Introduction to Cryptography

History of Cryptography

•Cryptography has roots that began around 2000 B.C. in Egypt

•Encryption methods evolved from being mainly for show into practical applications used to decorate tombto tell the story of the life of the deceased. The practice was not as much to hide the messages themselves, but to make them seem more noble, ceremonial, and majestic used to hide information from others.

•A Hebrew cryptographic method required the alphabet to be flipped so that each letter in the original alphabet is mapped to a different letter in the flipped alphabet. The encryption method was called atbash. An example of an encryption key used in the atbash encryption scheme is shown in following:

    ABCDEFGHI JK LMNOPQ R STU VW XYZ

    ZYXWVUTSR QP ONMLKJ I HGF ED CBA

•Around 400 B.C., the Spartans used a system of encrypting information by writing a message on a sheet of papyrus, which was wrapped around a staff. (This would look like a piece of paper wrapped around a stick or wooden rod.) The message was only readable if it was around the correct staff, which allowed the letters to properly match

What is Cryptography

•From Greek word Kryptos + graphy which means secret writing

•Art and science of achieving security by encoding messages to make them non-readable.

Basic Of Encryption :

Plain text -> Encryption -> Cipher Text -> Decryption -> Plain Text

Conceptual cryptography is to make readable message unreadable and so the opposite.

Goals of cryptosystem

•Cryptosystems can provide confidentiality, authenticity, integrity, and nonrepudiation services. It does not provide availability of data or systems. Confidentiality means that unauthorized parties cannot access information. Authenticity refers to validating the source of the message to ensure the sender is properly identified.  Integrity provide assurance that the message was not modified during transmission, accidentally or intentionally. Nonrepudiation means that a sender cannot deny sending the message at a later date, and the receiver cannot deny receiving it.

What is Cryptanalysis?

Technique of decoding messages from non-readable format back to readable format without knowing how they were initially converted from readable format to non-readable format.

Cryptology

•Combination of Cryptography and Cryptanalysis

•Cryptography + Cryptanalysis= Cryptology

Cryptography

•Plain text

    The original message or data that is fed into the algorithm as input.

    e.g -  This is the plain text.

• Encryption Algorithm

     The algorithm which converts the plain text

     into cipher text by various transpositions and substitutions.

•Secret Key

    The Key is a value independent of the plain text. The algorithm will produce a different output depending on the specific key being used at that time.

•Cipher Text

    The scrambled message produced as the output while changing the plain text as cipher text using encryption algorithm.

•Decryption Algorithm

    It takes the cipher text and the secret key and produces the original plain text.

Requirements for Secure Use of Encryption 

There are two requirements for secure use of encryption:

•Strong Encryption Algorithm

•

•Sender and Receiver must have copies of the secret key and should keep the key secure.

Cryptographic Systems are characterized  by three independent dimensions:

•Type of operations used for transforming plaintext to ciphertext.

•The number of keys used

The way in which the plaintext is processed

•Cryptanalysis

    Cryptanalytic attacks rely on nature of the algorithm

•Brute Force Attacks --> only possible combination are checked for

•Passwords : dictionary and brute force