CRYPTOGRAPHY AND NETWORK SECURITY, 2 Mark UnitII

1. Define symmetric encryption.

• A form of cryptosystem in which encryption and decryption are performed using the same key. Also known as conventional encryption.
• Symmetric encryption transforms plaintext into ciphertext using a secret key and an encryption algorithm. Using the same key and a decryption algorithm, the plaintext is recovered from the ciphertext.

2. What is the difference between differential and linear cryptanalysis?

            • In differential cryptanalysis, it breaks the DES in less 255 complexities.

            • In cryptanalysis, it finds the DES key given 247 plaintexts.

3. Define product cipher.

            Product cipher performs two or more basic ciphers in sequence in such a way that the final result or product is crypto logically stronger than any of the component ciphers.

4. What was the original set of criteria used by NIST to evaluate candidate AES cipher?

            The original set of criteria used by NIST to evaluate candidate AES cipher was:

                        • Security

                        • Actual Security

                        • Randomness

                        • Soundness

                        • Other security factors

                        • Cost

                        • Licensing Requirements

                        • Computational Efficiency

                        • Memory Requirements

                        • Algorithm And Implementation Characteristics

                        • Flexibility

                        • Hardware and software suitability

                        • Simplicity

5. What was the final set of criteria used by NIST to evaluate candidate AES ciphers?

            The final set of criteria used by NIST to evaluate candidate AES ciphers was:

                        • General Security

                        • Software Implementations

                        • Restricted-Space Environments

                        • Hardware Implementations

                        • Attacks On Implementations

                        • Encryption vs. Decryption

                        • Key Agility

                        • Other Versatility And Flexibility

                        • Potential for Instruction-Level Parallelism

 6. What is power analysis?

             Power analysis is the power consumed by the smart card at any particular time during the cryptographic operation is related to the instruction being executed and to the data being processed. Multiplication consumes more power than addition and writing 1s consumes are power than writing 0s.

7. What is the purpose of the State array?

             A single 128-bit block is depicted as a square matrix of bytes. This block is copied into the State array, which is modified at each stage of encryption or decryption. After the final stage, State is copied to an output matrix.

8. How is the S-box constructed?

            The S-box is constructed in the following fashion:

            Initialize the S-box with the byte values in ascending sequence row by row. The first row contains {00}, {01}, {02}, ……….., {0F}; the second row contains {10},{11},etc; and so on. Thus, the value of the byte at row x, column y is {x y}. Map each byte in the S-box to its multiplicative inverse in the finite field GF (28); the value {00} is mapped to itself. Consider that each byte in the S-box consists of 8 bits labeled (b7,b6,b5,b4,b3,b2,b1,b0).Apply the following transformation to each bit of each byte in the S-box.

 9. Briefly describe Sub Bytes.

 Sub byte uses an S-box to perform a byte-by-byte substitution of the block. The left most 4 bits of the byte are used as row value and the rightmost 4 bits are used as a column value. These row and column values serve as indexes into the S-box to select a unique 8-bit value.

10. Briefly describe Shift Rows.

            In shift row, a row shift moves an individual byte from one column to another, which is a linear distance of a multiple of 4 bytes. In Forward Shift Row, each row perform circular left shift. Second Row a 1-byte circular left shift is performed. Third Row a 2-byte circular left shift is performed. For the Fourth Row a 3-byte circular left shift is performed. In Inverse Shift Row, each row perform circular right shift.

11. How many bytes in State are affected by Shift Rows?

             Totally 6-bytes in state are affected by Shift Rows.

12. Briefly describe Mix Columns.

             Mix Column is substitution that makes use of arithmetic over GF(28).Mix Column operates on each column individually. Each byte of a column is mapped into a new value that is a function of all four bytes in the column. The Mix Column Transformation combined with the shift row transformation ensures that after a few rounds, all output bits depend on all input bits.

13. Briefly describe Add Round Key.

            In Add Round Key, the 128 bits of State are bit wise XORed with the 128 bits of the round key. The operation is viewed as a column wise operation between the 4 bytes of a State column and one word of the round key; it can also be viewed as a byte-level operation. The Add Round Key transformation is as simple as possible and affects every bit of State.

14. Briefly describe the Key Expansion Algorithm.

             The AES key expansion algorithm takes as input a 4-word(16-byte) key and produces a linear array of 44 words(156 bytes). This is sufficient to provide a 4-word round key for the initial Add Round Key stage and each of the 10 rounds of the cipher.

15. What is the difference between Sub Bytes and Sub Word?

• Sub Bytes: Sub Bytes uses an S-box to perform a byte-by-byte substitution of the block.
• Sub Word: Sub Word performs a byte substitution on each byte of its input word,using the Sbox.

 16. What is the difference between Shift Rows and Rot Word?

• Shift Rows: Shift Row is simple permutation. It shifts the rows circularly left or right.
• Rot Word:Rot word performs a one-byte circular left shift on a word. This means that an input word [b0,b1,b2,b3] is transformed into [b1,b2,b3,b0].

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

            Some block cipher modes of operation only use encryption because the input is set to some initialization vector and the leftmost bits of the output of the encryption function are XORed with the first segment of plain text p1 to produce the first unit of cipher text C1 and it is transmitted. While in decryption, the cipher text is XORed with the output of the encryption function to produce the plain text.

18. What is triple encryption?

 Tuchman proposed a triple encryption method that uses only two keys [TUCH79].

     The function follows an encrypt – decrypt – encrypt (EDE) sequence.

            C=Ek1[Dk2[Ek1[P]]] 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: C=Ek1[Dk2[Ek1[P]]] = Ek1[P]

 19. What is a meet-in-the-middle attack?

             Meet-in-the-middle attack, was first described in [DIFF77]. It is based on the observation that, if we have C=Ek2[Ek1[P]] Then X=Ek1[P]=Dk2[C] Given a known pair, (P,C), the attack proceeds as follows. First, encrypt P for all 256 possible values of K1. Store these results in a table and then sort the table by the values of X. Next, decrypt C using all 256 possible values of K2. As each decryption is produced, check the result against the table for a match. If a match occurs, then test the two resulting keys against a new known plaintext-ciphertext pair. If the two keys produce the correct ciphertext, accept them as the correct keys.

20. How many keys are used in triple encryption?

             Tuchman proposed a triple encryption method that uses only two keys [TUCH79].

21. What is the key size for Blowfish?

 Blowfish makes use of a key that ranges from 32 bits to 448 bits (one to fourteen 32-bit words). That key is used to generate 18 32-bit subkeys and four 8*32 S-boxes containing a total of 1024 32-bit entries. The total is 1042 32-bit values, or 4168 bytes.

22. Distinguish cipher and Bloch cipher

           

Cipher	Block Cipher
An algorithm for encryption and decryption. A cipher replaces a piece of information (an element in plaintext) with another object, with the intent to conceal meaning. Typically, the replacement rule is governed by a secret key.	A symmetric encryption algorithm in which a block of plaintext bits (typically 64 or 128) is transformed as a whole into a ciphertext block of the same length.

23. What is multiple encryption?

            Repeated use of an encryption function, with different keys, to produce a more complex mapping from plaintext to ciphertext.