1. Cryptography

2. Basic Terminology plaintext - original message
ciphertext - coded message
cipher - algorithm for transforming plaintext to ciphertext
key - info used in cipher known only to sender/receiver
encipher (encrypt) - converting plaintext to ciphertext
decipher (decrypt) - recovering ciphertext from plaintext
cryptography - study of encryption principles/methods
cryptanalysis (codebreaking) - study of principles/ methods of deciphering ciphertext without knowing key

3. Introduction
In a simplest form, encryption is to convert the data in some unreadable form.
This helps in protecting the privacy while sending the data from sender to receiver.
On the receiver side, the data can be decrypted and can be brought back to its original form.
The reverse of encryption is called as decryption.

4. Introduction

5. Introduction

6. Principles Authentication
This is another important principle of cryptography.
In a layman’s term, authentication ensures that the message was originated from the originator claimed in the message.

7. Principles
Integrity
Now, one problem that a communication system can face is the loss of integrity of messages being sent from sender to receiver.
This means that Cryptography should ensure that the messages that are received by the receiver are not altered anywhere on the communication path.
This can be achieved by using the concept of cryptographic hash.

8. Principles Non Repudiation
happens if Alice sends a message to Bob but denies that she has actually sent the message?
Cases like these may happen and cryptography should prevent the originator or sender to act this way.
One popular way to achieve this is through the use of digital signatures.

9. Basic Encryption Methods
Two categories:
Substitution Ciphers: each letter or group of letters is replaced by another letter or another group of letters. It preserves the order of the plaintext symbol but disguise them.
E.g., Caesar cipher; mono-alphabetic substitution (each letter map onto another letter; 26!=4x10^26 possible keys)

10. Basic Encryption Methods
Transposition Ciphers: Reorder the letters but do not disguise them.
E.g., Rail Fence cipher m=“HELLOWORLD” distribute the letter up and down between two rows from left to right; then output row-wise.  H L O O L  c=“HLOOLELWRD” E L W R D

11. Columnar Transposition Ciphers
A transposition cipher. Keyed by a phrase such as “MEGABUCK”. The letter in the key indicated the order of columns to be output. Plaintext horizontally read in, ciphertext read out column by column.

12. Keys Symmetric Asymmetric Same key for encryption and decryption
Key distribution problem
Asymmetric
Mathematically related key pairs for encryption and decryption
Public and private keys

13. Kerckhof’s Principle
“All crypto algorithms must be public; only the keys are secret”
Trying to keep the algorithm secret (security by obscurity principle?) never works. Reasons:
Logistic issue: Too much effort to invent, test, and install new algorithm
“Publish the algorithm and let academic cryptologists to break the system. If no one succeeded in 5 years, it must be pretty solid.”
Real secrecy in the key, its length a major design issue

14. Key Length and Work Factor
2 digit key  100 combinations.
6 digit key  1 million combinations.
64 bit keys to prevent kid brother from reading your .
128 bit keys for routine commercial use
>256 bits keep major governments at bay.

15. One Time Pad
The One Time Pad encryption method is a binary additive stream cipher
Here a stream of truly random keys is generated and then combined with the plain text for encryption or with the ciphertext for decryption by an ‘exclusive OR’ (XOR) addition.
There are few conditions to be followed.

16. One Time Pad The key must be as long as the plain text.
„The key must be truly random.
„The key must only be used once.
To decipher the message, a person must have a copy of the one-time pad to reverse the process.
A one-time pad should be used only once (hence the name) and then destroyed.

17. Types of Cryptography Crypto often uses keys: Public key cryptography
Algorithm is known to everyone
Only “keys” are secret
Public key cryptography
Involves the use of two keys
Symmetric key cryptography
Involves the use one key
Hash functions
Involves the use of no keys
Nothing secret: How can this be useful?

18. Symmetric Key Cryptography

19. Symmetric Key Cryptography
This type of cryptography technique uses just a single key.
The sender applies a key to encrypt a message while the receiver applies the same key to decrypt the message.
Since only single key is used so we say that this is a symmetric encryption.

20. Types of Symmetric Cryptography
Stream ciphers
encrypt one bit at time
Block ciphers
Break plaintext message in equal-size blocks
Encrypt each block as a unit

21. Public Key Cryptography

22. Public Key Cryptography
This type of cryptography technique involves two key crypto system in which a secure communication can take place between receiver and sender over insecure communication channel.
Since a pair of keys is applied here so this technique is also known as asymmetric encryption.

23. Hash Function This technique does not involve any key.
Rather it uses a fixed length hash value that is computed on the basis of the plain text message.
Hash functions are used to check the integrity of the message to ensure that the message has not be altered, compromised or affected by virus.

24. Hash Function
Hashing is the transformation of a string of characters into a usually shorter fixed-length value or key that represents the original string.
Hashes play a role in security systems where they're used to ensure that transmitted messages have not been tampered with.
Hashing is used in conjunction with authentication to produce strong evidence that a given message has not been modified.

25. Hash Function
Encoding is often confused with encryption and hashing. They are not the same. But before I go into the differences, I'll first mention the similarities:
All three transform data into another format.
Both encoding and encryption are reversible, unlike hashing.

26. Hash Function
Hashing serves the purpose of ensuring integrity, i.e. making it so that if something is changed you can know that it's changed.
Technically, hashing takes arbitrary input and produce a fixed-length string that has the following attributes:
The same input will always produce the same output.
Multiple disparate inputs should not produce the same output.

27. Hash Function
It should not be possible to go from the output to the input.
Any modification of a given input should result in drastic change to the hash.

28. Hashing Example