Sankalchand Patel College of Engineering, Visnagar B.E. Semester V (CE/IT) INFORMATION SECURITY Practical List

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1 1. IMPLEMENT CAESAR CIPHER WITH VARIABLE KEY It is an encryption technique in which each plaintext letter is to be replaced with one a fixed number of places (in following implementation, key) down the alphabet. This example is with a shift of three (key=3), so that a B in the plaintext becomes E in the ciphertext. 2. IMPLEMENT SIMPLE TRANSPOSITION - PERMUTATION It is an encryption technique in which plaintext letters are permuted in a predefined fashion. For example, as shown in implementation, if the key is , first letter is shifted to sixth position; second letter is shifted to fourth position and so on. Example: Message: welcome Key: Encrypted Text: lecemw Decrypted Text: welcome 3. IMPLEMENT RAIL FENCE CIPHER The Rail Fence cipher is a form of transposition cipher that gets its name from the way in which it is encoded. In the rail fence cipher, the plaintext is written downwards on successive "rails" of an imaginary fence, then moving up when we get to the bottom. The message is then read off in rows. For example, using three "rails" (level=3) and a message of 'SKY IS THE LIMIT, encipherment would be: S Y S H L M T K I T E I I And would be read as (One rail at a time from top to bottom): SYSHLMTKITEII Page 1

2 4. IMPLEMENT 6 X 6 PLAYFAIR CIPHER The technique encrypts pairs of letters (digraphs). The Playfair cipher uses a 5 by 5 table containing alphabetic characters. (6 x 6 Playfair includes all alphabet characters (a to z) and numbers (0 to 9)). To generate the key table, one would first fill in the spaces in the table with the letters of the keyword (dropping any duplicate letters), then fill the remaining spaces with the rest of the letters of the alphabet in order (usually omitting "Q" to reduce the alphabet to fit, other versions put both "I" and "J" in the same space). The key can be written in the top rows of the table, from left to right, or in some other pattern, such as a spiral beginning in the upper-left-hand corner and ending in the center. The keyword together with the conventions for filling in the 5 by 5 table constitutes the cipher key. To encrypt a message, one would break the message into digraphs (groups of 2 letters) such that, for example, "HELLOWORLD" becomes "HE LL OW OR LD", and map them out on the key table. The two letters of the digraph look like the corners of a rectangle in the key table. Note the relative position of the corners of this rectangle. Then apply the following 4 rules, in order, to each pair of letters in the plaintext: If both letters are the same (or only one letter is left), add an "X" after the first letter. Encrypt the new pair and continue. Some variants of Playfair use "Q" instead of "X", but any uncommon monograph will do. If the letters appear on the same row of your table, replace them with the letters to their immediate right respectively (wrapping around to the left side of the row if a letter in the original pair was on the right side of the row). If the letters appear on the same column of your table, replace them with the letters immediately below respectively (wrapping around to the top side of the column if a letter in the original pair was on the bottom side of the column). If the letters are not on the same row or column, replace them with the letters on the same row respectively but at the other pair of corners of the rectangle defined by the original pair. The order is important the first encrypted letter of the pair is the one that lies on the same row as the first plaintext letter. Example: Key= PLAYFAIREXAMPLE Table would be P L A Y F I R E X M B C D G H J K N O S T U V W Z Original Message: HIDETHEGOLDINTHETREESTUMP Word pairs are: HI DE TH EG OL DI NT HE TR EX ES TU MP Encryption: The pair HI forms a rectangle, replace it with BM The pair DE is in a column, replace it with ND The pair TH forms a rectangle, replace it with ZB The pair EG forms a rectangle, replace it with XD The pair OL forms a rectangle, replace it with KY The pair DI forms a rectangle, replace it with BE The pair NT forms a rectangle, replace it with JV Page 2

3 The pair HE forms a rectangle, replace it with DM The pair TR forms a rectangle, replace it with UI The pair EX (X inserted to split EE) is in a row, replace it with XM The pair ES forms a rectangle, replace it with MN The pair TU is in a row, replace it with UV The pair MP forms a rectangle, replace it with IF So the Encrypted Text: BM ND ZB XD KY BE JV DM UI XM MN UV IF 5. IMPLEMENT 3 X 3 HILL CIPHER Hill cipher is a polygraphic substitution cipher based on linear algebra. Each letter is first encoded as a number. Often the simplest scheme is used: A = 0, B =1,..., Z=25. A block of n letters is then considered as a vector of n dimensions, and multiplied by a n n matrix, modulo 26. Encryption: For Example, the key is given by the 3 x 3 matrix and the message="act". Since 'A' is 0, 'C' is 2 and 'T' is 19, the message is the vector: which corresponds to a ciphertext of 'POH'. Decryption: In order to decrypt, we turn the ciphertext back into a vector, then simply multiply by the inverse matrix of the key matrix. which gets us back to 'ACT'. Page 3

4 6. IMPLEMENT VIGENERE CIPHER To encipher, a table of alphabets as shown below is be used. It consists of the alphabet written out 26 times in different rows, each alphabet shifted cyclically to the left compared to the previous alphabet. Encryption: For example, suppose that the plaintext to be encrypted is: ATTACKATDAWN The person sending the message chooses a keyword and repeats it until it matches the length of the plaintext, for example, the keyword "LEMON": LEMONLEMONLE The first letter of the plaintext, A, is enciphered using the alphabet in row L, which is the first letter of the key. This is done by looking at the letter in row L and column A of the Vigenère square, namely L. Similarly, for the second letter of the plaintext, the second letter of the key is used; the letter at row E and column T is X. The rest of the plaintext is enciphered in a similar fashion: Plaintext: ATTACKATDAWN Key: LEMONLEMONLE Ciphertext: LXFOPVEFRNHR Decryption: Decryption is performed by finding the position of the ciphertext letter in a row of the table, and then taking the label of the column in which it appears as the plaintext. For example, in row L, the ciphertext L appears in column A, which taken as the first plaintext letter. The second letter is decrypted by looking up X in row E of the table; it appears in column T, which is taken as the plaintext letter. Page 4

5 7. IMPLEMENT AUTO-KEY CIPHER Encryption: c i = (m i + m i-1 ) mod s, m 0 = k (seed key). Decryption: m i = (c i - m i-1 ) mod s, m 0 = k. Example: Message: rendezvous Seed key = 8 Plaintext r e n d e z v o u s C i Ciphertext z d q t x w r f z r -C i Recovered IMPLEMENT VERNAM CIPHER Encryption: c i = m i XOR k i. Decryption: m i = c i XOR k i Example: Message: hello Key = Plaintext h e l l o Plaintext in Binary Key c i = m i XOR k i Ciphertext = : m i = c i XOR k i Recovered h e l l o Page 5

6 9. IMPLEMENT ONE TIME PAD (OTP) CIPHER In cryptography, the one-time pad (OTP) is an encryption algorithm where the plaintext is combined with a random key or "pad" that is as long as the plaintext and used only once. A modular addition is used to combine the plaintext with the pad. Example: Encryption: 7 (H) 4 (E) 11 (L) 11 (L) 14 (O) Original message + 23 (X) 12 (M) 2 (C) 10 (K) 11 (L) Random Key = message + key = 4 (E) 16 (Q) 13 (N) 21 (V) 25 (Z) message + key (% 26) >> ciphertext Decryption: 4 (E) 16 (Q) 13 (N) 21 (V) 25 (Z) ciphertext - 23 (X) 12 (M) 2 (C) 10 (K) 11 (L) key = ciphertext - key = 7 (H) 4 (E) 11 (L) 11 (L) 14 (O) ciphertext - key (mod 26) >> message 10. IMPLEMENT BRUTE-FORCE ATTACK. Page 6

7 11. Frequency Analysis Cryptanalysis 12. Implement frequency analysis attack on plain text Page 7

8 Encryption Tools 1. Meo Encryption Software ( Page 8

9 2. AES Tool 2.5 ( Page 9

10 3. DigiSecret ( Page 10

11 4. CrypTool ( Page 11