**Category:**

## Part A: Cryptography

### Encryption

**What is encryption? **Encryption is an activity of transforming electronic data or information into another form known as ciphertext that makes it unreadable by anyone except the intended recipient (Konheim, 2007).

**What is encryption used for?** Encryption is used for safeguarding sensitive information as it moves from the sender to the recipient (Konheim, 2007). With the utilization of proper encryption, such as private key encryption, confidential data transmitted over the Internet will be secure. Encryption is utilized for offering reliability as private data is transmitted via an open network (Konheim, 2007). The recipient will get the ciphertext from the sender, and he/she will retrieve the same data as the original plaintext.

**Advantages.** Encryption guarantees the protection of sensitive data (Stallings, 2013). For example, it offers a protection mechanism including Advanced Encryption Standards, which would require more effort from an intruder to decrypt the message. Besides, it provides protection flexibility on the devices connected to a public network (Stallings, 2013). Data encryption safeguards the confidential information that is copied on the desktop, smartphone, laptop, email server, or the organization’s network.

**Disadvantages.** Data encryption has certification problems (Stallings, 2013). Most of the public key systems utilize a third party to certify the authenticity of public keys. Nevertheless, in case the certification authority experiences technical issues, the intruder could deceive users into transmitting data to the wrong destination. Data encryption can experience direct compromise, especially working with public-key cryptography (Stallings, 2013). The attacker can locate loopholes in the underlying algorithms and manipulate it to break the cipher.

### Symmetric Key Cryptography

**What is symmetric key cryptography?** It is a cryptographic technique, which adopts a single, shared common key to encipher and decipher data (Konheim, 2007).

**Applications.** Symmetric key cryptography is utilized in the military sphere (Stallings, 2013), e.g. Caesar cipher algorithm, which selects a shared private key between the range of 1 and 25. In case the sent message is only text, the algorithm replaces each letter by the third letter. Promoting Data Encryption Standard (DES) algorithms is another key application (Stallings, 2013). The algorithms are particularly useful in the contemporary understanding of block ciphers and their approach to cryptanalysis.

**Advantages.** A symmetric key encryption method is relatively simpler and faster. It uses the private key that does not necessarily require sophisticated algorithms for message encryption and decryption (Konheim, 2007). Encoding and decoding are much easier than public-key encryption. It offers an extremely secure encryption system, which guarantees data confidentiality (Konheim, 2007). It uses a private key cipher that incorporates a secure algorithm. It ensures that the data sent on the public network is protected from intruders via the use of private key encryption.

**Disadvantages.** The encryption technique requires a common secret key that must be shared between the sender and the recipient (Stallings, 2013). This prerequisite demands a high level of confidentiality as the process of selecting, transmitting, and backing up private keys is challenging to achieve in a reliable and secure way. Symmetric key encryption does not provide data origin verification and data integrity safety (Stallings, 2013). The recipient lacks an appropriate mechanism to authenticate the sender, thus it is hard to verify that the decoded message conforms to the original texts.

### Asymmetric Key Cryptography

**What is it?** It is a cryptographic technique that uses a pair of keys to encode and decode a message making sure that the message safely reaches the recipient (Stallings, 2013).

**Why is it called public-key cryptography? **The sender uses an encryption key that is known to everyone on the public network (Konheim, 2007). He utilizes the recipient’s public key to encode the message, while the recipient uses his/her secret or a private key to decrypt the message.

**Applications.** Asymmetric key cryptography is used in the Rivest Shamir Adelson (RSA) algorithm to promote public key encryption (Stallings, 2013). It is secure for digital signing as well as encoding, and its long keys are utilized in modern electronic commerce. Integration of the Internet Supervisory Control and Data Acquisition (SCADA) is another essential application of asymmetric key cryptography (Krutz, 2006). It authenticates the data and secures the Internet SCADA systems to enable only users with private key access to the information.

**Advantages.** Asymmetric key cryptography solves the key distribution problem since the communicating parties do not need to exchange keys (Stallings, 2013). The sender uses the recipient’s public key to encode the message. The recipient only has to utilize his secret key to decode it. The principal advantage of asymmetric key cryptography is that it offers enhanced security (Stallings, 2013). The sender does not transmit secret or private keys via the public network. It guarantees the safety of the conveyed data since the recipient utilizes his/her private key to retain the original message.

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**Disadvantages.** The speed required to encrypt information is considerably low (Stallings, 2013). It is because public key cryptography or asymmetric key encryption uses extensive mathematical operations that make it slower than the asymmetric algorithms. It can experience direct compromise since the sender conveys public keys on the network (Mirzaee & Amiri, 2014). The attacker can locate the weaknesses of the algorithm and use this to break the cipher. The hacker can guess the underlying public key to decode the encryption.

## Part B: Two-Square Cipher

### Two-Square Cipher

A two-square cipher is a cryptographic extension of the Playfair cipher, which adopts two different keys and enciphers each combination of letters twice in order to get the original message (Department of the Army, 1997). A polygraphic substitution cipher functions similarly, i.e. split the original plaintext into a section of letters. The cipher utilizes five-by-five squares to arrange the pair of letters.

### Varieties of the Two-Square Cipher

1) Horizontal two-square cipher incorporates five-by-five matrices on each side. Each of the matrices incorporates the letters of the alphabet, which are positioned in the same location to allow decreasing the alphabet to fit (Mirzaee & Amiri, 2014). Additionally, the alphabets that are appearing in both squares are mostly random letters, which depend on some phrases or even keywords.

2) Vertical two-square cipher utilizes two five-by-five matrices that are positioned one above the others (Mirzaee & Amiri, 2014). However, the first alphabet of both plaintext and ciphertext digraphs adopts the top matrix, while the preceding alphabets utilize the bottom character.

The user should first fill the empty spaces in the matrices with the characters of a keyword or phrases in order to develop five-by-five two-square cipher matrices. He/she should eliminate all matching characters. This approach will allow one to fill the remaining empty sections with the rest of the characters in order. One should keenly do this by excluding “Q” to reduce the characters to fit in the matrices (Department of the Army, 1997). Moreover, the user can write the character in the top rows of the table, e.g. from the left to the right and also integrate the other key formats including spiral beginning appearing in the uppermost left corner and the alphabets structures concluding in the center. As a result, the cipher key will consist of characters and the rules of filling in the five-by-five matrices table.

### Description of the Two-Square Encryption Steps

Before performing encryption, it is vital to develop two tables utilizing alphabets or even longer expressions that can be used as private or secret keys (Department of the Army, 1997). The two tables should use a dimension of five-by-five characters and involve 25 characters of the Latin alphabet. Furthermore, the Latin alphabet consists of 26 characters, and thus one commonly omits one of the unique characters, for instance, *x *or* q* or uses “j” and* *“I” as one character.

Utilizing secret keys is another imperative encryption step. This procedure should happen when one is filling the spaces between the two formulated tables (Mirzaee & Amiri, 2014). First, all duplicated characters in the spaces need to be omitted, and only the letters that appeared first should be used. Second, the user should add the remaining letters to these two tables using the original convention. In other words, the sender should add the letters of the keyword to both tables. Third, the communicating parties need to agree on the order that the table should be filled with the remaining character letters, commonly in the alphabetical order. For instance, if someone uses the identity of both parents of the Roman Emperor Vespasian as private keywords: Titus Flavius Sabinus and the second’s parent identity as Vespasia Polla (Mirzaee & Amiri, 2014). With the use of these Latin characters, counting both characters “I” and “j” together as one, adding the remaining letters into the tables row by row, from the left to the right and from the top to the bottom, one will develop the following tables:

T | I | U | S | F |

L | A | V | B | N |

C | D | E | G | H |

K | M | O | P | Q |

R | W | X | Y | Z |

V | E | S | P | A |

I | O | L | B | C |

D | F | G | H | K |

M | N | Q | R | T |

U | W | X | Y | Z |

The two tables should be positioned side by side to ensure that the lines of the entire rows are maintained (Mirzaee & Amiri, 2014).

The next encryption step involves splitting the plaintext into considerable parts. Each part of the plaintext should incorporate two subsequent letters (Mirzaee & Amiri, 2014). It is recommended that a rare character or letter may be added to the original plain text in order to prevent people from breaking or quickly guessing the algorithm.

Locating the first character of each pair in the first table and determining the second character of each pair in the second table is another crucial encryption step. A person needs to develop a rectangle that accommodates two tables and has edges in cells, which are defined by the two letters (Department of the Army, 1997). Principally, encryption will involve changing the two letters with the other two unique characters. The other two edges of the rectangle will determine these two unique characters. Additionally, one should substitute all characters with letters chosen based on the two tables.

The participating parties need to agree on the convention the new characters need to be added to the ciphertext (Mirzaee & Amiri, 2014). For instance, the first character would be the one from the right table and the second basic letter should be obtained from the left table.

### Description of the Two-Square Cipher Decryption Steps

The recipient performs decryption using the same criteria. First, the recipient should develop the same two tables, which conforms to the sender. Second, he/she decrypts the pairs of letters within the two tables via analogous mechanisms (Department of the Army, 1997). Thus, he/she locates two ciphertext characters in the created tables and then establishes rectangle edges or corners in the reverse order. Finally, he/she identifies two new plaintext characters and reads the message.

### Advantages of Two-Square Cryptosystem

The main advantage of the two-square cryptosystem is that it guarantees information confidentiality between the communicating parties (Konheim, 2007). The recipient develops two tables, which are similar to the senders. He only needs to identify the private keyword to determine the original meaning of the message from plaintext fragments. This activity promotes data privacy since the recipient is the only one who knows the private keyword.

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The cryptography of two-square cipher is harder to crack than common substitution ciphers since it changes the pairs of characters or diagraphs rather than single characters. Additionally, if one uses a frequency analysis of English pairs of characters or digraphs to encrypt a message, he/she can utilize the same pairs of characters in monograph frequencies (Konheim, 2007). This cryptographic technique makes it harder to hack than typical substitution ciphers.

### Disadvantages of Two-Square Cryptosystem

The pieces of plaintexts are mostly visible in its ciphertext since it uses five-by-five letters in generating algorithm. This feature makes it vulnerable to intruders because of the underlying frequency assessment and guessing plaintext fragments (Stallings, 2013). The hacker can gradually guess and uncover larger pieces of the plaintext of the original message.

Determining the candidate plaintext fragments is much easier since the structure of its algorithm is similar to that of four-square cipher. The double Playfair digraph and its reverse procedure, for instance, CD and DC will decipher the same alphabetical pattern in the plaintext, for instance, RE and ER (Department of the Army, 1997). Similarly, some words contain these letters, such as a receiver. As a result, identifying the original plaintext from its fragments will be easier for the attacker and may change the plaintext.

### Part C

#### 1

12 letters of the name are* KHALIDABDULL*

Steps of preparing the plaintext:

1. Split the letters of the name *KHALIDABDULL* into pairs of two to generate plaintext.

KHALIDABDULL KH AL ID AB DU LL

#### 2

Develop five-by-five matrices using the keywords *retreats* and *enemies*.

Usually, either* Q* is omitted, or *J and I *are put together to minimize the number of characters from 26 to 25. In this case, I have put “J” and “I” together.

Keyword: Retreats

R | E | T | A | S |

B | C | D | F | G |

H | I | K | L | M |

N | O | P | Q | U |

V | W | X | Y | Z |

Keyword: Enemies

E | N | M | I | S |

A | B | C | D | F |

G | H | K | L | O |

P | Q | R | T | U |

V | W | X | Y | Z |

#### 3

The plain text: KH AL ID AB DU LL

So, the letter we are seeking is* K*

R | E | T | A | S |

B | C | D | F | G |

H | I | K |
L | M |

N | O | P | Q | U |

V | W | X | Y | Z |

E | N | M | I | S |

A | B | C | D | F |

G | H |
K | L | O |

P | Q | R | T | U |

V | W | X | Y | Z |

Find the second letter of the paired letters in the lower/ right text matrix

Therefore, the letter we are searching for is *H*

*KH* is encrypted to *KH*, since both alphabets are on the same record.

The next letter we are searching for is* A* on the first matrix and* L* on the second matrix

R | E | T | A |
S |

B | C | D | F | G |

H | I | K | L | M |

N | O | P | Q | U |

V | W | X | Y | Z |

E | N | M | I | S |

A | B | C | D | F |

G | H | K | L |
O |

P | Q | R | T | U |

V | W | X | Y | Z |

From the above, a rectangle is defined by two plaintext characters, and the opposite corner defines the ciphertext pair of letters.

*AL *is encrypted to *LI*

Using the same process:

*ID* is encrypted to* CL*

*AB* is encrypted to *FN*

*DU* is encrypted to *PF*

*LL *is encrypted to *LL*

Plaintext: KH AL ID AB DU LL

Ciphertext: KH LI CL FN PF LL