Dr. Susan Al Naqshbandi
S.alNaqshabandi@uvt.nl
 The word “Cryptography” is derived from Greek words
κρυπτός kryptós meaning “hidden” and γράφω gráfo
meaning “to write”.
 Cryptography is the science of using mathematics to
encrypt and decrypt data. Cryptography enables you to
store sensitive information or transmit it across insecure
networks (like the Internet) so that it cannot be read by
anyone except the intended recipient.
 Cryptography is the practice and study of hiding
information.
 In other words,
Cryptography is a method to enlock and unlock a
document using a personal key.
 Encryption
 Decryption
 Key
 The process of disguising a message in such a way as to hide
its substance is encryption.
 An encrypted message is cipher text.
 The process of turning cipher text back into plaintext is
decryption.
Plain Text
Encryption
Cipher Text
Decryption Plain Text
 A key is a value that works with a cryptographic algorithm
to produce a specific cipher text. Keys are basically really,
really, really big numbers. Key size is measured in bits; In
public key cryptography, the bigger the key, the more
secure the cipher text.
 Based on the type of key used, Cryptography is broadly
categorized into:
 Symmetric Key Cryptography (Private, Conventional)
 Asymmetric Key Cryptography(Public)
For a sender and recipient to communicate securely using
conventional encryption, they must agree upon a key and keep
it secret between themselves.
 Caesar Cipher:
An extremely simple example of conventional cryptography is a
substitution cipher. A substitution cipher substitutes one piece of
information for another.
 Suppose you have a message as:
“I HAVE SENT YOU THREE EUROS”
with a key = 3.
Assign Numerical equivalent to each letter:
A
B
C
D
E
F
G
H
I
J
K
L
0
1
2
3
4
5
6
7
8
9
10 11 12
N
O
P
Q
R
S
T
U
V
W X
Y
M
Z
13 14 15 16 17 18 19 20 21 22 23 24 25
 Now add your key to your message
from assigned numerical values from table
Key
T =19 + 3 = 22 = W (Converted letter)
Entire converted message would appear as:
“I HAVE SENT YOU THREE EUROES”
Encrypted Message
“L KDYH VHQW BRX WKUHH HXURHV”
Public key cryptography is an asymmetric scheme that uses a pair of
keys for encryption: a public key, which encrypts data, and a
corresponding private, or secret key for decryption.
PRIVATE KEYS
1)Only one key is used for
encryption & decryption.
2)Private keys are shared.
3)Known to only
communicating parties.
PUBLIC KEYS
1)Two keys are used, one for
Encryption and another is
used for decryption.
2)Public key pair is only
generated, not shared.
3)One is publicly exposed &
other is kept secret.
A cryptographic algorithm, plus all possible keys and all the
protocols that make it work comprise a cryptosystem. PGP is a
cryptosystem.
PGP (Pretty Good Privacy)
What is PGP?
PGP is an open source freely available software package for e-mail
security.
How PGP works
PGP combines some of the best features of both conventional and public key
cryptography. PGP is a hybrid cryptosystem. When a user encrypts plaintext with
PGP, PGP first compresses the plaintext. Then creates a session key, which is a onetime-only secret key. This key is a random number generated from the random
movements of your mouse and the keystrokes you type. This session key works
with a very secure, fast conventional encryption algorithm to encrypt the plaintext;
the result is cipher text. Once the data is encrypted, the session key is then
encrypted to the recipient's public key. This public key-encrypted session key is
transmitted along with the cipher text to the recipient.
Decryption works in the reverse. The recipient's copy of PGP uses
his or her private key to recover the temporary session key, which
PGP then uses to decrypt the conventionally-encrypted cipher text.
"There are two kinds of cryptography in this world:
cryptography that will stop your kid sister from reading
your files, and cryptography that will stop major
governments from reading your files.”
 Cryptographic strength is measured in the time and
resources it would require to recover the plaintext. The
result of strong cryptography is cipher text that is very
difficult to decipher without possession of the
appropriate decoding tool.
 One would think, then, that strong cryptography
would hold up rather well against even an extremely
determined cryptanalyst. Who's really to say? No one
has proven that the strongest encryption obtainable
today will hold up under tomorrow's computing
power.

E-business dimensions are expanding day by
day Its only cryptography which can provide
immunity to those transactions which are
likely to toll hundreds of billions of dollars per
year.
Thank You