Definition of cryptography
Cryptography
is the study of mathematical techniques related to aspects of information
security, such as confidentially or privacy, data integrity and entity
authentication. Cryptography is not the only means of providing information
security, but rather one set of techniques.

Cryptography 
Categories of cryptographic algorithm
There are main two types of
cryptographic algorithm.
1:  Symmetric key
2:  Asymmetric key
Symmetric key
1.
Sender and Receiver share a key.
2.
A secret piece of information used to encrypt or
decrypt the message.
3.
If a key is secret, than nobody other than
sender or receiver can read the
message
4.
If Alice
and bank each has secret key, than they may send each other
private
message.
5.
The task of privately choosing a key before
communication, however can
be problematic.
Asymmetric key
1.
Solves the key exchange problem by defining an
algorithm which uses
two keys, each of which can be use to encrypt the message.
2.
If one is used to encrypt a message, another key
must be used to
decrypt it.
3
This makes it possible to receive secure message
by simply publishing one
key (public key) and keeping another
secret (private key).
4
Any one may encrypt a message using public key,
but only the owner of
the public key is able to read it.
5
In this way Alice may send private message to owner of a
keypair (the
bank) by encrypting it using their
publickey. Only bank can decrypt it.
Goals of cryptography
The main
goals of cryptography are
1:  Confidentially or privacy
2:  Data integrity
3:  Authentication
4:  Nonrepudiation
Characteristics of a cryptographic algorithm
The main
characteristics of cryptographic algorithm are
1:  Level of security
2:  Performance
3:  Ease of implementation