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Basic Techniques used in Cryptography

SUBSTITUTION:
This is amongst the simplest of techniques. The characters are mapped to different characters for example 'p' in a text message may be replaced by z. Such maps are generally reversible and hence no two characters(alphabets) would correspond to a same character.

PERMUTATION:
The alphabets are permuted in a fixed manner by first breaking the text message into blocks of equal size.
Example: secret permutation: 1-->2-->4-->1 . 3-->3
Text message: This is easy
Cipher: sTih siyesa
In this case, the spaces are also counted as characters. But mostly, we do not permute the spaces.

SHIFT:
Each alphabet will be shifted by some fixed number. Example shift by 3 gives:
a-->d, b-->e, . . .
Note: This is a special case of substitution.

ADDITION MODULO 26
Example:
text: ADDING
secret key: CAT
cipher: DEXLMA
Here, A+C=D,D+A=E,....,T+G(mod26) = A (When the addition of two alphabets crosses Z, take the remainder, i.e., again start from A as in : Y + C= B)

OTHER SIMPLE TECHNIQUES OF INFORMATION HIDING
Keep thinking. As you proceed, you will get the clear picture. :)
Note: A combination of more than one of he above mentioned techniques can also be used.

WAYS TO ATTACK
Often you will find some weaknesses in text. While substitution ciphers can be attacked by frequency analysis of alphabets(see wiki) permutations can be attacked by observing 'mini-anagrams'. Out of a cipher, one may be lucky enough to guess a word or its part permutations can be attacked by observing 'mini-anagrams'. Work backwards and try getting the key assuming the guess was right. If you start getting something meaningful, you are nearly done. Continue observing and thinking.

Have a good time.

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