Substitution ciphers : Here each letter reains it's position, but is substituted by another letter. A three place shift left forms the cipher in the example below.

Plain alphabet : a b c d e f g h i j k l m n o p q r s t u v w x y z

Cipher alphabet : d e f g h i j k l m n o p q r s t u v w x y z a b c

Message : this is our secret

Encrypted message : vjku ku qwt ugetgv

Crypt analyst uses short cut techniques like frequency analysis to decrypt (decipher) a message. The frequency of occurance of English alphabets in a message are as etaonrlshdifcmugpywbvkrjqz.

So 'e' is the most frequently used alphabet, while 'z' is used less. Code books are used to decode messages. Normally, there will be an understanding between the sender and receiver about the code of the algorithm used in encrypting.

e.g.- Lion means Head of the organization, off shoot means department.