it to B.send it to B.
With the message X and the encryption key PUb asWith the message X and the encryption key PUb as
input, A forms the cipher text Y = [Y1, Y2,..., YN]:input, A forms the cipher text Y = [Y1, Y2,..., YN]:
Step 2:Step 2: B decrypt the massage using it’s private key PRb.B decrypt the massage using it’s private key PRb.
Using it’s private key PRb and the cipher text Y itUsing it’s private key PRb and the cipher text Y it
obtain the original massage Xobtain the original massage X

Authentication In A Public
Key Encryption :
Step 1:Step 1: A prepares a message to B and encrypts it usingA prepares a message to B and encrypts it using
A's private key before transmitting it.A's private key before transmitting it.
Step 2:Step 2: B can decrypt the message using A's public key.B can decrypt the message using A's public key.
Because the message was encrypted using A'sBecause the message was encrypted using A's
private key, only A could have prepared theprivate key, only A could have prepared the
message.message.

Comparing Secrecy and
Authentication
• In authentication technique :It is impossible to alter the
message without access to A's private key, so the message
is authenticated both in terms of source and in terms of
data integrity. But secrecy doesn't provide this advantage.
•
•
• Thus the authentication is much more confidential and
secure in terms of alteration of the massage.

Authentication & Security:
Contd.Contd.1/9/17 ABHISHEK SRIVASTAVA (CSE)

DIGITAL SIGNATURE
• An authentication mechanism that enables the creator of a
message to attach a code that acts as a signature.
• In situations where there is not complete trust between sender
and receiver, something more than authentication is needed.
• 1. It must verify the author and the date and time of the
signature.
• 2. It must to authenticate the contents at the time of the
signature.
• 3. It must be verifiable by third parties, to resolve disputes.
• Thus, the digital signature function includes the authentication
function.
• A variety of approaches has been proposed for the digital
signature function. These approaches fall into two
categories: direct and arbitrated

Direct Digital Signature :
The direct digital signature
involves only the communicating parties (source,
destination). It is assumed that the destination knows the
public key of the source. A digital signature may be
formed by encrypting the entire message with the sender's
private key or by encrypting a hash code of the message
with the sender's private key.

Arbitrated Digital Signature :
• The problems associated with direct digital signatures can
be addressed by using an arbiter.
• As with direct signature schemes, there is a variety of
arbitrated signature schemes. In general terms, they all
operate as follows. Every signed message from a sender X
to a receiver Y goes first to an arbiter A, which check it’s
origin and context and then sent to Y.
•

Applications For Public-key
Cryptosystems
Public key cryptosystem used in many systems such as:
1.Decision support system
2. RSA Algorithm
3. Elliptic Curve
4. Diffie-Hellman key exchange

COMPUTATIONAL COST
• It is computationally easy for a party B to generate a pair
(public key PUb, private key PRb).
• It is computationally easy for a sender A, knowing the
public key and the message to be encrypted, M, to
generate the corresponding cipher text:
• C = E(PUb, M)
• It is computationally easy for the receiver B to decrypt
the resulting cipher text using the private key to
recover the original message:
• M = D(PRb, C) = D[PRb, E(PUb, M)]
• It is computationally infeasible for an adversary,
knowing the public key, PUb, to determine the
private key, PRb.
•

WEAKNESSES
ComputatioComputatio
nal costnal cost
Keys in public-key cryptography, due to theirKeys in public-key cryptography, due to their
unique nature, are more computationally costlyunique nature, are more computationally costly
than their counterparts in secret-keythan their counterparts in secret-key
cryptography.cryptography.
VulnerableVulnerable
to bruteto brute
forceforce
attacksattacks
Keys in asymmetric cryptography are moreKeys in asymmetric cryptography are more
vulnerable to brute force attacks than in secret-vulnerable to brute force attacks than in secret-
key cryptography.key cryptography.
VulnerableVulnerable
to the manto the man
in thein the
middlemiddle
attackattack
Public-key cryptography also has vulnerabilitiesPublic-key cryptography also has vulnerabilities
to attacks such as the man in the middle attack.to attacks such as the man in the middle attack.
In this situation, a malicious third partyIn this situation, a malicious third party
intercepts a public key on its way to one of theintercepts a public key on its way to one of the
parties involvedparties involved

Unit 4

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