e for each segment, implying that, if the data are four equal segments then the result will also have four equal segments. II. Triple DES is an algorithm compatible with DES that uses three DES blocks and 56-bit keys and operates on eight character segments. The encryption process in triple DES uses an encryption-decryption-encryption combina- tion of DES while the decryption process in triple DES uses a decryption- encryption-decryption combination of DES (see Figure 31.4). There are many other symmetric key algorithms like AES (Advanced Encryption Standard), RC4, IDEA (International Data Encryption Algorithm), Twofish, Serpent, Blowfish, and CAST5. Fundamentals of EMS, NMS and OSS/BSS 64-bit plaintext 56-bit key Complex set of 48-bits cipher blocks processor 64-bit ciphertext Figure 31.3 Block view of DES. 64-bit plaintext 64-bit plaintext Encrypt DES Decrypt DES Decrypt DES Encrypt DES Encrypt DES Decrypt DES 64-bit ciphertext 64-bit ciphertext Encryption triple DES Decryption triple DES Figure 31.4 Block view of triple DES. 31.3 Public Key Cryptography In this technique there are two keys: the private key is kept by the receiver and the public key is announced to the public. In public key cryptograghy, different keys are used for encryption and decryption. So the methodology uses asymmetric key algorithms. In the public key encryption or decryption process, each entity creates a public key and a private key SO that communication to a third party is possible through the public key. The numbers of keys that are used in the public key cryptography are far smaller when compared to symmetric key cryptography. The complexity of the public key cryptography algorithm restricts its use for large Secure Communication Message digest function (fixed length) Message (variable length) Figure 31.5 Making message digest. amount of texts. Also the relationship between an entity and its public key must be verified in the public key method. The digital signature that is used for authenticating the sender of documents and messag