Sample Solution SYMMETRIC ENCRYPTION - Essay Prowess

# Sample Solution SYMMETRIC ENCRYPTION

FIT3031-Tutorial 2 Sample Solution SYMMETRIC ENCRYPTION

Q1. What are the essential ingredients of a symmetric cipher?

Ans:        Plaintext, encryption algorithm, secret key, ciphertext, decryption algorithm.

Q2. What are the two basic functions used in encryption algorithms Ans:               Permutation and substitution.

Q3. How many keys are required for two people to communicate via a symmetric cipher? How many keys are required for n people to communicate with each other securely?

Ans:        One secret key. n(n-1)/2 keys

Q4. What is the difference between a block cipher and a stream cipher? a.Why is it not desirable to reuse a stream cipher key?

1. Why do some block cipher modes of operation only use encryption while others use both encryption and decryption?

Ans:        A stream cipher is one that encrypts a digital data stream one bit or one byte at a time. A block cipher is one in which a block of plaintext is treated as a whole and used to produce a ciphertext block of equal length.

1. If two plaintexts are encrypted with the same key using a stream cipher, then cryptanalysis is often quite simple. If the two ciphertext streams are XORed together, the result is the XOR of the original plaintexts. If the plaintexts are text strings, credit card numbers, or other byte streams with known properties, then cryptanalysis may be
2. In some modes, the plaintext does not pass through the encryption function, but is XORed with the output of the encryption function. The math works out that for decryption in these cases, the encryption function must also be

Q5. What are the two general approaches to attacking a cipher?

Ans:        Cryptanalysis and brute force.

Q6. List and briefly define types of cryptanalytic attacks based on what is known to the attacker.

Ans:

 Type of Attack Known to Cryptanalyst Ciphertext only • Encryption algorithm • Ciphertext to be decoded Known plaintext • Encryption algorithm • Ciphertext to be decoded • One or more plaintext-ciphertext pairs formed with the secret key Chosen plaintext • Encryption algorithm • Ciphertext to be decoded • Plaintext message chosen by cryptanalyst, together with its corresponding ciphertext generated with the secret key Chosen ciphertext • Encryption algorithm • Ciphertext to be decoded • Purported ciphertext chosen by cryptanalyst, together with its corresponding decrypted plaintext generated with the secret key Chosen text • Encryption algorithm • Ciphertext to be decoded • Plaintext message chosen by cryptanalyst, together with its corresponding ciphertext generated with the secret key • Purported ciphertext chosen by cryptanalyst, together with its corresponding decrypted plaintext generated with the secret key

Table 2.1 Types of Attacks on Encrypted Message

Q7. What is triple encryption? Why is the middle portion of 3DES a decryption rather than an encryption?

Ans:     With triple encryption, a plaintext block is encrypted by passing it through an encryption algorithm; the result is then passed through the same encryption algorithm again; the result of the second encryption is passed through the same encryption algorithm a third time. Typically, the second stage uses the decryption algorithm rather than the encryption algorithm.

There is no cryptographic significance to the use of decryption for the second stage. Its

only advantage is that it allows users of 3DES to decrypt data encrypted by users of the older single DES by repeating the key. Hence the middle portion of 3DES is a decryption rather than an encryption

Q8.     List ways in which secret keys can be distributed to two communicating parties.

Ans:      For two parties A and B, key distribution can be achieved in a number of