Notes

Cryptography..
scientific study of techniques for securing digital information, transactions, and distributed computations.

In the private-key setting, two parties share some secret information called a key, and use this key when they wish to communicate secretly with each other. A party sending a message uses the key to encrypt ( or "scramble" ) the message before it is sent, and the· receiver uses the same key to decrypt ( or "unscramble" ) and recover the message upon receipt.
Same key is used to encrypt and decrypt, reason why its known as symmetric key settings.

Syntax of encryption:
1. Key generation algorithm.
2. Encryption Algorithm.
3. Decryption Algorithm.

KERCKHOFF principle:
The cipher method must not be required to be secret, and it must be able to fall into the hands of the enemy without inconvenience.
According to this, even if the adversary knows the algorithm and every thing else the encryption is secure till the key is a secret.
ARGUMENTS in favor of above:
1. It will be easy to keep a key secret then the whole algorithm.
2. If the key is exposed it would be much easier to change the key than the whole algorithm.
3. If many pairs of people need to encrypt and communicate it would be much easier for them to use different keys rather the using a different algorithm.


ADVANTAGE of open Crypto design:
1. We cannot come up with a good encryption algorithm every time and the open one is tried and tested.
2. Security flaws to be reveled by ethical hackers then by malicious parties.
3. If security of system lies in security of code then reverse engineering might be a risk so better use a key which is irrelevant to the algorithm.

ATTACK scenarios:
1. Ciphertext only attack: Most basic adversary knows the cipher text and try guessing the original plain text.
2. Known Plaintext attack : Adversary came to know about few plain text cipher text pair and tries to know the plain text of the other cipher text under same encryption.
3. Chosen Plaintext attack: Adversary have the ability to convert any plain text of his choice to the cipher text and then tries to guess.
4.Chosen Cipher text attack : Adver­sary have capability to obtain the decryption of ciphertexts of its choice. Aim is to determine the plain­ text that was encrypted in some other ciphertext.


Ceaser Cipher: Shift the position of letters.--> encryption is done the same way and there is no secret key.
Shift Cipher: General case of caeser cipher.


Sufficient Key Space principle: Any encryption must have sufficient key space so that its nor vulnerable to exhaustive search.
Vigenere Cipher poly alphabet shift: Cipher using a word as key, and shift according to corresponding letter in key.
BREAKING:
if we know the length then after every n ( length ) a letter is encrypted using the same shift so can be done.
Kasiski method :If the text is long enough then identifying common bigrams and trigrams, and then if they are mapped to same word then the difference between the initial and final i will be a multiple of the key length.