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Symmetric key cryptography, also known as secret key cryptography or conventional cryptography, refers to an encryption system in which the sender and receiver share a single common key that is used to encrypt and decrypt the message. The process is shown in figure 1.3. The used algorithm is known as the symmetric algorithm or secret key algorithm. The key is defined as a piece of information (a parameter) that determines the functional output of a cryptographic algorithm or cipher. The key used for encrypting and decrypting a message has to be known to all the authentic recipients or else the message could not be decrypted by conventional means [6]. The examples of symmetric key cryptography are discussed below.

 Data encryption standard (DES): the DES was published in 1977 by the US National Bureau of Standards. It uses a 56-bit key and maps a 64-bit input block of plaintext onto a 64-bit output block of ciphertext. 56 bits is a rather small key for today’s computing power.

 Triple DES: it is an improved version created after overcoming the shortcomings of DES. Since it is based on the DES algorithm, it is very easy to modify existing software to use Triple DES. It has the advantage of proven reliability and a longer key length that eliminates many of the shortcut attacks that can be used to reduce the amount of time it takes to break the DES.

 Advanced encryption standard (AES): the AES is an encryption standard adopted by the US Government. The standard comprises three block ciphers, AES-128, AES-192, and AES-256. Each AES cipher has a 128-bit block size with key sizes of 128, 192, and 256 bits, respectively. The AES ciphers have been analyzed extensively and are now used worldwide.

 International data encryption algorithm (IDEA): the IDEA was developed in 1991. It uses a 128-bit key to encrypt a 64-bit block of plaintext into a 64-bit block of ciphertext. IDEA’s general structure is very similar to DES. It performs 17 rounds, each round taking 64 bits of input to produce a 64-bit output, using per-round keys generated from the 128-bit key.

Figure 1.3. Symmetric key cryptography.

Key management in symmetric key systems

The symmetric key systems are simpler and faster but their main drawback is that the two parties must somehow exchange the key in a secure way and keep it secure after that. The key management caused a nightmare for the parties using the symmetric key cryptography. The worry was about how to get the keys safely and securely across all users so that the decryption of the message would be possible. This gave the chance for third parties to intercept the keys in transit to decode the secret messages. Thus, if the key was compromised, the entire coding system was compromised and a ‘secret’ would no longer remain a ‘secret’.