Cryptography is a method of storing and transmitting data in a particular form so that only those for whom it is intended can read and process it.So these slides give you an introduction to cryptography and types of ciphers.

2. It is the practice and study of techniques for secure
communication in the presence of third parties.
Science and art of transforming messages to make them
secure and immune to attack.
The art of protecting information by transforming it ie;
encrypting it into an unreadable format is called
ciphertext.Only those who possess a secret key can decipher
the message into plain text.

3. Substitution ciphers
Transposition ciphers

4. It is a method of encoding by which units of plaintext are
replaced with ciphertext ,according to a fixed system;the
“units” may be single letters,pairs of letters,triplets of
letters,mixtures of the above and so forth.
The receiver deciphers the text by performing the inverse
substitution.

5. It is a simple data encryption scheme in which
plaintext characters are shifted in some regular pattern
to form cipher text.
In transposition ciphers letters are jumbled up together.
Highly secure

6. SUBSTITUTION CIPHERS
CAESAR
CIPHERS
AUTOKEY
CIPHERS
AFFINE
CIPHERS
VIGNERE
CIPHERS

7. Caesar cipher involves replacing each letter of alphabet
with the letter standing three places further down the
alphabet.
Encryption
Algorithm: C= (p+3)mod26
Decryption
Algorithm: P= (c-3)mod26

8. In an Autokey cipher the plain text itself is used
as a keyword with a slight modification.
Encryption
We have to start the keyword with a short seed,
Generally a plain text followed by a single letter.
Then write that seed letter below to first letter and plain
text itself is written below all other letters. Then sum up
the corresponding digital equivalence of the letters, and
convert it into cipher text.

9. Decryption
Decryption is done by rewriting to each numerical term of
both plaintext and cipher text. Suppose the plain text has
digital equivalent cipher text is c1,c2,……..cn.
If S indicate the numeral for C then the plain text number is,
P1 = (c1-S) mod26
Pk = (ck- P k-1)mod 26

10. Example of Monoalphabetic Cipher.
Here the encryption process is substantially mathematical.
Encryption
The first step in the encryption process is to transform each
of the letters in the plaintext alphabet to the corresponding
integer in the range 0 to m-1. With this done, the
encryption process for each letter is given by
E(x) = (ax + b) mod m Where a and b are the key for the cipher.

11. Decryption
In deciphering the cipher text, we must perform the
opposite (or inverse) functions on the cipher text to
retrieve the plaintext. Once again, the first step is to
convert each of the cipher text letters into their integer
values. We must now perform the following calculation
on each integer
D(x) = c(x - b) mod m
Where c is the modular multiplicative inverse of a.

12. The Vigenere Cipher is an adaptation of the Trithemius
Cipher, but instead of systematically progressing through the
cipher text alphabets in the Tabula Recta, it uses a keyword to
pick which columns to use.

13. Encryption
A sequence of n letters with numerical equivalence b1,b2,...bn will serve as
keyword. The plane text message is expressed as p1,p2....pn say pi.Then
conversion to cipher text using the congruence relation.
Ci=Pi +bi (mod26) ; i=1,2....n
For eg: plaintext message is “a simple example”
Decryption
Deciphering is carried out by the relation
Pi =Ci –bi (mod26)
Plain text a s i m p l e e x a m p l e
Key
stream
b a t t i s t a b a t t i s

14. TRANSPOSITION CIPHERS
RAIL
FENCE
CIPHERS
ROUTE
CIPHERS
COLUMNAR
TRANSPOSITION
CIPHER
MYSZKOWSKI
TRANSPOSITION
CIPHER

15. The Rail fence cipher is an easy to apply transposition cipher
that jumbles up the order of the letters of a message in a quick
convenient way.
It also has the security of a key to make it a little bit harder to
break.
The Rail Fence cipher works by writing your message on
alternate lines across the page, and then reading off each line in
turn.

16. Encryption
Write the plaintext message in zigzag lines across the page,
and then read off each row.
 Firstly, we need to have a key, which for this cipher is the
number of rows you are going to have. then start writing the
letters of the plaintext diagonally down to the right until you
reach the number of rows specified by the key. then bounce
back up diagonally until we hit the first row again. This
continues until the end of the plaintext.
Eg:

17. Decryption
We start writing the message, but leaving a dash in place of
the spaces yet to be occupied. Gradually, you can replace all
the dashes with the corresponding letters, and read off the
plaintext from the table.
We start by making a grid with as many rows as the key is,
and as many columns as the length of the cipher text. We then
place the first letter in the top left Square, and dashes
diagonally downwards where the letters will be. When we get
back to the top row, we place the next letter in the cipher text.
Continue like this across the row, and start the next row when
you reach the end.

18. The Route Cipher is a transposition cipher where the key is which
route to follow when reading the cipher text from the block created
with the plaintext. The plaintext is written in a grid, and then read
off the following route chosen
Encryption
 First we write the plaintext in a block of reasonable size for the
plaintext. Part of your key is the size of this grid, so you need to decide
on either a number of columns or number of rows in the grid before
starting. Once the plaintext is written out in the grid, you use the Route
assigned. This could be spiralling inwards from the top right corner in a
clockwise direction, or zigzagging up and down.

19. Decryption
To decrypt a message received that has been encoded with the
Route Cipher, we need to know the route used and the width
or height of the grid. We then start by constructing a blank
grid of the right size, and then place the cipher text letters in
the grid following the route specified.

20.  Columnar Transposition involves writing the plaintext out in rows, and then reading
the cipher text off in columns.
 In its simplest form, it is the Route cipher where the route is to read down each
column in order.
Encryption
 We first pick a keyword for our encryption. We write the plaintext out in a grid
where the number of columns is the number of letters in the keyword. We then title
each column with the respective letter from the keyword. We take the letters in the
keyword in alphabetical order, and read down the columns in this order. If a letter
is repeated, we do the one that appears first, then the next and so on.
 Eg:

21. Decryption
The decryption process is significantly easier if nulls have been
used to pad out the message in the encryption process. Below
we shall talk about how to go about decrypting a message in
both scenarios.
Firstly, if nulls have been used, then you start by writing out the
keyword and the alphabetical order of the letters of the
keyword. You must then divide the length of the cipher text by
the length of the keyword. The answer to this is the number of
rows you need to add to the grid. You then write the cipher text
down the first column until you reach the last row. The next
letter becomes the first letter in the second column (by the
alphabetical order of the keyword), and so on.

22.  The same methodology as for Columnar Transposition is
used, where the plaintext is written out in rows under the
keyword. The only difference is that when there are repeated
Letters in the keyword, rather than number them from left to
right, give all the same letters the same number.
 Then read across columns which have the same number in
the keyword.

23. Encryption
We have to choose our keyword for the encryption process first.
We then write out the plaintext in a grid, where the number of
columns in the grid is the number of letters in the keyword. We
then number each letter in the keyword with its alphabetical
position, giving repeated letters the same numbers. We then start at
number 1 (the first letter alphabetically in the keyword), and if it is
the only appearance of 1, we read down the column just like
in Columnar Transposition. If, however, the number 1 appears
more than once, we read from left to right all the first letters of the
columns headed by 1. Then we move to the next row, and read
across, left to right, the letters in the rows headed by 1. Once
complete, we move on to the number 2, and so on.

24. Decryption
The decryption process is very similar to Columnar
Transposition. We shall go through how to do it if nulls have
been used to fill spaces. By comparing this method with that
given in Columnar Transposition when nulls are not used, you
should be able to work out what to do.
We start by writing out the keyword, and the alphabetical order
of the letters, remembering to give repeated letters the same
number. We then divide the length of the cipher text by the
length of the keyword to work out how many rows we need to
add to our grid. We then have to systematically put the cipher
text back in to the grid. Start at number 1, and continue to the
highest number. If the number only appears once, we fill down
the column. If the number appears twice, we move from left to
right across the columns with that number heading them.