Introduction
There are
basically two types of symmetric cipher: Substitution Cipher, Transposition
Cipher.
Substitution
Cipher: A substitution is a technique in which each letter or bit of
the plaintext is substituted or replaced by some other letter, number or symbol
to produce cipher text. For Example, ABC ⟹ XYZ.
Types of
Substitution Cipher: Caesar Cipher, Monoalphabetic Cipher, Vigenère
Cipher, Playfair Cipher, One time pad cipher (Vernam cipher), Hill Cipher.
Transposition Cipher: In transposition technique, there is no replacement of alphabets or numbers occurs instead their positions are changed or reordering of position of plain text is done to produce cipher text. For Example, ABCDE ⟹ BADEC.
Types of Transposition Cipher: Rail Fence Cipher, Columnar Transposition Cipher.
One Time Pad Cipher
One-time
pad (OTP), also called Vernam-cipher or the perfect cipher, is a crypto
algorithm where plaintext is combined with a random key. The key is at least as
long as the message or data that must be encrypted.
Each key
is used only once, and both sender and receiver must destroy their key after
use. There should only be two copies of the key: one for the sender and one for
the receiver.
Rules of encryption is as follows
Step-1: Assign a number to each character like (a = 0, b = 1, c = 2, … z = 25). As per given table.
Step-2: Assign a number to each character of plain text and key according to alphabetical order. (From table of step-1)
Step-3: Add both the number (Corresponding plain-text character number and Key character number).
Step-4: Subtract the number from 26 if the added number is greater than 26. otherwise left it. Assign alphabets of numbers, it produces cipher text.
Practical issues in one time pad
cipher
The best
example of one-time pad is recharge voucher of any mobile company. All recharge
voucher having different key or code printed on it. Once that code entered into
mobile, customer will get talk time according to the voucher cost. If any
customer trying to use same code of voucher, he/she will get failure message. The
company is regenerating all keys or code in such a way that every recharge
voucher having new and unique code on it called one-time pad.
Large
random key cannot be generated. Key distribution and generation of keys can be
problematic.
To learn more about One time pad cipher, Click here
thanks sir
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