CIS3360: Security In Computing 2

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CIS3360: Security in ComputingHomework 51.( 15 points) Knowledge-based Question:a.What are the differences between public/symmetric key cryptography? The advantageand disadvantage of each?a.For English alphabet based texts regardless of upper or lower case (i.e., assuming eachletter has only 26 possible value), if the length of the key using Vigenere Cipher is 4, howmany possible encryption keys exist? (hint: we all know that Caesar cipher has 26possible encryption keys)b.What is the usage of a cryptographic hash function? Can two documents have the samehash value?Answer:1. Public vs Symmetric Key Cryptography•Public Key (Asymmetric):oUses two keys: public (shared) andprivate (secret).oAdvantages: Easier key distribution, non-repudiation.oDisadvantages: Slower, more computationally intensive.•Symmetric Key:oUses one keyfor both encryption and decryption.oAdvantages: Faster, simpler.oDisadvantages: Key distribution is harder, not scalable.2. Vigenère Cipher with Key Length 4•There are26^4 = 456,976possible encryption keys.3. Cryptographic Hash Function•Usage: Ensures data integrity, stores passwords securely, creates digital signatures.•Collisions: While rare, two documents can technically have the same hash value (this iscalled a collision).

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2.(10 points) Determine whether a statement is True or False. If a statement is false, you mustexplain why.a.Digitalsignature of a message m is H(m), where H() represent a cryptographic hashfunction such as MD5 or SHA-1.b.Cesar cipher is a kind of substitution cipher, while Vigenere cipher is a kind oftransposition cipher.Answer:Let's go through each statement:a. Digital signature of a message m is H(m), where H() represents acryptographic hash function such as MD5 or SHA-1.•False.Adigital signatureis not just the hash of a message (H(m)). Instead, it is the result ofapplying aprivate keyto the hash of the message (H(m)) using a signing algorithm (likeRSA or ECDSA). The process involves signing the hash (H(m)) to create the signature,not just computing the hash itself. So, a digital signature typically involves hashing themessage and then encrypting the hash with a private key, not just the hash alone.b. Caesar cipher is a kind of substitution cipher, while Vigenère cipher is a kindof transposition cipher.•False.Both theCaesar cipherandVigenère cipherare types ofsubstitution ciphers, nottransposition ciphers.oCaesar ciphershifts each letter by a fixed number of positions in the alphabet (asimple form of substitution).oVigenère cipheruses a keyword to shift letters in the plaintext by differentamounts, effectively creating a more complex substitution cipher.Atransposition cipherinvolves rearranging the positions of characters in the plaintext, which isnot what the Vigenère cipher does.3.(20 points)Cipher Generation:a.What are the substitutions for the (decimal) numbers 12, 7, and 2 using the S-boxshown in the textbook as Figure 8.3.b.What is the encryption of the following string “THELAZYFOX” using the Caesarcipherwith a key of “3c.What is the Hill cipher key matrixKthat can realize the following permutation:π: (1,2,3,4,5)→(3,5,1,4,2)

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