Cryptography
Jill Tolentino
Introduction
•
Cryptography in Greek means “secret writing.”
•
Today, it refers to the science and art of transforming
messages to make them secure and immune to attacks.
•
Cryptography is used to achieve:
–
confidentiality .............. only authorized persons can access information
–
integrity ...................... information that was sent is what was received
–
authentication ............. guarantee of originator and of electronic
transmission
–
non

repudiation ........... originator of information cannot deny content or
transmission
Cryptography components
•
Original message is called
plaintext.
•
An
encryption algorithm
transforms the plaintext to
ciphertext
.
•
A
decryption algorithm
tranforms the ciphertext back to
plaintext.
•
Sender uses an encryption
algorithm and the receiver uses
a decryption algorithm.
Encryption and Decryption
•
An encryption/decryption
algorithm is referred to as
ciphers
.
•
Through the use of public
ciphers with secret keys, one
cipher can serve millions of
communicating pairs.
•
A
key
is a number (value) that
the cipher, as an algorithm,
operates on.
•
The encryption and decryption
algorithms are public; the keys
are secret.
Cryptography
Cryptography algorithms are divided into
two groups:
•
Symmetric

key (secret

key) cryptography
algorithms
•
Public

key(asymmetric) cryptography algorithms
Symmetric

key Cryptography
•
The same key is shared and used by
the sender and receiver to encrypt
and decrypt the message.
•
The decryption algorithm is the
inverse of the encryption algorithm.
•
Example: If encryption algorithm
uses a combination of addition and
multiplication, the decryption
algorithm uses a combination of
division and subtraction.
Symmetric

key Cryptography
Advantages:
•
Efficient

takes less time to encrypt a
message using a symmetric

key algorithm
than a public

key algorithm.
•
Key is smaller
•
Can encrypt and decrypt long messages
Symmetric

key Cryptography
Disadvantages:
•
Each pair of users must have a unique
symmetric key.
This means if N people in the world want to
use this method, there needs to be N(N

1)/2 symmetric keys. For
example, for 1 million people to communicate, 500 billion symmetric
keys are needed.
•
Distribution of the keys between two parties
can be difficult.
Symmetric

key Cryptography
•
Traditional ciphers use a character or symbol as the unit of
encryption/decryption.
•
Block ciphers use a block of bits as the unit of
encryption/decryption.
•
Traditional ciphers involve either substitution or
transposition.
•
Substitution Cipher
–
bit

level encryption method in which n bits
substitute for another n bits as defined by P

boxes, encoders, and
decoders.
•
Transposition Cipher
–
A character

level encryption method in
which the position of the character changes.
Examples:
Public

key Cryptography
•
There are two keys:
a private key and a
public key.
•
The sender uses the
public key to
encrypt the
message and the
receiver uses the
private key to
decrypt the
message.
Public

key Cryptography
Advantages:
•
More efficient for short messages.
•
Removes the restriction of a shared symmetric key
between two entities.
•
Each entity is independent, and the pair of keys
created can be used to communicate with any
other entity.
•
Number of keys needed is reduced: For 1 million
users to communicate, only 2 million keys are
needed, not 500 billion, as in symmetric

key
cryptography.
Public

key Cryptography
Disadvantages
:
•
Complexity of the algorithm…the algorithm needs
large numbers to be effective.
•
Association between entity and its public key,
must be verified. (Can be overcome using a
certification authority)
Public

key Cryptography
•
The most common public

key algorithm is
called the RSA method after its inventors
(Rivest, Shamir, and Adleman).
•
The sender uses the following algorithm to
encrypt the message: C=P
e
mod N
•
The receiver uses the following algorithm to
decrypt the message: P=C
d
mod N
Example:
Enter the password to open this PDF file:
File name:

File size:

Title:

Author:

Subject:

Keywords:

Creation Date:

Modification Date:

Creator:

PDF Producer:

PDF Version:

Page Count:

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