
: Cryptography :
By definition cryptography is the process of converting recognisable data into
an encrypted code for transmitting it over a network (either trusted or
untrusted). Data is encrypted at the source, i.e. sender's end and decrypted
at the destination, i.e. receiver's end.
In all cases, the initial unencrypted data is referred to as plaintext.
It is encrypted into ciphertext, which will in turn (usually) be decrypted
into usable plaintext using different encryption algorithms.
The Purpose :
* Authentication : The process of proving one's identity.
* Privacy/confidentiality : Ensuring that no one can read
the message except the intended receiver.
* Integrity : Assuring the receiver that the received message
has not been altered in any way from the original.
* Nonrepudiation : A mechanism to prove that the sender
really sent this message.
In general cryptographic algorithms are classified into three categories
as follows :
1) Secret Key Cryptography (SKC) : Uses a single key for
both encryption and decryption.
2) Public Key Cryptography (PKC) : Uses one key for encryption
and another for decryption.
3) Hash Functions : Uses a mathematical transformation
to irreversibly "encrypt" information.
Secret Key Cryptography :
With secret key cryptography, a single key is used for both encryption and
decryption. Because a single key is used for both functions, secret key
cryptography is also called symmetric encryption.
Secret key cryptography algorithms that are in use today include :
1) Data Encryption Standard (DES) :
DES is a blockcipher employing a 56bit key that operates on 64bit blocks.
DES uses a key of only 56 bits, and thus it is now susceptible to "brute
force" attacks.
TripleDES (3DES) and DESX are the two important variants that strengthen
DES.
2) Advanced Encryption Standard (AES ) :
The algorithm can use a variable block length and key length; the latest
specification allowed any combination of keys lengths of 128, 192, or 256
bits and blocks of length 128, 192, or 256 bits.
3 ) International Data Encryption Algorithm
(IDEA) : Secretkey cryptosystem written by Xuejia Lai and
James Massey, in 1992 and patented by Ascom; a 64bit SKC block cipher using
a 128bit key. Also available internationally.
4) Rivest Ciphers : Named
for Ron Rivest, a series of SKC algorithms.
RC1 : Designed on paper but never implemented.
RC2 : A 64bit block cipher using variablesized keys designed
to replace DES. It's code has not been made public although many companies
have licensed RC2 for use in their products. Described in RFC 2268.
RC3 : Found to be breakable during development.
RC4 : A stream cipher using variablesized keys; it is
widely used in commercial cryptography products, although it can only be
exported using keys that are 40 bits or less in length.
RC5 : A blockcipher supporting a variety of block sizes,
key sizes, and number of encryption passes over the data. Described in RFC
2040.
RC6 : An improvement over RC5, RC6 was one of the AES Round
2 algorithms.
5) Blowfish : A symmetric
64bit block cipher invented by Bruce Schneier; optimized for 32bit processors
with large data caches, it is significantly faster than DES on a Pentium/PowerPCclass
machine. Key lengths can vary from 32 to 448 bits in length. Blowfish, available
freely and intended as a substitute for DES or IDEA, is in use in over 80
products.





