Cryptography and its Types

Running head: CRYPTOGRAPHY
CRYPTOGRAPHY
Name of the Student
Name of the University
Author Note

CRYPTOGRAPHY
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1. Encrypt the message “coursework” with Caesar cipher using 7 as the key.
Solution:s
2. Decrypt the ciphertext “wjanxnts” with Caesar cipher using 5 as the key.
Solution:
3. Using Caesar cipher, what key shall we use to encrypt “Julius Caesar” to “qbspbz
jhlzhy”?
Solution: Using the cipher text and plaintext we can determine the shifts for the each alphabet.
As it is a Caesar cipher encryption, assuming all the shifts should be same for all letters.

CRYPTOGRAPHY
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Plaintext: Julius Caesar
Cipher text: qbspbz jhlzhy
Determining shifts: J->q= 7 shifts, u->b= 7 shifts, l->s= 7 shifts, i->p= 7 shifts, u->b=7shifts,
s->z= 7 shifts, c->j=7shifts, a->h=7 shifts, e->l= 7 shifts, s->z= 7 shifts, a->h= 7 shifts, r->y 7
shifts.
Hence, the key used is 7. Ans.
4. Encrypt the message “coursework” with Vigenère cipher using “test” as the key.
Solution: Message = COURSEWORK, Key = TEST.
Draw a table for the letters of the plaintext (message), and repeat the letters of the key so that the
no of letters present in the plain text becomes the equal to the no of letters in the key.
C O U R S E W O R K
T E S T T E S T T E

CRYPTOGRAPHY
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To determine the letters of cipher text the 26x26 table of English alphabet is used.

CRYPTOGRAPHY
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By using the table,
The cipher text will be = VSMKLIOHKO
5. Decrypt the ciphertext “smes zwgl” with Vigenère cipher using “with” as the key.
Solution: Cipher text = SMES ZWGL, Key = WITH
Draw a table for the letters of the plaintext (message), and repeat the letters of the key so that the
no of letters present in the plain text becomes the equal to the no of letters in the key.
S M E S Z W G L
W I T H W I T H

CRYPTOGRAPHY
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To determine the letters of plain text, the 26x26 table of English alphabet is used.

CRYPTOGRAPHY
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By using the table, match the key with column or row and find the respective cipher text letter to
the column if row is selected and row if column is selected.
Hence the plaintext will be = WELLDONE.
6. Consider a case insensitive shift cipher, with the following distribution over M [𝑀 = 𝑎] =
0.2 , 𝑃𝑟[𝑀 = 𝑏] = 0.2 , 𝑃𝑟[𝑀 = 𝑐] = 0.3 , 𝑃𝑟[𝑀 = 𝐷] = 0.3
a. What is the probability that the ciphertext is y?
Probability of ciphertext is y = Pr[C=y]

CRYPTOGRAPHY
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Where the probability of each letter depends on the probability plaintext and used key.
Hence, the plain text letters along with the keys will be,
Either, M=a and key = 2
Either, M=b and key = 3
Either, M=c and key = 4
Or M=D and key = 5
So the formula will lead to
Pr[C=z] = Pr[M=a]*Pr[K=2]+ Pr[M=b]*Pr[K=3]+ Pr[M=c]*Pr[K=4] +
Pr[M=D]*Pr[K=5]
Pr[C=y] = 0.2*1/26 + 0.2*1/26 + 0.3*1/26 + 0.3*1/26
As, for all k ∈{0,...,25}, Pr[K=k] = 1/26]
Pr[C=y] =1/26 Ans.
b. What is the probability that the ciphertext is z?
Probability of ciphertext is z = Pr[C=z]
Where the probability of each letter depends on the probability plaintext and used key.
Hence, the plain text letters along with the keys will be,
Either, M=a and key = 1
Either, M=b and key = 2
Either, M=c and key = 3