Chapter 2
Cryptographic Tools

Basic Terminology
Plaintext
The original message
Ciphertext
The coded message
Enciphering or encryption
Converting
plaintext to ciphertext
Deciphering or decryption
Restoring the plaintext from the ciphertext
2

2 Types of Encryption
Symmetric
Asymmetric

Symmetric Encryption
•
The universal technique for providing
confidentiality for transmitted or stored data
•
Also referred to as conventional encryption or
single-key encryption
•
Two requirements for secure use:
•
Need a strong encryption algorithm
•
Sender and receiver must have obtained copies
of the secret key in a secure fashion
and must
keep the key secure

Symmetric Encryption
•
The only type of encryption in use prior to the
development of public-key encryption in the 1970s
•
Remains by far the most widely used of the two
types of encryption
5

Plaintext
input
Y
= E[
K
,
X
]
X
= D[
K
,
Y
]
X
K
K
Transmitted
ciphertext
Plaintext
output
Secret key shared by
sender and recipient
Secret key shared by
sender and recipient
Encryption algorithm
(e.g., DES)
Decryption algorithm
(reverse of encryption
algorithm)
Figure 2.1
Simplified Model of Symmetric Encryption

Security through obscurity
and Kerchoffs’ assumption
•
S
ecurity through obscurity
The
secrecy of the design or
implementation
is the main method of providing
security.
•
Kerchoffs’ assumption
The adversary knows all details
of the encryption function
except the secret
key
7

Cryptographic
Protocols
•
Design features:
Two types of
operations used
for transforming
plaintext to
ciphertext
Two types of
operations used
for transforming
plaintext to
ciphertext
Substitutio
n
(confusion)
Transpositi
on
(diffusion)
Number of keys
used
Number of keys
used
Symmetric
single-key
Asymmetri
c
two-keys
public encryption
key
private decryption
key
The way in which
the plaintext is
processed
The way in which
the plaintext is
processed
Block
cipher
Stream
cipher
8

Security of Encryption
•
Unconditionally secure
o
No matter how much time or computational power an opponent
has, it is impossible to decrypt the ciphertext
•
Computationally secure
o
The cost of breaking the cipher exceeds the value of the
encrypted information
o
The time required to break the cipher
exceeds the useful
lifetime of the
information
9

Attacking Symmetric
Encryption
Cryptanalytic Attacks
Brute-Force Attacks
Rely on:
Nature of the algorithm
Some knowledge of the general
characteristics of the plaintext
Some sample plaintext-
ciphertext pairs
Exploits the characteristics of
the algorithm to attempt to
deduce a specific plaintext or
the key being used
If successful all future and past
messages encrypted with that
key are compromised
Try all possible keys on some
ciphertext until an intelligible
translation into plaintext is
obtained
On average half of all
possible keys must be tried to
achieve success
Some degree of knowledge
about the expected plaintext
is needed, and some means
of automatically
distinguishing plaintext from
garble

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