.2). Symmetric key or secret key cryptogra- phy algorithms and the public key or asymmetric cryptography algorithms are the two groups of cryptography algorithms. Fundamentals of EMS, NMS and OSS/BSS Sender Receiver Plaintext Plaintext Encryption Network Decryption Ciphertext Ciphertext Figure 31.1 Transformation of message from plaintext to ciphertext. Encryption key Decryption key Encryption Decryption Plaintext algorithm Ciphertext Ciphertext Plaintext algorithm Encryption Decryption Figure 31.2 Encryption and decryption process. 31.2 Symmetric Key Cryptography The symmetric key (secret key cryptography) is named SO because the same key is used by both the sender and the receiver to perform the encryption and decryp- tion process. The sender encrypts the data using the symmetric key along with the encryption algorithm. The receiver decrypts the data using the same symmetric key using the decryption algorithm. The algorithm used for decryption is the inverse of the algorithm used for encryption in symmetric key cryptography. Symmetric key cryptography is used to encrypt and decrypt long messages because the key is smaller. It is efficient and takes little time in encryption process. However it should be noted that the sender as well as the receiver must have the same unique symmet- ric key and the distribution of the keys between the two parties can be difficult. Some of the cipher types are: A. Substitution cipher: Using the substitution method, the substitution cipher substitutes a symbol with another. The characters present in the plaintext can be replaced with another character. It includes monoalphabetic and polyalphabetic substitution. Monoalphabetic substitution: A character in the plaintext is always converted to the same character in the ciphertext regardless of its position in the text. For example, if the algorithm says character C in the plaintext must be changed to character F, then every character C is changed to character F regardless of its position in the text. There is a one-to-one rela- tions