Cryptography

• 1.
  • CRYPTOGRAPHY

• • Ms. Date Saroj

• • ME CSE (Part Time )

2.

• CRYPTOGRAPHY comes from the Greek words for secretwriting.

• It is the science of devising methods that allow information to be sent in a secure form in such a way that the only person, who possess the key can read it.

• It is the collection of algorithm & associated procedures for hiding & unhiding information

3.

• Plaintext :Messages to be encrypted.

• Cipher text: Output of encryption process.

• Key

• Intruder( Active & Passive)

• Channel

4. Encryption Model,E Channel Decryption Model,D Introduction to Cryptography Ek DK Intruder p L A I N T E XT P L A I N T E X T CIPHERTEXT Active Intruder PassiveIntruder FIG.THE ENCRYPTION MODEL(For a Symmetric-Key Cipher) 5.

• The messages to be encrypted ,known asPlaintext , are transformed by a key.

• The output of encryption process, known asCipher text , is the transmitted through thechannel .

• We assume that theintruder hears & accurately copies down the complete Cipher text. However, unlike the intended receiver, the intruder does not know what the decryption key is and so cant decrypt the Cipher text easily.

Cryptography 6. Cryptography

• Passive intruders can only listen to the communication channel

• Active intruders can modify the legitimate messages before they get to the receiver.

• Cryptanalysis:The art of breaking ciphers.

• Cryptography:The art of devising ciphers.

• &Cryptology means-----------

CRYPTOLOGY CRYPTANALYSIS CRYPTOGRAPHY = + 7.

• It will be often useful to have a notationfor relating plaintext, cipher text & Keys.

• Lets assume that ,the plaintext has n characters , drawn from an alphabet X.

• We represent the plaintext msg. as a vector of n symbolsX={x1,x2,x3,xn}

• Before transmitting, the sender Encrypts the msg.

• This is done by a rule that maps X into a sequence of new characters called as Cipher text.

• Y={y1,y2,y3,..,yn}

Notation: 8.

• The mapping from X to Y is based on anencryption rule,which we may write as

• Y=E k(X)

• The subscript k (Key) denotes a particular rule, employed in aciphersystem to determine the cipher text.

• The cipher text msg. is transmitted to thereceiver .

• In order to read the original msg.,the receiver must use another rule ,called adecryption rule .

Notation:Contd.. 9.

• This decryption rule can be looked as an inverse mappingdefined by encryption rule.

• X=Dk (Y)

Notation:Contd. Y = Ek (X) X = Dk (Y) X = Dk(Ek(X)) 10.

• Lets look at the methods by which cryptosystems can be attacked:

• 1. Cipher text-only attack

• 2. Known plaintext attack.

• 3. Chosen plaintext attack

• 4. Chosen cipher text attack.

• Regardless of any attack intruders task is to determine the key. If the key is known, the cryptosystem is broken.

11.

• Encryptions methods have historically been divided into two categories:

• 1. Substitution Ciphers

• 2.Transposition Ciphers

12.

• In a substitution cipher each letter or group of letters is replaced by another letter or group of letters.

• One of the oldest known ciphers is theCaesar Cipher . In this method a becomes D, b becomes E,.. , z becomes C.

• For exampleattack becomes DWWDFN.

• A generalization is shift cipher text alphabets by k letters.

13. Transposition Ciphers

• Substitution ciphers preserves the order of alphabets.

• Transposition ciphers reorder the letters .

14. Some simple cipher systems

• 1. The shift Cipher

• 2. The Vigenere Cipher

• 3. The permutation Cipher

• 4. The Substitution Cipher

15. Codes

• It is also possible to increase the security of encryption by encrypting the entire words, phrases, sentences, before the applying to the encryption model.

• Secret codebook is shared between sender & receiver.

• Even if the cipher has been broken , Intruderwill still face the problem of decoding it.

• Provides data compression to the plaintext.

16. Codescontd.

• The combination of coding & ciphering, if used properly, can produce a cryptosystem that is far more difficult to break than a cryptosystem that uses either.

• DISADVANTAGE

• To prevent from the attacks, the codebook needs to be changed frequently. But changing & securely communicating codebooks is difficult challenge. So, codes are not well adapted in modern communication systems.

17.

• THANK YOU !........