# Symmetric-Key Cryptography (Private Key Encryption)

Symmetric key encryption is the oldest and best-known technique. It can be utilized to secure communication between two or more parties with a shared same secret key. A secret key, which can be a number, a word, or just a string of random letters, is applied to the text of a message to change the content in a particular way. This might be as simple as shifting each letter by a number of places in the alphabet.

Symmetric-key cryptography uses the same key for encryption clear text and decryption of ciphertext. The keys may be unique and identified it can be shared between two or more parties in a secure manner that can use to perform private information communication. In case someone tries to intercepts the ciphertext it won’t be useful to that person. Because a secret key is needed to decrypt the ciphertext. So the message will be a secret.

Symmetric key algorithms subsist in two categories,

- Block ciphers – these operate on groups of bits called blocks; usually fixed block size that is multiple of 8 bits is used to block ciphers. 64 bits and 128 bits are the common block sizes of a block cipher.
- Stream ciphers – these operate on single bits of data, The security of block ciphers depends on the ability of the algorithm to create an output that is indistinguishable from truly random data and on the randomness of the key.

### Few algorithms applied to private key encryption.

#### Data Encryption Standard (DES)

DES can be categorized under the symmetric-key algorithm. It is applied for encryption of electronic data. it was most important in the improvement of modern cryptography. But now consider insecure. DES was developed by IBM during 1970. Originally the DES algorithm made use of 64-bit keys, but for integrity checking some of these key bits are used. So 56 bits are only used for encrypting and decrypting data. This kind of DES algorithm is considered outdated and no longer should be used. In many applications, it is said to be insecure for use.

##### Triple-DES algorithm (TDES)

The Triple-DES algorithm (TDES) can be introduced as a modern derivative of the original DES. Three DES keys have been used in the TDES algorithm which is based on encrypt, decrypt, and encrypt mode respectively. The Triple-DES can work with either the 168-bit keys (3TDEA, which has three separate DES keys) or 112-bit keys (2TDEA, in which the first and third DES key are the same). Due to some properties in the TDES algorithm, only 112 bits are the effective key-strength in the 168-bit key and only 80 bits are the effective key-strength in the 112-bit key. Due to this reason, NIST does not recommend the use of 2TDEA after 2010.

DES and TDES are block ciphers. 64 bits (8 bytes) cipher block is used in TDES. DES and TDES are commonly used, but now they use of that algorithm is decreased because of the weakness of them. This algorithm has some significant characters that make efficient hardware. Because of that DES widely used in embedded systems. The cryptography community is thoroughly inspected AES but there is no considerable attacks have been detected to date.

According to the National Institute of Standards and Technology (NIST); AES is considered as the most secure one beyond 2030. Even though DES which was the forerunner of AES specifically built insensitive manner but not for secret information. US CNSS has been recommended AES for encrypting official materials marked as a secret with 128,192 and 256-bit keys moreover recommended for official materials marked as top secret with 192 and 256-bit keys.

AES and TDES are the most frequently used algorithm, the below table shows the strength of the keys of the above discussed AES and TDES.

Algorithm | Bits of key strength |

2-key Triple DES | 80 bits |

3-key Triple DES | 112 bits |

128-bit AES | 128 bits |

192-bit AES | 192 bits |

256-bit AES | 256 bits |

#### Blowfish

Blowfish algorithm can be categorized under symmetric-key block cipher. It was developed by Bruce Schneier in 1993 incorporated into a substantial number of cipher suites and encryption products. There has been not found any effective weak point in Blowfish to date since it gives the best encryption rate in software. Blowfish is also one of the fastest block ciphers in public use.

#### International Data Encryption Algorithm (IDEA)

Known as The International Data Encryption Algorithm can be categorized under a symmetric-key block cipher invented by James Massey in 1991. It utilizes a block cipher with a 128-bit key and is by and large thought to be extremely secure. It is considered among the best openly known algorithms. In the quite a while that it has been being used, no viable assaults on it have been distributed in spite of various endeavors to discover a few. The thought is patented in the United States and in a large portion of the European nations.

#### RC4

Known as Rivest Cipher 4, developed by Ronald Rivest. In the past this algorithm has been used in many applications such as SSL and WiFi encryption (in the WEP protocol). Because of multiple vulnerabilities had been detected in RC4 algorithm therefore RC4 algorithm is not used in nowadays.

### Advantages and Disadvantages of using Symmetric-key Strengths of Symmetric-key cryptography

- Symmetric-key cryptography is faster. One disadvantage of the public key encryption systems can be seen as they require a lot of mathematics which is complicated to make the encryption work and this method is also very computationally intensive. The encryption and decryption of symmetric keys much easier and gives good read and write performance. So many Solid State Drives (SSD) that are extremely fast made use of symmetric key encryption inside the drive for storing data and these drives are also faster compared to traditional hard drives that have unencrypted data.
- Encrypted information can be exchanged on the link utilizing Symmetric-key cryptography regardless of whether there is a probability that the information will be captured. Since there is no key transmitted with the information, the odds of information being decrypted are invalid
- In symmetric-key cryptosystem is used as a password to ensure the authentication of the user.
- There is one and the only way of decrypting an encrypted message is a secret key that is used to encrypt a message.
- Symmetric-key cryptography which is employed in the private keys is sturdy since it cannot easily crack by brute force attacks. The One Time Pad (OTP) which consists of plain text with a random key is completely risk-free from any attacker nevertheless of time and computing power. Private-key algorithms are basically more difficult to crack than public-key algorithms. Less computing power is required in private key algorithms with comparing to the same private-keys in public-key cryptography.
- symmetric key encryption can become extremely secure when a secure algorithm is utilized. The most widely used symmetric key encryption is the AES (Advanced Encryption Standard) that was designed by the US Government. when using the 256-bit key length, roughly a billion years would take for a 10 petaflop computer to crack the key with brute-force attack. As of November 2012, the world’s fastest computer runs at 17 petaflops. So 256-bit AES is mostly unbreakable.

### Weaknesses of Symmetric-key cryptography

- The main disadvantage of Symmetric cryptosystems is the key transportation. The secret key is to be transmitted to the getting system before the genuine message is to be transmitted. In this way, the main secure method for transferring keys would transfer them personally.
- An appropriate exchange of private keys in a secure manner is the necessary efficient utilization of symmetric-key algorithms. A long time ago this transaction had been done through face to face discussions which were unrealistic. The exchange of keys turns into more complex if someone needs the secret exchange of data at first due to their riskiness of security to start. The symmetric key is able to decrypt everything encrypted with it. When symmetric key encryption is used in two-way communications; both parties able to compromise. In an asymmetric key if someone gets your private key, enable to decrypt messages sent to you but not able to decrypt what you send to the other party because that is encrypted with a different key pair.
- There is another issue raises in increasing participants in the transaction of symmetric key cryptography. When the number of participants increases; with regard to that risk of compromise and the consequences of compromise increase rapidly. Each and every extra user creates a weak point in the system which can be an advantage to an attacker. By chance, an attacker able to take control of one private key; Entire users or only a few users in that group have to suffer for that.
- It cannot give digital signatures that cannot be repudiated.