Question

i	j(i)	i	j(i)	i	j(i)	i	j(i)
0	-0.0499	7	-0.08539	13	0.144812	19	0.08021
1	0.107506	8	0.062922	14	-0.0499	20	-0.30103
2	-0.06719	9	-0.04444	15	-0.18366	21	-0.33834
3	-0.04717	10	0.219422	16	-0.02898	22	0.058373
4	-0.09176	11	0.083849	17	0.08021	23	0.79083
5	-0.25918	12	-0.02261	18	-0.14271	24	0.130254
6	0.055643					25	-0.10177

1. (3 points) Please encipher the following plaintext with Caesar Cipher and the encryption key of 10: TODAYISTUESDAY;

2. (3 points) In a Caesar cipher, the encryption key is 5, and the ciphertext is LTTIQZHPBNYMDTZWJCFR, please decipher it;

3. (6 points) Decipher the following ciphertext, which was enciphered using the Caesar cipher: TEBKFKQEBZLROPBLCERJXKBSBKQP.

4. (4 points) Let k be the encipherment key for a Caesar cipher. The decipherment key differs; it is 26 – k. One of the characteristics of a public key system is that the encipherment and decipherment keys are different. Why then is the Caesar cipher a classical cryptosystem, not a public key cryptosystem? Be specific.

Answer 1

In Caesar Cipher ,input text/plain text is shifted particular number of places down the alphabet.

Encryption

For example, if plain text = ABC and key = 2

A is shifted 2 places ahead in alphabet set to become C; Similarly B is shifted to become D and C to E.
Therefore ciphertext = CDE

Decryption

It is exactly opposite of encryption. if cipher text = CDE and key = 2

C is shifted 2 places backward in alphabet set to become A; Similarly D is shifted to become B and E to C.
Therefore ciphertext = ABC

a) Please encipher the following plaintext with Caesar Cipher and the encryption key of 10: TODAYISTUESDAY
Following the principles as mentioned above, for plaintext = TODAYISTUESDAY; key =10
Cipher text = DYNKISCDEOCNKI

b) In a Caesar cipher, the encryption key is 5, and the ciphertext is LTTIQZHPBNYMDTZWJCFR, please decipher it
Following the principles as mentioned above, for ciphertext = LTTIQZHPBNYMDTZWJCFR; key =5
Plain text = GOODLUCKWITHYOUREXAM

c) Decipher the following ciphertext, which was enciphered using the Caesar cipher: TEBKFKQEBZLROPBLCERJXKBSBKQP.

Using hit and trial method, we use all the keys from 1 to 26 one by one to decipher the cipher text as in b) until we find the decrypted text having some meaning.
So we observe for key = 23; Decrypted Text = WHENINTHECOURSEOFHUMANEVENTS which denotes something meaningful.

d) Let k be the encipherment key for a Caesar cipher. The decipherment key differs; it is 26 – k. One of the characteristics of a public key system is that the encipherment and decipherment keys are different. Why then is the Caesar cipher a classical cryptosystem, not a public key cryptosystem? Be specific.

In public key cryptosystem, one should not be able to derive the decipherment key from the encipherment key. But in Caesar cipher if we know the encipherment key say 'k', we can also tell what the decipherment key will be. It is 26-k. Hence any attacker can apply brute force to decipher the ciphertext . Moreover Caesar cipher can have only 26 keys. Therefore, Caesar cipher is a classical cryptosystem, not a public key cryptosystem.