Chapter 2:-BASIC CRYPTOGRAPHY

What's Cryptography?

Cryptography Definition:

Cryptography is defined as a Method of Transmitting and Storing Information/data in an unreadable format, to ensure the Confidentiality of the data. Main objective of Cryptography is to hide the data from unauthorised persons.

Cryptography Concept:
The Encryption concept is shown below. Here the plain text is modified in to a Cipher Text. Human and Machines can't read the Cipher text.
To read the data, we need to convert this Cipher text into plain text format. The process of converting Cipher text to Plain text is called Decryption

STEGANOGRAPHY

Cryptography Standards

• Encryption (Asymmetric and Symmetric)
• Hashing
• Public Key Infrastructure
• Digital Signature
• Wirereless Security

Asymmetric Key Encryption Algorithms

• Elliptical Curve Cryptography
• RSA
• DSA
• Diffie-Hellman
• El Gamal

Symmetric Key Encryption Algorithms

• DES, 3DES
• AES
• Blowfish,
• RC4, RC5 and RC6

Hashing Algorithms

• MD4
• MD5
• SHA
• HAVAL
• Digital Signature Standard
• DSS
• RSA
• PKI Standards
• X.509 Public Key Certificates
• Wireless Security Standard
• WEP
• WPA
• WPA2

There are two types of Encryption algorithms based on the Key used for encryption and decryption. They are,

#symmetric (single key, or private-key encryption)
#asymmetric (two-keys, or public-key encryption)

 

Cryptanalytic Attacks

*ciphertext only -only know algorithm & ciphertext, is statistical, know or can identify plaintext
*known plaintext -know/suspect plaintext & ciphertext
*chosen plaintext -select plaintext and obtain ciphertext
*chosen ciphertext -select ciphertext and obtain plaintext
*chosen text -select plaintext or ciphertext to en/decrypt
 
Brute Force Search

 

Caesar Ciphers

*One of the earliest substitution cipher described by Julius Caesar in the Gallic Wars.

*In this cipher each of the letters A to W is encrypted by being represented by the letter that occurs three places after it in the alphabet.
*Although Caesar used a ‘shift’ of 3, a similar effect could have been achieve using any number from 1 to 25.
*In fact any shift is now commonly regarded as defining a Caesar Cipher.
*The encryption key and decryption key are both determined by a shift but the encryption and decryption rules are different.
*We could have changed the formulation slightly to make the two rules coincide and have different encryption and decryption keys.
*A shift of 26 has the same effect as a shift of 0 and, for any shift from 0 to 25, encryption with that shift is the same as decryption with the new shift obtained by subtracting the original shift from 26.
E.g: encryption with shift 8 is the same as decryption with shift 26 - 8 =18.


Example:
YMJ KPJQ UWNHJ BNQQ NSHWJFXJ YT WH KTZW GD SJCY BJJP

*A single key search may not identify the key uniquely.

*It is much more likely merely to limit the number of possibilities by eliminating some obviously wrong ones.
*An exhaustive search for the encryption key for cryptogram HSPPW yields two possibilities that lead to complete English words for the assumed message.
*These shifts are 4, that gives DOLLS, and 11, that gives WHEEL.
*When this happens we need more information, possibly the context of the message, or some extra ciphertext, before we can determine the key uniquely.


How to make it more secure
#rather than just shifting the alphabet
#could shuffle (jumble) the letters arbitrarily
#each plaintext letter maps to a different random ciphertext letter
#hence key is 26 letters long

Plain: abcdefghijklmnopqrstuvwxyz
Cipher: DKVQFIBJWPESCXHTMYAUOLRGZN
Plaintext: ifwewishtoreplaceletters
Ciphertext: WIRFRWAJUHYFTSDVFSFUUFYA

English Letter Frequencies

Use in Cryptanalysis

*key concept - monoalphabetic substitution ciphers do not change relative letter frequencies

*discovered by Arabian scientists in 9th century
*calculate letter frequencies for ciphertext
*compare counts/plots against known values
*if caesar cipher look for common peaks/troughs
#peaks at: A-E-I triple, NO pair, RST triple
#troughs at: JK, X-Z
*for monoalphabetic must identify each letter
#tables of common double/triple letters help


Example Cryptanalysis

Chipertext:

UZQSOVUOHXMOPVGPOZPEVSGZ

WSZOPFPESXUDBMETSXAIZVUEPHZHM

DZSHZOWSFPAPPDTSVPQUZWYMXU

ZUHSXEPYEPOPDZSZUFPOMBZWPF

UPZHMDJUDTMOHMQ

*count relative letter frequencies (see text)

*guess P & Z are e and t
*guess ZW is th and hence ZWP is the
*proceeding with trial and error finally get:
-it was disclosed yesterday that several informal but direct contacts have been made with political representatives of the viet cong in moscow

Vigenère Ciphers

Vigenere Table