This slide helps you to find the idea of Symmetric and asymmetric key cryptography.

1. NORTH WESTERN UNIVERSITY
Computer Science & Engineering
Course Titel: Computer Networks
Course Code: CSE-3205
Presentation on: Symmetric and Asymmetric
key cryptography. Submitted by:
Name : MD. MONIRUL ISLAM
ID: 20151116010
Submitted to:
Name : Romana Rahman
Senior Lecturer
Computer Science & Engineering
North Western University , Khulna

2. INDEX:
* General Introduction of Cryptography.
* Objectives
* Types of Cryptography.
* Symmetric Cryptosystems.
* Limitations of Symmetric Cryptography
* Asymmetric Cryptosystems.
* Limitations of Asymmetric Cryptography.
* References.

3. General Introduction of Cryptography
Cryptography is a method of protecting information and communications through the use of
codes, so that only those for whom the information is intended can read and process it.
It has two main Components:
1. Encryption
– Practice of hiding messages so that they can not be read by anyone other than
the intended recipient
2. Authentication
– Ensuring that users of data/resources are the persons they claim to be and that
a message has not been surreptitiously altered

4. How does it work?
 Two functions are needed:
Fig 01: Encoder & Decoder

5. Example
 encoder function is next letter in the alphabet.
 decoder function is …
Fig 02: Encoder & Decoder

6. Encryption and Decryption
- Plain text
- Cipher text
 Encoding :- The contents of the message (the plain text) in such a way that hides
its contents from outsiders is called encryption.
 The process of retrieving the plaintext from the cipher-text is called
decryption.
 Encryption and decryption usually make use of a key, and the coding method is
such that decryption. Can be performed only by knowing the proper key.

7. Objectives
Cryptography has four main objectives:
 Confidentiality of the message: Only the authorized recipient should be able to extract the
content of the cypher. In addition, obtaining information about the content of the message
(such as a statistical distribution of certain characters) should not be possible, once the
cryptographic analysis becomes easier.
 Message integrity: The recipient must be able to determine if the message was altered
during transmission.
 Authentication of the sender: The recipient should be able to identify the sender and verify
if it was him who sent the message.
 Irrevocability of the sender: It should not be possible to deny the authorship of the
message.

8. Types of Cryptography
Two kinds of Cryptography:
Symmetric
1) Alice and Bob agree on a
cryptosystem
2) Alice and Bob agree on a key.
3) Alice takes her plaintext
message and encrypts it using
the encryption algorithm and the
key. This creates a cipher text
message.
4) Alice sends the cipher text
message to Bob.
5) Bob decrypts the cipher text
message with the same algorithm
and key and reads it.
Asymmetric
1) Alice and Bob agree on a
public-key cryptosystem.
2) Bob sends Alice his public
key.
3) Alice encrypts her message
using Bob’s public key and
sends it to Bob.
4) Bob decrypts Alice’s message
using his private key.

9. Symmetric Cryptosystem
Both the sender and receiver share a single key. The sender uses this key to encrypt plaintext and send
the cipher text to the receiver. On the other side the receiver applies the same key to decrypt the
message and recover the plain text.
Material to
keep secret:
Text
Data
Audio
Video
Graphics
Material to
keep secret:
Text
Data
Audio
Video
Graphics
Lock
Encryption
Algorithm
Decryption
Algorithm
Shared Key
Sender and recipient Must both know
the key.
This is a weakness!
Fig 03: Symmetric Cryptosystem

10. Traditional ciphers
Traditional cipher are two types
 Simple substitution Cipher.
 Transposition Cipher.
Fig 04: Traditional Ciphers

11. Simple Substitution Ciphers
It is an improvement to the Caesar Cipher. Instead of shifting the alphabets by some number, this
scheme uses some permutation of the letters in alphabet.
For example, A.B…..Y.Z and Z.Y……B.A are two obvious permutation of all the letters in
alphabet. Permutation is nothing but a jumbled up set of alphabets.
With 26 letters in alphabet, the possible permutations are 26! (Factorial of 26) which is equal to
4x1026. The sender and the receiver may choose any one of these possible permutation as a cipher
text alphabet. This permutation is the secret key of the scheme.

12. Process of Simple Substitution Cipher
1.Write the alphabets A, B, C,...,Z in the natural order.
2. The sender and the receiver decide on a randomly selected permutation of the letters of the alphabet.
3. Underneath the natural order alphabets, write out the chosen permutation of the letters of the alphabet. For
encryption, sender replaces each plaintext letters by substituting the permutation letter that is directly beneath it in
the table. This process is shown in the following illustration. In this example, the chosen permutation is K,D, G, ...,
O. The plaintext ‘point’ is encrypted to ‘MJBXZ’.
Here is a jumbled Cipher text alphabet, where the order of the cipher text letters is a key.
On receiving the cipher text, the receiver, who also knows the randomly chosen permutation, replaces each cipher
text letter on the bottom row with the corresponding plaintext letter in the top row. The cipher text ‘MJBXZ’ is
decrypted to ‘point’.

13. Mono alphabetic Cipher
Mono alphabetic cipher is a substitution cipher in which for a given key, the cipher alphabet for
each plain alphabet is fixed throughout the encryption process.
A simple example is where each letter is encrypted as the next letter in the alphabet: "a simple
message" becomes "B TJNQMF NFTTBHF". In general, when performing a simple
substitution manually, it is easiest to generate the cipher text alphabet first, and encrypt by
comparing this to the plaintext alphabet. The table below shows how one might choose to, and
we will, lay them out for this example.

14. Polyalphabetic Cipher
In a polyalphabetic cipher, each occurrence of a character can have a different substitute. The relationship
between a character in the plaintext to a character in the cipher text is a one-to-many relationship.
As an example we shall encrypt the plaintext "Leon Battista albert". To keep with the convention of writing cipher text in
uppercase, we shall invert Alberta's own rule, and use lowercase letters to signify the change.
We start by referencing the starting position of the cipher disc, which in this case is "a" is encrypted as "V", so we start the
Cipher text with a lowercase "v". We then encrypt the first few letters as a Caesar Shift, using the cipher text alphabet
given below.
The 1st shift used, as the disk in shown
Plaintext: leonbat...
Cipher text: vGZJIWVOg...

15. The uppercase letters above encrypt the plaintext letters given. The "v" indicates the starting position of the disc, and
the "g" indicates that we need to change the position so that "G" is beneath "a". We then get the new cipher text
Alphabet as shown below.
Plaintext: ...tistaa...
Cipher text: ...gZOYZGGm...
This time the plaintext letters are encrypted to the cipher text letters using the cipher text alphabet above, and the "m“
indicates that we are changing alphabet again to get the final cipher text alphabet below.
Plaintext: ...lberti
Cipher text: ...mXNQDFU
So we get the final cipher text "vGZJIWVOgZOYZGGmXNQDFU".

16. Transposition Cipher
Transposition Ciphers are a bit different to Substitution Ciphers. Whereas Substitution ciphers replace each
letter with a different letter or symbol to produce the cipher text, in a Transposition cipher, the letters are just
moved around.
The letters or words of the plaintext are reordered in some way, fixed by a given rule (the key).
One example of a transposition cipher, is to reverse the order of the letters in a plaintext. So "a simple
example" becomes "ELPMAXE ELPMIS A". Another, similar, way to encrypt a message would be to reverse
the letters of each word, but not the order in which the words are written. In this case "a simple example“
becomes "A ELPMIS ELPMAXE". Both of these are available in the activity at the bottom of the page.

17. Limitations of Symmetric Cryptography
 Many people need to communicate with a server (many-to-one communications).
 cannot keep server key secret for long.
 Once the secret key is compromised, the security of all subsequent.
messages is suspect and a new key has to be generated.
 Authentication service must know private key.
 privacy implications---someone else knows your key.
 two possible points of attack.
 changing authentication service requires a new key.
 Digital signatures are difficult.
 Crossrealm authentication.
 accessing services outside the domain or realm of your authentication server is problematic.
 requires agreement and trust between authentication services.
 introduces another potential point of attack.

18. Asymmetric Cryptosystems
In asymmetric-key cryptography, users do not need to know a symmetric shared key; everyone
• shields a private key and
• advertises a public key
Material to
keep secret:
Text
Data
Audio
Video
Graphics
Material to
keep secret:
Text
Data
Audio
Video
Graphics
Lock
Encryption
Algorithm
Decryption
Algorithm
Decryption keyEncryption Key
Sender knows public key
Recipient knows private key.
Fig 05: Asymmetric Cryptosystem

19. RSA Cryptography
RSA algorithm is a public key encryption technique and is considered as the most secure way of encryption.
It was invented by Rivest, Shamir and Adleman in year 1978 and hence name RSA algorithm.
Process of RSA
 Choose two large prime numbers p & q
 Compute n= p q and z=(p-1)(q-1)
 Choose number e, less than n, which has no common factor (other than 1) with z
 Find number d, such that e d – 1 is exactly divisible by z Keys are generated using n, d, e
-Public key is (n, e)
-Private key is (n, d)
 Encryption: c = me mod n
-m is plain text
-c is cipher text
 Decryption: m = cd mod n
 Public key is shared and the private key is hidden

20. P=5 & q=7
n=5*7=35 and z=(4)*(6) = 24
e = 5
d = 29 , (29x5 –1) is exactly divisible by 24
Keys generated are
-Public key: (35,5)
-Private key is (35, 29)
Encrypt the word love using (c = me mod n)
-Assume that the alphabets are between 1 & 26

21.  Decrypt the word love using (m = cd mod n)
-n = 35, c=29

22. Limitations of Asymmetric Cryptography
 It is a slow process.
 Its public keys are not authenticated.
 It risks loss of private key, which may be irreparable.
 It risks widespread security compromise.

23. References
[1] Thomas H. Cormen, Charles E. Leiserson, Ronald L. Rivest, and Clifford Stein. Introduction to Algorithms. MIT Press
and McGraw-Hill, 2001. ISBN 0-262-03293-7. Section 31.7: The RSA public-key cryptosystem, pp.881–887
[2] Bruce Schneier. Applied Cryptography. John Wiley & Sons, Inc. 1996. (ISBN: 0471128457)
[3] Bob Thibadeau http://dollar.ecom.cmu.edu/sec/lec02.ppt.
[4] [2]Data Communications and Networking By Behrouz A.Forouzan.

24. THANK YOU