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# Block Ciphers and Cryptography

There are numerous methods for safeguarding and securing data. Cryptography is one of these methods, and it is described as the process of transforming plaintext (regular text) to ciphertext by encryption and then converting ciphertext back to plaintext via decryption. This process ensures data integrity while also ensuring data security.

The amount of space needed to store a table in a 64-bit block with 80-bit keys. Block ciphers must be kept in tables, and they were formerly stored in 64-bit blocks. These blocks could store 264 possibilities, which could store 264 plaintext and memory space as well. Currently, the block size is 128-bits.

Rounds of DES.

The rounds in which DES permutates is 16 rounds where each round uses a separate 48-bit key. A DES (Data Encryption Standard) is a lock cipher employing a 56-bit key that operates on 64-bit blocks. Block sizes are 64 bit or 128-bits key size of 56-bits (+8 parity bits). It has a structure of Balanced Fiestel Network (Ferguson, Schneier and Kohno 54).

How 3DES works as a function of DES

3DES comprises of three processes: Encryption, Decryption, and Encryption. It works in a way that you take three separate 56-bit keys and encrypt them with key one (K1) and then decrypt with key two (K2). After decryption, encrypt with key three (K3) to finish the 3DES process. K1=K3 which has a two key version therefore, K1=K2=K3 in that K3 is a decrypt key similar to K1 and hence 3DES is single DES.

Possible length of Advanced Encryption Standard (AES)

AES encryption is a widely used method for Block ciphers and used 128-bits, 192-bits or 256-bits. The block size block sizes of 128-bits. The AES works rounds depending on the key size used in which vary from 128,192 and 256 bits. It has a structure of Substitution Permutation Network.

Situations to Prefer 3DES to the DES

3DES is less vulnerable to side channel attacks though it is not immune to attacks whereas AES is harder to get right due to its internal mathematical design (Ferguson, Schneier and Kohno 59). 3DES has 8-byte block cipher size in comparison to the 16- byte of AES, which can lead to overhead when using it in a block mode encryption. AES however due to its internal design can be more secure than 3DES. It also has faster execution depending on the block cipher.

Exhaustive Search Approach

Time for a single processor to execute is 2n-1 where n is the number of bits and using the exhaustive search approach, the process runs through all nodes in the block cipher and decrypts single node exhaustively. The hours that would take a 56-bit key to be deciphered, would be 22 hours done on a single processor made in 1998.

DES Complementation Property

Examples of systems that use DES Encryption with a single key k can fully define a multi-user file system, which can be prone to attacks. Given that the values of m in EK(m) and key k in a DES that has been encrypted with key2 K, The formulae to get the values is EK(m) = Ek (m).

Attacks That Utilize Complimentary Property

An attacker sends 2 64-bit blocks which should be written to a file system A and B. These particular encrypted blocks will be stored to physical memory. EK (A) and Ek (B). For each key candidate y, compute E (B). This is a single cryptographic operation of which the value is equal to E (B) then y=k hence the key was found. The attack then checks 2 key candidates for each operation thus reducing the attack on 256 possible DES keys in half to 255.

Encrypted text

The following is the encrypted block of data. zGxZtpLHduVBkh/k8m6jm5KEVC2e34ARMRfHI6wFUPk0q2JyWtrUijVi0N7GJVjKF7gZZNVM+tU8bwpI6TS7sg

The minimum padding should be 8 or 16 depending on the block size variations. Padding is when you have data that may be separate or in multiple blocks for a cipher that is extended in order to have equal portions of the data. Most nodes require the length of the padding of plaintext, is to be a multiple of the block size.

4.2 Security and Performance variant of CBC mode

Random IV is very secure due to the randomization of the key inserted in the block to produce ciphertext. In CBC the ciphertext block being used to randomize the pali text block and take note of the first block which we have to use the IV. Encrypting data using this method makes it hard to decipher and requires a lot of computing power.

However the Random IV has a slight disadvantage in that the ciphertext is one block longer than plaintext. This makes it complicated for short messages in context due to the increase computing power for one block.

Nonce -Generated IV variant works in two steps where first the message is encrypted with key that is given a number called “nonce” short form of “number used once”. The second step is to encrypt this first message with number to construct unique nonce. this makes the message secure and uses the same computation power as encrypting a message twice.

If an attacker was to happen and know the nonce and can use it to decipher the encryption, then the nonce should always be protected by encrypting the nonce with a separate different key

Fixed IV has a major advantage in which it uses less computation power hence increase in the performance. However the fixed IV introduces the ECB problem in that the first block in each message encrypted starts with the same ciphertext. this is dangerous as that the attacker may decipher the text easily if just compared the fixed IV key.

Counter IV assigns the values 0 and 1 to the block of plaintext before encryption. his helps t identify the bock and reduce the computation time of this encryptions to give a ciphertext block similar to the plaintext block During sending two messages the first one is labelled IV=0 and the second one is labelled IV= 1. if an attacker was to spot this security would be compromised and the message would be decoded by using a simple counter of the first and second block.

4.3 Counter (CTR) mode.

CTR is a form of DES though not a standardized version despite its longevity being used.. This mechanism works with blocks of plaintext (P) and ciphertext (C) and generally concentrates on the nonce with a counter value to encrypt it to form a block of the key stream. Since this value has no nonce the counter will encrypt to form a block cipher similar in size with key.

4.4 Cipher Block Chaining (CBC).

In CBC, the IV is run with a XOR function (⊕) with the plaintext run through the block cipher. The operation that performs the encryption of any plaintext in cbc mode is CA=Ek(IVA⊕PA)=Ek(IVA⊕(IVA ⊕IV B⊕ ”false”)) The IVA and the ⊕IVA cancel each other out making the equation CA=Ek(IVB⊕”false”). Since the Random IV is unpredictable it makes this algorithm safe from attacks. CBC mode of operation involves a sequence of bits in a block cipher which is encrypted as a single unit or block with a cipher key applied to the whole block. The decrypted text using CBC method with random IV is Hy8Wq_ow7v5>\Y&oR-pp.

4.5 Encrypted key for the hex digits provided.

Xepg8mRhd7Yvd+uIYHlp/2NuT7tZXJFZJlI7+HSRaTHqXTZmB+eH6K6ycj6zG/7TdFkarHi+KjgLCxtRg+XTm0NXdc+zeC8dDwwMY3EKlqVd6nx4/Lbc/vgKggPYJ1fdb9K/5wsdE8L8VRNswlGG7GRRhcRd9unZEimGqisMYVW69uW30+EKeV7vr2qHEiLKhX1SWZv9XrmLCO6YM0HMaQ

Works Cited

Ferguson, Niels, Bruce Schneier and Tadayoshi Kohno. Cryptography Engineering: Design Principles and Practical Applications. John Wiley & Sons, 2011. Print.