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==[[2015_Summer_Cryptography_Paar|The Paar Lectures on Introductory Cryptography]]==
[http://slectures.projectrhea.org Slectures] by [[User:Agarwa65|Divya Agarwal]] and [[User:Khmarsh|Katie Marsh]] 
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<center><font size= 4>2. Introduction to Cryptography</font size>

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==Introduction to cryptography ==

The block diagram below shows an overview of the field of cryptology.
[[File:OverviewCrypt.png|400px|thumb|left|Fig 1: Overview of field of Cryptology]]

'''Cryptography''' is the science of secret writing with the goal to hide the orginal message.

'''Cryptanalysis''' is the science and sometimes art of breaking cryptosystems. 

Cryptography itself splits into three main branches:

==== 1. Symmetric Algorithms : ====
* Also known as private-key, single-key or secret-key cryptography

[[File:SymCrypt.png|400px|thumb|left|Fig 2: Symmetric Cryptography Basics]]

* In Fig 2, x is the plaintext message which Alice wants to send, y is the ciphertext message which has been encrypted, K is the key to encrypt or decrypt the message.
** Encryption equation : <math>y = e_k(x)</math>  , where <math>e_k(x)</math> is the encryption function  and,
** Decryption equation : <math>x = d_k(y)</math>  , where <math>d_k(x)</math> is the decryption function 

* Encryption and decryption are inverse operations if the same key K is used on both sides : 
  <math> d_k(y)= d_k(e_k(x))= x </math>

* '''However, the system is only secure if an attacker does not learn the key K!'''

==== 2. Asymmetric Algorithms : ====

* Also known as public-key cryptography.
* Unlike Symmetric cryptography, user possesses a secret key as well as a public key.
* We do not discuss this in detail in our slecture but use the references to learn more.

==== 3. Cryptographic Protocols : ====

* Roughly speaking, crypto protocols deal with the application of cryptographic algorithms. More details in the reference book.

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==Conclusion==

The main focus of slecture is on basics of cryptography uptil symmetric algorithms inlcuding understanding the maths behind it.
 
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==References==
* C. Paar. Understanding Cryptography. Lecture Notes. Dept. of Electr. Eng. and Information Sciences, Ruhr University.
* C. Paar and J. Pelzl. Understanding Cryptography. A textbook for Student and Practitioners. Springer 2010.
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==[[2015_Summer_Cryptography_Paar_Introduction to Crpytography_Divya Agarwal and Katie Marsh_comments | Questions and comments]]==
If you have any questions, comments, etc. please post them [[2015_Summer_Cryptography_Paar_Introduction to Crpytography_Divya Agarwal and Katie Marsh_comments|here]].
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