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NETWORK SECURITY:CRYPTOGRAPHY-I


Traditional Cryptography:

Cryptography is the study of message secrecy.Cryptology term has been derived from Greek κρυπτός kryptós "hidden," and the verb γράφω gráfo "write".
Cryptography referred almost exclusively only to encryption, the process of converting ordinary information (plaintext) into unintelligible gibberish (ie, ciphertext). Decryption is the reverse, moving from unintelligible ciphertext to plaintext. A cipher (or cypher) is a pair of algorithms which perform this encryption and the reversing decryption. The detailed operation of a cipher is controlled both by the algorithm and, in each instance, by a key. This is a secret parameter (known only to the communicants) for a specific message exchange context. Keys are important as ciphers without variable keys are trivially breakable, and so rather less than useful for most purposes. Historically, ciphers were often used directly for encryption or decryption, without additional procedures such as authentication or integrity checks.

THE PURPOSE OF CRYPTOGRAPHY:

Cryptography is the science of writing in secret code and is an ancient art; the first documented use of cryptography in writing dates back to circa 1900 B.C. when an Egyptian scribe used non-standard hieroglyphs in an inscription. Some experts argue that cryptography appeared spontaneously sometime after writing was invented, with applications ranging from diplomatic missives to war-time battle plans. It is no surprise, then, that new forms of cryptography came soon after the widespread development of computer communications. In data and telecommunications, cryptography is necessary when communicating over any untrusted medium, which includes just about any network, particularly the Internet.

TRADITIONAL CRYPTOGRAPHY:

In earlier days,Cryptography was done by two process:1)Substitution Ciphers,and 2)Transposition Ciphers.

Substitution Ciphers: In this technique of cryptography,each letter or group of letters is replaced by another letter or group of letters to disguise it.
Example:
plain text: abcdefghijklmz
cipher text:qwertyuiopasd (set of letters of keyboard.)

Transposition Ciphers: Substitution ciphers preserve the order of the plaintext symbols but disguise them.Transposition ciphers,in contrast,reorder the letters but don't disguise them

Principles Of Cryptography:
The first principle is that all encrypted messages must contain some redundancy,that is,information not needed to understand message,ie some extra bytes are reserved ,except from the data byte memory storage.
The second Cryptographic principle is that some measures must be taken to prevent active intruders from playing back old messages.
How to crack the simple matrix transposition ciphers:

1)Count how many letters are in the ciphertext (for this example, assume the ciphertext is 99 letters long)
2)Make all of the matrices that would fit such a length (e.g. 2x50, 3x33, 4x25, 5x20, 6x17, 7x15, 8x13, 9x11, 10x10). Use TWO of each size.
3)For each size matrix, write out the ciphertext across the rows on one copy. On the other copy, write out the ciphertext down the columns.
4)At each stage, see if you can find anything legible, reading perpendicular to how you put the ciphertext in.

A harder version of the matrix transposition cipher is the column-scrambled matrix transposition cipher. Just like the ones above, you find a matrix of suitable dimensions and write your text in row-by-row. If there are blank cells left, fill them in with a dummy character (sometimes an 'X'). However, before writing down the ciphertext from the columns, you first scramble the columns. This generates a new matrix of the same size. Now read off the text down the columns, as before. This is a harder cipher, but there is a systematic way to crack it.


Now try to break this:


YRANHOVEHOOOTRIOOORYMCBKOITOEROAUEAIDUURENRDGEFOORINSEUSTU
SHBKSESLYAGEEEAORDENTSAMEGYARTCSHDBKMCSTDREEAGHI


Answer will be published in next posting......

1 comments:

  nuaim

May 3, 2007 at 10:40 PM

thanks for this info..you make a good job...i have to take a few minute to study about your writing..its too usefull information..thanks aman