ABSTRACT
Before 1976, secure communications were achieved by encrypting transmitted
messages. There was no separate process for encryption and authentication,
i.e., there was only one operation: encryption. The authentication was in-
tended to be achieved by encryption, because if the message is altered, then
decryption will fail to output any meaningful text (here it uses the language
information to assist the verification). However, some communication systems
may not need to protect confidentiality, but authenticity and integrity of me-
dia must be protected, i.e., authentication only. On the other hand, for those
secure communications, at the receiver side, it should possess the same key
for decrypting the ciphertext as was used in the encryption. This requests to
distribute the keys among all communication entities. During the middle of
1970, since communication networks have been in practice, key distribution
has become a heavy burden to communication networks.
