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Assignment #1

Security in Computing COSC2536/2537

 

Q1

(a) Decrypt the ciphertext

 

IAUTMOCSMNIMREBOTNELSTRHEREOAEVMWIH

TSEEATMAEOHWHSYCEELTTEOHMUOUFEHTRFT

 

This message was encrypted with a double transposition (of the type  discussed in the lecture and tute) using a matrix of 7 rows and 10 columns. Hint:  The first word is “there.”

(Marks 1.5)

 

(b) Find the plaintext and the key, given the cipher-text

CSYEVIXIVQMREXIH.

(Marks 1)

 

(c) Decrypt the following message that was encrypted using a simple substitution cipher:
GBSXUCGSZQGKGSQPKQKGLSKASPCGBGBKGUKGCEUKUZKGGBSQEICA
CGKGCEUERWKLKUPKQQGCIICUAEUVSHqKGCEUPCGBCGQOEVSHUNSU
GKUZCGQSNLSHEHIEEDCUOGEPKHZGBSNKCUGSUKUASERLSKASCUGB
SLKACRCACUZSSZEUSBEXHKRGSHWKLKUSQSKCHQTXKZHEUQBKZAEN
NSUASZFENFCUOCUEKBXGBSWKLKUSQSKNFKQQKZEHGEGBSXUCGSZQ
GKGSQKUZBCQAEIISKOXSZSICVSHSZGEGBSQSAHSGKHMERQGKGSKR
EHNKIHSLIMGEKHSASUGKNSHCAKUNSQQKOSPBCISGBCqHSLIMQGKG
SZGBKGCGQSSNSZXQSISQQGEAEUGCUXSGBSSJCqGCUOZCLIENKGCA
USOEGCKGCEUqCGAEUGKCUSZUEGBHSKGEHBCUGERPKHEHKHNSZKGGKAD

(Marks 2)

 

Q2 Using the letter encodings in Table I, the following two cipher-text messages were encrypted with the same one-time pad:

 

KHHLTK and KTHLLE

 

Find all possible plaintexts for each message and the corresponding one-time pad.

 

Table I Abbreviated Alphabet

(Marks 3)

 

 

Q3   Bob is a receiver and Alice is a sender. Bob generates public and private keys using RSA encryption algorithm and sends the public key to Alice. Alice has a message M=8596 to send.  Bob uses parameter p=89 and q=191, and chooses a small public key parameter e.  What are the values of suitable public and private keys?  How would Alice encrypt message M=8596? How would Bob decrypt the encrypted message C with the private key?

(Marks 2.5)

 

Q4 Alice has a message M=37 to send to Bob securely using ElGamal encryption algorithm. Bob chooses p=8081, g=2849, x=53. Alice chooses r=78. Show the encryption and decryption steps.

(Marks 2.5)

 

Q5 Alice has a message M=186 to send to Bob securely using Paillier encryption algorithm. Bob chooses p=11, q=17,and selects an integer g =188. Alice selects a random number r=97. Show the encryption and decryption steps.

(Marks 2.5)

 

 

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